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Handbook Of Magnetic Compass Adjustment


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HANDBOOK OF MAGNETIC COMPASS ADJUSTMENT AGENCY BETHESDA MD 2004 Formerly PubNo 226 AS ORIGINALLY PUBLISHED BY DEFENSE MAPPING AGENCY CENTER WASHINGTON DC 1980 INTRODUCTION This document has been prepared in order to present all pertinent information regarding the practical procedures ofmagnetic compass adjustment in one text As such it treats of the basic principles of compass deviations and their correctionand not of the details of particular compass equipment Although this text is presented as a systematic treatise on compass adjustment ships personnel who are inexperienced withcompass correction will find sufficient information in Chapters I and XIV to eliminate compass errors satisfactorily study of the entire text Reference should also be made to figure 318 for condensed information regarding thevarious compass errors and their correction In this handbook the term compass adjustment refers to any changes of permanent magnet of soft iron correctors wherebynormal compass errors are reduced The term compass compensation refers to any change in the current supplied to coils whereby the errors due to degaussing are reduced The basic text is the outgrowth of lecture notes prepared by Nye S Spencer and George F Kucera while presenting coursesof instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station NavalOperating Base Norfolk Virginia CHAPTER I PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT CHECKOFF LISTNOTE If the magnetic adjustment necessitates a movement of degaussing compensating coils or b a change of Flindersbar length the coil compensation must be checked Refer to Chapter XIV101 Dockside tests and adjustments 1 Physical checks on the compass and binnacle a Remove any bubbles in compass bowl article 402 b Test for moment and sensibility of compass needles article 403 c Remove any slack in gimbal arrangement d Magnetization check of spheres and Flinders bar article 404 e Alignment of compass with foreandaft line of ship article 405 f Alignment of magnets in binnacle g Alignment of heeling magnet tube under pivot point of compass h See that corrector magnets are available 2 Physical checks of gyro azimuth circle and peloruses a Alignment of all gyro repeater peloruses or dial peloruses with foreandaft line of ship article 405 b Synchronize gyro repeaters with master gyro c Make sure azimuth circle and peloruses are in good operating condition 3 Necessary data a Past history or log data which might establish length of Flinders bar articles 407 and 607 b Azimuths for given date and observers position Chapter VIII c Ranges or distant objects in vicinity local charts d Correct variation local charts e Degaussing coil current settings for swing for residual deviations after adjustment and compensation ships Degaussing Folder 4 Precautions a Determine transient deviations of compass from gyro repeaters doors guns etc Chapter X b Secure all effective magnetic gear in normal seagoing position before beginning adjustments c Make sure degaussing coils are secured before beginning adjustments Use reversal sequence if necessary d Whenever possible correctors should be placed symmetrically with respect to the compass articles 318 and 613 5 Adjustments a Place Flinders bar according to best available information articles 407 608 and 609 b Set spheres at midposition or as indicated by last deviation table c Adjust heeling magnet using balanced dip needle if available Chapter XI Applies when system other than gyro is used as heading reference 1102 Adjustments at sea These adjustments are made with the ship on an even keel and after steadying on each headingWhen using the gyro swing from heading to heading slowly and check gyro error by suns azimuth or ranges on each headingif desired to ensure a greater degree of accuracy article 706 Be sure gyro is set for the mean speed and latitude of thevessel Note all precautions in article 1014 above OSCAR QUEBEC international code signal should be flown toindicate such work is in progress Chapter VII discusses methods for placing the ship on desired headings 1 Adjust the heeling magnet while the ship is rolling on north and south magnetic heading until the oscillations of the compass card have been reduced to an average minimum This step is not required if prior adjustment has been made using a dip needle to indicate proper placement of the heeling magnet 2 Come to an east 090 cardinal magnetic heading Insert foreandaft B magnets or move the existing B magnets in such a manner as to remove all deviation 3 Come to a south 180 magnetic heading Insert athwartship C magnets or move the existing C magnets in such a manner as to remove all deviation 4 Come to a west 270 magnetic heading Correct half of any observed deviation by moving the B magnets 5 Come to a north 000 magnetic heading Correct half of any observed deviation by moving the C magnets The cardinal heading adjustments should now be complete 6 Come to any intercardinal magnetic heading eg northeast 045 Correct any observed deviation by moving the spheres in or out 7 Come to the next intercardinal magnetic heading eg southeast 135 Correct half of any observed deviation by moving the spheres The intercardinal heading adjustments should now be complete although more accurate results might be obtained by correcting the D error determined from the deviations on all four intercardinal heading as discussed in article 501 8 Secure all correctors before swinging for residual deviations 9 Swing for residual undegaussed deviations on as many headings as desired although the eight cardinal and intercardinal headings should be sufficient 10 Should there still be any large deviations analyze the deviation curve to determine the necessary corrections and repeat as necessary steps 1 through 9 above Chapter V 11 Record deviations and the details of corrector positions on standard Navy Form NAVSEA 31204 and in the Magnetic Compass Record NAVSEA 31203 article 901 12 Swing for residual degaussed deviations with the degaussing circuits properly energized Chapter XIV 13 Record deviations for degaussed conditions on standard Navy Form NAVSEA 31204103 The above checkoff list describes a simplified method of adjusting compasses designed to serve as a simple for the novice who chooses to follow a stepbystep procedure The Dockside Tests and Adjustments are essentialas a foundation for the Adjustments at Sea and if neglected may lead to spurious results or needless repetition of theprocedure at sea Hence it is strongly recommended that careful considerations be given these dockside checks prior tomaking the final adjustment so as to allow time to repair or replace faulty compasses anneal or replace magnetized spheres orFlinders bar realign binnacle move gyro repeater if it is affecting the compass or to make any other necessary It is further stressed that expeditious compass adjustment is dependent upon the application of the various correctorsin a logical sequence so as to achieve the final adjustment with a minimum number of steps This sequence is incorporated inthe above checkoff list and better results will be obtained if it is adhered to closely Figure 318 presents the various compasserrors and their correction in condensed form The table in figure 103 will further clarify the mechanics of placing thecorrector magnets spheres and Flinders bar Chapter IV discusses the more efficient and scientific methods of in addition to a more elaborate treatment of the items mentioned in the checkoff list Frequent should be made to determine the constancy of deviations and results should be systematically changes in deviation will indicate the need for readjustment To avoid Gaussin error article 1003 when adjusting and swinging ship for residuals the ship should be steady on thedesired heading for at least 2 minutes prior to observing the deviation 2 Foreandaft and athwartship magnets Quadrantal spheres Flinders bar E on NEly W on NEly Deviation change E on Ely and W W on Ely and E Easterly on east Westerly on east Deviation Deviation W on SEly E on SEly with change in on Wly when sail on Wly when sail and westerly on and easterly on E on SWly W on SWly latitude ing toward equator ing toward equator west west and and from N latitude or from N latitude or Magnets Spheres B error B error W on NWly E on NWly Bar away from equator away from equator D error D error to S latitude to S latitudeNo fore and aft Place required Place magnets red Place magnets red No spheres on Place spheres Place spheres fore Place required magnets in No bar in holder amount of bar forward aft binnacle athwartship and aft amount of bar aft binnacle forward Fore and aft Spheres at Move spheres Move spheres Bar forward of Increase amount Decrease amount magnets red Raise magnets Lower magnets athwartship toward compass or outward or binnacle of bar forward of bar forward forward position use larger spheres remove Move spheres Move spheres Fore and aft Spheres at fore Bar aft of Decrease amount Increase amount Lower magnets Raise magnets outward or toward compass ormagnets red aft and aft position binnacle of bar forward of bar forward remove use larger spheres E on Nly W on Nly W on Ely and E E on Ely and W Easterly on north Westerly on north Deviation Deviation W on Ely E on Ely Bar on Wly when sail on Wly when sail and westerly on and easterly on E on Sly W on Sly ing toward equator ing toward equator south south and and Deviation change from S latitude or from S latitude or Magnets Spheres C error C error W on Wly E on Wly with change in away from equator away from equator E error E error latitude to N latitude to N latitude Place spheres at Place spheres atNo athwartship Place athwartship Place athwartship No spheres on port forward and starboard forward Heeling magnet magnets in magnets red magnets red port binnacle starboard aft inter and port aft inter Adjust with changes in magnetic latitude binnacle starboard cardinal positions cardinal positions Slew spheres Athwartship Spheres at Slew spheres If compass north is attracted to high side of ship when rolling magnets red Raise magnets Lower magnets athwartship clockwise through raise the heeling magnet if red end is up or lower the heeling through required starboard position required angle magnet if blue end is up angle If compass north is attracted to low side of ship when rolling Slew spheres Slew spheres lower the heeling magnet if red end is up or raise the heeling Athwartship Spheres at fore Lower magnets Raise magnets clockwise through magnet if blue end is upmagnets red port and aft position through required required angle NOTE Any change in placement of the heeling magnet will angle affect the deviations on all headings Figure 103 Mechanics of magnetic compass adjustment 3 CHAPTER II MAGNETISM201 The magnetic compass The principle of the present day magnetic compass is in no way different from that of thecompass used by the ancients It consists of a magnetized needle or array of needles pivoted so that rotation is in ahorizontal plane The superiority of the present day compass results from a better knowledge of the laws of magnetismwhich govern the behavior of the compass and from greater precision in Magnetism Any piece of metal on becoming magnetized that is acquiring the property of attracting small particles ofiron or steel will assume regions of concentrated magnetism called poles Any such magnet will have at least two poles ofunlike polarity Magnetic lines of force flux connect one pole of such a magnet with the other pole as indicated in figure202 The number of such lines per unit area represents the intensity of the magnetic field in that area If two such magneticbars or magnets are placed side by side the like poles will repel each other and the unlike poles will attract each other Figure 202 Lines of magnetic force about a magnet203 Magnetism is in general of two types permanent and induced A bar having permanent magnetism will retain itsmagnetism when it is removed from the magnetizing field A bar having induced magnetism will lose its magnetism whenremoved from the magnetizing field Whether or not a bar will retain its magnetism on removal from the magnetizing fieldwill depend on the strength of that field the degree of hardness of the iron retentivity and also upon the amount of physicalstress applied to the bar while in the magnetizing field The harder the iron the more permanent will be the Terrestrial magnetism The accepted theory of terrestrial magnetism considers the earth as a huge magnet surroundedby lines of magnetic force that connect its two magnetic poles These magnetic poles are near but not coincidental with thegeographic poles of the earth Since the northseeking end of a compass needle is conventionally called a red pole north poleor positive pole it must therefore be attracted to a pole of opposite polarity or to a blue pole south pole or negative poleThe magnetic pole near the north geographic pole is therefore a blue pole south pole or negative pole and the magnetic polenear the south geographic pole is a red pole north pole or positive pole205 Figure 205 illustrates the earth and its surrounding magnetic field The flux lines enter the surface of the earth atdifferent angles to the horizontal at different magnetic latitudes This angle is called the angle of magnetic dip andincreases from zero at the magnetic equator to 90 at the magnetic poles The total magnetic field is generally considered ashaving two components namely H the horizontal component and Z the vertical component These components change asthe angle changes such that H is maximum at the magnetic equator and decreases in the direction of either pole Z is zero atthe magnetic equator and increases in the direction of either pole 4 Figure 205 Terrestrial magnetism206 Inasmuch as the magnetic poles of the earth are not coincidental with the geographic poles it is evident that a compassneedle in line with the earths magnetic field will not indicate true north but magnetic north The angular difference betweenthe true meridian great circle connecting the geographic poles and the magnetic meridian direction of the lines of magneticflux is called variation This variation has different values at different locations on the earth These values of may be found on the compass rose of navigational charts The variation for most given areas undergoes an annualchange the amount of which is also noted on all charts See figure 206 Figure 206 Compass rose showing variation and annual change 5207 Ships magnetism A ship while in the process of being constructed will acquire magnetism of a permanent natureunder the extensive hammering it receives in the earths magnetic field After launching the ship will lose some of thisoriginal magnetism as a result of vibration pounding etc in varying magnetic fields and will eventually reach a more orless stable magnetic condition This magnetism which remains is the permanent magnetism of the ship208 The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placedin a magnetic field such as the earths field The amount of magnetism induced in any given piece of soft iron is dependentupon the field intensity the alignment of the soft iron in that field and the physical properties and dimensions of the ironThis induced magnetism may add to or subtract from the permanent magnetism already present in the ship depending onhow the ship is aligned in the magnetic field The softer the iron the more readily it will be induced by the earths magneticfield and the more readily it will give up its magnetism when removed from that field209 The magnetism in the various structures of a ship which tends to change as a result of cruising vibration or aging butdoes not alter immediately so as to be properly termed induced magnetism is called subpermanent magnetism Thismagnetism at any instant is recognized as part of the ships permanent magnetism and consequently must be corrected assuch by means of permanent magnet correctors This subpermanent magnetism is the principal cause of deviation changes ona magnetic compass Subsequent reference to permanent magnetism in this text will refer to the apparent that includes the existing permanent and subpermanent magnetism at any given instant210 A ship then has a combination of permanent subpermanent and induced magnetism since its metal structures are ofvarying degrees of hardness Thus the apparent permanent magnetic condition of the ship is subject to change fromdeperming excessive shocks welding vibration etc and the induced magnetism of the ship will vary with the strength ofthe earths magnetic field at different magnetic latitudes and with the alignment of the ship in that field211 Resultant induced magnetism from earths magnetic field The above discussion of induced magnetism andterrestrial magnetism leads to the following facts A long thin rod of soft iron in a plane parallel to the earths field H will have a red north pole induced in the end toward the north geographic pole and a blue south poleinduced in the end toward the south geographic pole This same rod in a horizontal plane but at right angles to the field would have no magnetism induced in it because its alignment in the magnetic field is such that there will be notendency toward linear magnetization and the rod is of negligible cross section Should the rod be aligned in some between those headings that create maximum and zero induction it would be induced by an amount that is afunction of the angle of alignment If a similar rod is placed in a vertical position in northern latitudes so as to be aligned withthe vertical earths field Z it will have a blue south pole induced at the upper end and a red north pole induced at the lowerend These polarities of vertical induced magnetization will be reversed in southern latitudes The amount of horizontal orvertical induction in such rods or in ships whose construction is equivalent to combinations of such rods will vary with theintensity of H and Z heading and heel of the ship 6 CHAPTER III THEORY OF MAGNETIC COMPASS ADJUSTMENT301 Magnetic adjustment The magnetic compass when used on a steel ship must be so corrected for the ships that its operation approximates that of a nonmagnetic ship Ships magnetic conditions create deviations of themagnetic compass as well as sectors of sluggishness and unsteadiness Deviation is defined as deflection of the card needlesto the right or left of the magnetic meridian Adjustment of the compass is the arranging of magnetic and soft iron the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship thus reducingthe deviations and eliminating the sectors of sluggishness and unsteadiness The magnetic conditions in a ship which affect a magnetic compass are permanent magnetism and induced magnetism asdiscussed in Chapter II302 Permanent magnetism and its effects on the compass The total permanent magnetic field effect at the compass maybe broken into three components mutually 90 apart as shown in figure 302a The effect of the vertical is the tendency to tilt the compass card and in the event of rolling or pitching of the ship to create of the card Oscillation effects that accompany roll are maximum on north and south compass headings and thosethat accompany pitch are maximum on east and west compass headings The horizontal B and C components of cause varying deviations of the compass as the ship swings in heading on an even keel Plotting these compass heading will produce sine and cosine curves as shown in figure 302b These deviation curves are curves because they reverse direction in 180 Figure 302a Components of permanent magnetic Figure 302b Permanent magnetic deviation effects field at the compass303 The permanent magnetic semicircular deviations can be illustrated by a series of simple sketches representing a ship onsuccessive compass headings as in figures 303a and 303b304 The ships illustrated in figures 303a and 303b are pictured on cardinal compass headings rather than on headings for two reasons 1 Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzed mathematically This can be visualized by noting that the ships component magnetic fields are either in line with or perpendicular to the compass needles only on cardinal compass headings 2 Such a presentation illustrates the fact that the compass card tends to float in a fixed position in line with the magnetic meridian Deviations of the card to right or left east or west of the magnetic meridian result from the movement of the ship and its magnetic fields about the compass card 7 Figure 303a Force diagrams for foreandaft permanent B magnetic field Figure 303b Force diagrams for athwartship permanent C magnetic field305 Inasmuch as a compass deviation is caused by the existence of a force at the compass that is superimposed upon thenormal earths directive force H a vector analysis is helpful in determining deviations or the strength of deviating fields Forexample a ship as shown in figure 305 on an east magnetic heading will subject its compass to a combination of namely the earths horizontal field H and the deviating field B at right angles to the field H The compass needlewill align itself in the resultant field which is represented by the vector sum of H and B as shown A similar analysis on theship in figure 305 will reveal that the resulting directive force at the compass would be maximum on a north heading andminimum on a south heading the deviations being zero for both conditions The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H resulting from permanent magnetic fields will vary with the magnetic latitude of the ship 8 Figure 305 General force diagram306 Induced magnetism and its effects on the compass Induced magnetism varies with the strength of the the mass of metal and the alignment of the metal in the field Since the intensity of the earths magnetic field variesover the earths surface the induced magnetism in a ship will vary with latitude heading and heel of the ship307 With the ship on an even keel the resultant vertical induced magnetism if not directed through the compass itself willcreate deviations that plot as a semicircular deviation curve This is true because the vertical induction changes magnitudeand polarity only with magnetic latitude and heel and not with heading of the ship Therefore as long as the ship is in thesame magnetic latitude its vertical induced pole swinging about the compass will produce the same effect on the compass asa permanent pole swinging about the compass Figure 307a illustrates the vertical induced poles in the structures of a ship Figure 307a Ships vertical induced magnetism Figure 307b Induced magnetic deviation effects Generally this semicircular deviation will be a B sine curve as shown in figure 307b since most ships are the centerline and have their compasses mounted on the centerline The magnitude of these deviations will change withmagnetic latitude changes because the directive force and the ships vertical induction both change with magnetic latitude308 The masses of horizontal soft iron that are subject to induced magnetization create characteristic deviations as indicatedin figure 307b The D and E deviation curves are called quadrantal curves because they reverse polarity in each of the fourquadrants 9309 Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated infigure 309 Figure 309 Symmetrical arrangements of horizontal soft iron310 The deviation resulting from the earths field induction of these symmetrical arrangements of horizontal soft iron areillustrated in figure 310 showing the ship on various compass headings The other heading effects may be similarly studiedSuch a D deviation curve is one of the curves in figure 307b It will be noted that these D deviations are maximum on headings and zero on the cardinal headings Figure 310 Effects of symmetrical horizontal D induced magnetism 10311 Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those infigure 311 Figure 311 Asymmetrical arrangements of horizontal soft iron312 The deviations resulting from the earths field induction of these asymmetrical arrangements of horizontal soft iron areillustrated in figure 312 showing the ship on different compass headings The other heading effects may be similarly studiedSuch an E deviation curve is one of the curves in figure 307b It will be observed that these E deviations are maximum oncardinal headings and zero on the intercardinal headings Figure 312 Effects of asymmetrical horizontal E induced magnetism313 The quadrantal deviations will not vary with latitude changes because the horizontal induction varies the directive force H314 The earths field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant Adeviation curve The magnetic A and E errors are of smaller magnitude than the other errors but when encountered aregenerally found together since they both result from asymmetrical arrangements of horizontal soft iron In addition to thismagnetic A error there are constant A deviations resulting from 1 physical misalignments of the compass pelorus or gyro2 errors in calculating the suns azimuth observing time or taking bearings 11315 The nature magnitude and polarity of all these induced effects are dependent upon the disposition of metal thesymmetry or asymmetry of the ship the location of the binnacle the strength of the earths magnetic field and the angle ofdip316 Certain heeling errors in addition to those resulting from permanent magnetism are created by the presence of bothhorizontal and vertical soft iron which experience changing induction as the ship rolls in the earths magnetic field This partof the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship Oscillation roll are maximum on north and south headings just as with the permanent magnetic heeling errors317 Adjustments and correctors Since some magnetic effects remain constant for all magnetic latitudes and others varywith changes of magnetic latitude each individual effect should be corrected independently Further it is apparent that thebest method of adjustment is to use 1 permanent magnet correctors to create equal and opposite vectors of fields at the compass and 2 soft iron correctors to assume induced magnetism the effect of which will be equaland opposite to the induced effects of the ship for all magnetic latitude and heading conditions The compass for the support of the compass and such correctors Study of the binnacle in figure 317 will reveal that suchcorrectors are present in the form of 1 Vertical permanent heeling magnet in the central vertical tube 2 Foreandaft B permanent magnets in their trays 3 Athwartship C permanent magnets in their trays 4 Vertical soft iron Flinders bar in its external tube 5 Soft iron spheres The heeling magnet is the only corrector that corrects for both permanent and induced effects and consequently must bereadjusted occasionally with radical changes in latitude of the ship It must be noted however that any movement of theheeling magnet will require readjustment of other correctors Figure 317 Binnacle with compass and correctors 12318 The tabular summary of Compass Errors and Adjustments figure 318 summarizes all the various magnetic conditions in a ship the types ofdeviation curves they create the correctors for each effect and headings on which each corrector is adjusted Correctors should be under all but exceptional conditions discussed in detail later and as far away from the compass as possible to preserve uniformity ofmagnetic fields about the compass needle array Other details of corrector procedure are emphasized in chapter VI Fortunately each magnetic effect hasa slightly different characteristic curve that makes identification and correction convenient A complete deviation curve can be analyzed for its and thus the necessary corrections anticipated A method for analyzing such curves is described in chapter V Compass Coefficient Type deviation curve Causes of such errors Corrections for such errors Magnetic or compass headings of headings on which to apply maximum correctors deviation Human error in calculations Check methods and calculations A Constant Same on all Any Physical compass gyro pelorus alignment Check alignments Magnetic asymmetrical arrangements of horizontal soft iron Rare arrangement of soft iron rods Foreandaft component of permanent magnetic field Foreandaft B magnets B Semicircular sin 090 090 or 270 Induced magnetism in asymmetrical vertical iron 270 forward or aft of compass Flinders bar forward or aft Athwartship component of permanent magnetic field Athwartship C magnets C Semicircular cos 000 000 or 180 Induced magnetism in asymmetrical vertical iron port or 180 starboard of compass Flinders bar port or starboard 045 Induced magnetism in all symmetrical arrangements of Spheres on appropriate axis D Quadrantal sin 2 045 135 225 or 315 135 horizontal soft iron athwartship for D 225 fore and aft for D 315 See sketch a 000 Induced magnetism in all asymmetrical arrangements Spheres on appropriate axis E Quadrantal cos 2 000 090 180 or 270 090 of horizontal soft iron port forward starboard aft for E 180 starboard forward port aft for E 270 See sketch b Heeling Oscillations with roll 000 Change in the horizontal component of the induced or Heeling magnet must be readjusted 090 or 270 with dip roll or pitch 180 permanent magnetic fields at the compass due to for latitude changes needle Deviations with 090 rolling or pitching of the ship 000 or 180 while rolling pitch constant list 270 Deviation A B sin C cos D sin 2 E cos 2 compass heading Figure 318 Summary of Compass Errors and Adjustments 13319 Compass operation Figure 319 illustrates a point about compass operation Not only is an uncorrected compass subjectto large deviations but there will be sectors in which the compass may sluggishly turn with the ship and other sectors inwhich the compass is too unsteady to use These performances may be appreciated by visualizing a ship with deviations asshown in figure 319 as it swings from west through north toward east Throughout this easterly swing the compass deviationis growing more easterly and whenever steering in this sector the compass card sluggishly tries to follow the shipSimilarly there is an unsteady sector from east through south to west These sluggish and unsteady conditions are by the positive and negative slopes in a deviation curve These conditions may also be associated with themaximum and minimum directive force acting on the compass see article 305 It will be observed that the occurs at the point of average directive force and that the zero deviations occur at the points of maximum andminimum directive force Figure 319 Uncompensated deviation curve320 Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compassfor all headings of the ship Correction could be achieved however by applying correctors so as to equalize the across the compass position for all headings of the ship The deviation method is more generally used because itutilizes the compass itself to indicate results rather than some additional instrument for measuring the intensity of Occasionally the permanent magnetic effects at the location of the compass are so large that they overcome the force H This condition will not only create sluggish and unsteady sectors but may even freeze the compass to onereading or to one quadrant regardless of the heading of the ship Should the compass be so frozen the polarity of themagnetism which must be attracting the compass needles is indicated hence correction may be effected simply by theapplication of permanent magnet correctors in suitable quantity to neutralize this magnetism Whenever such adjustments aremade it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will beimmediately evident For example a ship whose compass is frozen to a north reading would require foreandaft B with the red ends forward in order to neutralize the existing blue pole that attracted the compass If made on an eastheading such an adjustment would be practically complete when the compass card was freed so as to indicate an eastheading322 Listed below are several reasons for correcting the errors of the magnetic compass 1 It is easier to use a magnetic compass if the deviations are small 2 Although a common belief is that it does not matter what the deviations are as long as they are known this is in error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make the compass operationally unsatisfactory This is the result of unequal directive forces on the compass as the ship swings in heading 3 Furthermore even though the deviations are known if they are large they will be subject to appreciable change with heel and latitude changes of the ship323 Subsequent chapters will deal with the methods of bringing a ship to the desired heading and the methods of effects and of minimizing interaction effects between correctors Once properly adjusted the magnetic should remain constant until there is some change in the magnetic condition of the vessel resulting from shock from gunfire vibration repair or structural changes Frequently the movement of nearby guns doors gyrorepeaters or cargo affects the compass greatly 14 CHAPTER IV PRACTICAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENTNOTE If the adjuster is not familiar with the theory of magnetic effects the methods of analyzing deviation curves and themethods of placing a ship on any desired heading it is recommended to review Chapters II V and VII respectively adjustment401 Dockside tests and adjustments Chapter I Procedures for Magnetic Compass Adjustment is in general and brings to the attention of the adjuster many physical checks which are desirable before beginning anadjustment The theoretical adjustment is based on the premise that all the physical arrangements are perfect and much timeand trouble will be saved while at sea if these checks are made before attempting the actual magnet and soft iron A few of these checks are amplified below402 Should the compass have a small bubble compass fluid may be added by means of the filling plug on the side of thecompass bowl If an appreciable amount of compass liquid has leaked out a careful check should be made on the conditionof the sealing gasket and filling plug US Navy compass liquid may be a mixture of 45 grain alcohol and 55 distilledwater or a kerosenetype fluid These fluids are NOT The compass should be removed from the ship and taken to some place free from all magnetic influences except theearths magnetic field for tests of moment and sensibility These tests involve measurements of the time of vibration and theability of the compass card to return to a consistent reading after deflection These tests will indicate the condition of thepivot jewel and magnetic strength of the compass needles See NAVSEA 31203 for such test data on standard Navycompass equipment404 A careful check should be made on the spheres and Flinders bar for residual magnetism Move the spheres as close tothe compass as possible and slowly rotate each sphere separately Any appreciable deflection 2 or more of the compassneedles resulting from this rotation indicates residual magnetism in the spheres This test may be made with the ship on anysteady heading The Flinders bar magnetization check is preferably made with the ship on steady east or west compassheading To make this check a note the compass reading with the Flinders bar in the holder b invert the Flinders bar inthe holder and again note the compass reading Any appreciable difference 2 or more between these observed residual magnetism in the Flinders bar Spheres or Flinders bars that show signs of such residual magnetism shouldbe annealed ie heated to a dull red and allowed to cool slowly405 Correct alignment of the lubbers line of the compass gyro repeater and pelorus with the foreandaft line of the ship isof major importance Such a misalignment will produce a constant A error in the curve of deviations All of these instrumentsmay be aligned correctly with the foreandaft line of the ship by using the azimuth circle and a metal tape measure Shouldthe instrument be located on the centerline of the ship a sight is taken on a mast or other object on the centerline In the caseof an instrument off the centerline a metal tape measure is used to measure the distance from the centerline of the ship to thecenter of the instrument A similar measurement from the centerline is made forward or abaft the subject instrument andreference marks are placed on the deck Sights are then taken on these marks Standard compasses should always be aligned so that the lubbers line of the compass is parallel to the foreandaft line ofthe ship Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A errorpresent as discussed in article 411406 In addition to the physical checks listed in Chapter I there are other precautions to be observed in order to satisfactory compass operation These precautions are mentioned to bring to the attention of the adjuster that might disturb compass operation They are listed in Chapter I and are discussed further in Chapter X Expeditious compass adjustment is dependent upon the application of the various correctors in an optimum sequence so asto achieve the final adjustment with a minimum number of steps Certain adjustments may be made conveniently at docksideso as to simplify the adjustment procedures at sea407 Inasmuch as the Flinders bar is subject to induction from several of the other correctors and since its adjustment is notdependent on any single observation this adjustment is logically made first This adjustment is made by one of the 1 Deviation data obtained at two different magnetic latitudes may be utilized to calculate the proper length of Flinders bar for any particular compass location Details of the acquisition of such data and the calculations involved are presented in articles 605 to 609 inclusive 15 2 If the above method is impractical the Flinders bar length will have to be set approximately by a Using an empirical amount of Flinders bar that has been found correct for other ships of similar structure b Studying the arrangement of masts stacks and other vertical structures and estimating the Flinders bar length requiredIf these methods are not suitable the Flinders bar should be omitted until data is acquired The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the topWooden spacers are used at the bottom of the tube to achieve such spacing408 Having adjusted the length of Flinders bar place the spheres on the bracket arms at the best approximate position If thecompass has been adjusted previously place the spheres at the best position as indicated by the previous deviation table Inthe event the compass has never been adjusted place the spheres at midposition on the bracket arms409 The next adjustment is the positioning of the heeling magnet by means of a properly balanced dip needle as discussedin Chapter XI410 These three adjustments at dockside Flinders bar spheres and heeling magnet will properly establish the conditionsof mutual induction and shielding on the compass such that a minimum of procedures at sea will complete the adjustment411 Expected errors Figure 318 Summary of Compass Errors and Adjustment lists six different coefficients or types ofdeviation errors with their causes and corresponding correctors A discussion of these coefficients follows The A error is more generally caused by the miscalculation of azimuths or by physical misalignments rather than of asymmetrical arrangements of horizontal soft iron Thus if the physical alignments are checked at dockside and ifcare is exercised in making all calculations the A error will be insignificant Where an azimuth or bearing circle is used on astandard compass to determine deviations any observed A error will be solely magnetic A error This results from the factthat such readings are taken on the face of the compass card itself rather than at the lubbers line of the compass On asteering compass where deviations are obtained by a comparison of the compass lubbers line reading with the shipsmagnetic heading as determined by pelorus or gyro any observed A error may be a combination of magnetic A andmechanical A misalignment These facts explain the procedure wherein only mechanical A is corrected on the by realignment of the binnacle and both mechanical A and magnetic A errors are corrected on the steering compassby realignment of the binnacle see article 405 On the standard compass the mechanical A error may be isolated from themagnetic A error by making the following observations simultaneously 1 Record a curve of deviations by using an azimuth or bearing circle An A error found will be solely magnetic A 2 Record a curve of deviations by comparison of the compass lubbers line reading with the ships magnetic heading as determined by pelorus or by gyro Any A error found will be a combination of mechanical A and magnetic AThe mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total Afound in the second instance and is corrected by rotating the binnacle in the proper direction by that amount It is nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyromay be removed by rotating the binnacle in the proper direction by that amount The B error results from two different causes namely the foreandaft permanent magnetic field across the compass and aresultant asymmetrical vertical induced effect forward or aft of the compass The former is corrected by the use of foreandaft B magnets and the latter is corrected by the use of the Flinders bar forward or aft of the compass Inasmuch as theFlinders bar setting has been made at dockside any B error remaining is corrected by the use of foreandaft B magnets The C error has two causes namely the athwartship permanent magnetic field across the compass and a vertical induced effect athwartship of the compass The former is corrected by the use of athwartship Cmagnets and the latter by the use of the Flinders bar to port or starboard of the compass but inasmuch as this effect is very rare the C error is corrected by athwartship C magnets only The D error is due only to induction in the symmetrical arrangements of horizontal soft iron and requires correction byspheres generally athwartship of the compass The existence of E error of appreciable magnitude is rare since it is caused by induction in the asymmetrical arrangementsof horizontal soft iron When this error is appreciable it may be corrected by slewing the spheres as described in Chapter VI As has been stated previously the heeling error is most practically adjusted at dockside with a balanced dip needle SeeChapter XI412 A summary of the above discussion reveals that certain errors are rare and others have been corrected by adjustments atdockside Therefore for most ships there remain only three errors to be corrected at sea namely the B C and D errorsThese are corrected by the use of foreandaft B magnets athwartship C magnets and quadrantal spheres respectively 16413 Study of adjustment procedure Inspection of the general B C and D combination of errors pictured in figure 413 willreveal that there is a definite isolation of the deviation effects on cardinal compass headings Figure 413 B C and D deviation effects For example on 090 or 270 compass headings the only deviation which is effective is that due to B This isolation andthe fact that the B effect is greatest on these two headings make these headings convenient for B correction Correction of theB deviation on a 090 heading will correct the B deviation on the 270 heading by the same amount but in the and naturally it will not change the deviations on the 000 and 180 headings except where B errors are largeHowever the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090 or270 deviation correction but only by seventenths 07 of that amount since the sine of 45 equals 0707 The same convenient isolation of effects and corrections of C error will also change the deviations on all the by the seventenths rule as before It will now be observed that only after correcting the B and C errors on thecardinal headings and consequently their proportional values of the total curve on the intercardinal headings can the D errorbe observed separately on any of the intercardinal headings The D error may then be corrected by use of the spheres on heading Correcting D error will as a rule change the deviations on the intercardinal headings only and not onthe cardinal headings Only when the D error is excessive the spheres are magnetized or the permanent magnet correctorsare so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as aresult of sphere adjustments Although sphere correction does not generally correct deviations on cardinal headings it doesimprove the stability of the compass on these headings414 If it were not for the occasional A or E errors which exist the above procedure of adjustment would be quite sufficientie adjust observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal headingHowever figure 414 showing a combination of A and B errors will illustrate why adjusting procedure must deviations on more than the three essential headings If the assumption were made that no A error existed in the curve illustrated in figure 414 and the total deviation of 6E onthe 090 heading were corrected with B magnets the error on the 270 heading would be 4E due to B overcorrection If thenthis 4E error were taken out on the 270 heading the error on the 090 heading would then be 4E due to B Figure 414 A and B deviation effects 17 The proper method of eliminating this toandfro procedure and also correcting the B error of the ship to the best possibleflat curve would be to split this 4E difference leaving 2E deviation on each opposite heading This would in effect correctthe B error leaving only the A error of 2E which must be corrected by other means It is for this reason that 1 splitting isdone between the errors noted on opposite headings and 2 good adjustments entail checking on all headings rather than onthe fundamental three415 Before anything further is said about adjustment procedures it is suggested that care be exercised to avoid moving thewrong corrector Not only will such practice be a waste of time but it will also upset all previous adjustments Throughout an adjustment special care should be taken to pair off spare magnets so that the resultant field aboutthem will be negligible To make doubly sure that the compass is not affected by stray fields from them they should be keptat an appropriate distance until one or more is actually to be inserted into the binnacle416 Adjustment procedures at sea Before proceeding with the adjustment at sea the following precautions should beobserved 1 Secure all effective magnetic gear in the normal seagoing position 2 Make sure the degaussing coils are secured using the reversal sequence if necessaryThe adjustments are made with the ship on an even keel swinging from heading to heading slowly and after steadying oneach heading for at least 2 minutes to avoid Gaussin error article 1003 Chapter VII discusses methods of placing a ship onthe desired heading417 Most adjustments can be made by trial and error or by routine procedure such as the one presented in Chapter IHowever it is more desirable to follow some analytical procedure whereby the adjuster is always aware of the magnitude ofthe errors on all headings as a result of his movement of the different correctors Two such methods are presented 1 A complete deviation curve can be taken for any given condition and an estimate made of all the approximate coefficients See Chapter V for methods of making such estimates From this estimate the approximate coefficients are established and the appropriate corrections are made with reasonable accuracy on a minimum number of headings If the original deviation curve has deviations greater than 20 rough adjustments should be made on two adjacent cardinal headings before recording curve data for such analysis The mechanics of applying correctors are presented in figure 103 A method of tabulating the anticipated deviations after each correction is illustrated in figure 417a The deviation curve used for illustration is the one that is analyzed in Chapter V Analysis revealed these coefficients A 10E B 120E C 80E D 50E E 15E Anticipated Anticipated Anticipated Anticipated Anticipated Original curve after curve after curve after curve after Heading by curve after deviation next next next next compass first correcting curve correcting correcting correcting correcting A 10E B 120E C 80E D 50E E 15E 000 105E 95E 95E 15E 15E 00 045 200E 190E 106E 50E 00 00 090 115E 105E 15W 15W 15W 00 135 12W 22W 106W 50W 00 00 180 55W 65W 65W 15E 15E 00 225 80W 90W 06W 50E 00 00 270 125 W 135W 15W 15W 15W 00 315 68W 78W 06E 50W 00 00 Figure 417a Tabulating anticipated deviations Analysis method 182 More often it is desirable to begin adjustment immediately eliminating the original swing for deviations and the estimate or approximate coefficients In this case the above problem would be solved by tabulating data and anticipating deviation changes as the corrections are made Figure 417b illustrates such procedure It will be noted that a new column of values is started after each change is made This method of tabulation enables the adjuster to calculate the new residual deviations each time a corrector is changed so that a record of deviations is available at all times during the swing Arrows are used to indicate where each change is made Anticipated Anticipated Anticipated Anticipated Anticipated Original curve after curve after curve after curve after Heading by curve after deviation next next next next compass first correcting curve correcting correcting correcting correcting A 10E B 120E C 80E D 50E E 15E 000 105E 25E 25E 15E 00 045 64E 14E 04E 04E 090 115E 00 00 00 10W 05E 135 92W 36W 14E 04E 04E 180 55W 25E 25E 15E 00 225 00 56E 06E 04W 04W 270 10W 10W 10W 20W 05W 315 12E 44W 06E 04W 04W Figure 417b Tabulating anticipated deviations Oneswing method Since the B error is generally greatest it is corrected first hence on a 090 heading the 115E deviation is corrected zero by using foreandaft B magnets A lot of time need not be spent trying to reduce this deviation to exactlyzero since the B coefficient may not be exactly 115E and some splitting might be desirable later After correcting on the090 heading the swing would then be continued to 135 where a 92W error would be observed This deviation is recordedbut no correction is made because the quadrant error is best corrected after the deviations on all four cardinal headings havebeen corrected The deviation on the 180 heading would be observed as 55W Since this deviation is not too large andsplitting may be necessary later it need not be corrected at this time Continuing the swing to 225 a 00 deviation would beobserved and recorded On the 270 heading the observed error would be 10W which is compared with 00 deviation onthe opposite 090 heading This could be split leaving 05W deviation on both 090 and 270 but since this is so small itmay be left uncorrected On 315 the observed deviation would be 12E At 000 a deviation of 105E would be observedand compared with 55W on 180 Analysis of the deviations on 000 and 180 headings reveals an 80E C error whichshould then be corrected with athwartship C magnets leaving 25E deviation on both the 000 and 180 headings All thedeviations in column two are now recalculated on the basis of such an adjustment at 000 heading and entered in columnthree Continuing the swing the deviation on 045 would then be noted as 64 E Knowing the deviations on all it is now possible to estimate the approximate coefficient D D is 50E so the 64E deviation on 045 is correctedto 104E and new anticipated values are recorded in another column This anticipates a fairly good curve an estimate ofwhich reveals in addition to the B of 05E which was not considered large enough to warrant correction an A of 10E andan E of 15E These A and E errors may or may not be corrected as practical If they are corrected the subsequent stepswould be as indicated in the last two columns It will be noted that the ship has made only one swing all corrections havebeen made and some idea of the expected curve is available418 Deviation curves The last step after completion of either of the above methods of adjustment is to secure allcorrectors in position and to swing for residual deviations These residual deviations are for undegaussed conditions of theship which should be recorded together with details of corrector positions on the standard Navy Form NAVSEA 31204 andin the Magnetic Compass Record NAVSEA 31203 Article 901 discusses the purposes of the various NAVSEA RecordForms more fully Navy Form NAVSEA 31204 is complete and desirable in the interest of improved Flinders bar correctionand shielding conditions Figure 418 illustrates both sides of form NAVSEA 31204 with proper instructions and and Flinders bar data Should the ship be equipped with degaussing coils a swing for residual deviations underdegaussed conditions should also be made and data recorded on NAVSEA 31204 On these swings extreme care should be exercised in taking bearings or azimuths and in steadying down on each headingsince this swing is the basis of standard data for that particular compass If there are any peculiar changeable errors such asmovable guns listing of the ship or anticipated decay from deperming which would effect the reliability of the compassthey should also be noted on the deviation card at this time Chapter X discusses these many sources of error in detail If the 19Flinders bar adjustment is not based on accurate data as with a new ship it would be well to exercise particular care inrecording the conventional Daily Compass Log data during the first cruise on which a considerable change of occurs Figure 418 Deviation table NAVSEA 31204419 In order to have a reliable and uptodate deviation card at all times it is suggested that the ship be swung to checkcompass deviations and to make readjustments if necessary after 1 Radical changes in magnetic latitude 2 Deperming Delay adjustment several days if possible after treatment 3 Structural changes 4 Long cruises or docking on the same heading such that the permanent magnetic condition of the vessels has changed 5 Magnetic equipment near the binnacle has been altered 6 Reaching the magnetic equator in order to acquire Flinders bar data See Chapter VI 7 At least once yearly to account for magnetic decay etc 8 Appreciable change of heeling magnet position if Flinders bar is present 9 Readjustment of any corrector 10 Change of magnetic cargo 11 such reasonable care the compass should be a reliable instrument requiring little attention 20 CHAPTER V TYPICAL DEVIATION CURVE AND THE ESTIMATION OF APPROXIMATE Simple analysis The data for the deviation curve illustrated in figure 501 is as follows Ships compass heading Deviation N 000 105E NE 045 200E E 090 115E SE 135 12W S 180 55W SW 225 80W W 270 125W NW 315 68W Figure 501 Typical deviation curve and its individual components Since A is the coefficient of constant deviation its approximate value is obtained from the above data by estimating themean of the algebraic sum of all the deviations Throughout these computations the sign of east deviation is considered plusand west deviation is considered minus 8A 105 200 115 12 55 80 125 68 8A 420 340 8A 80 A 10 10E Since B is the coefficient of semicircular sine deviation its value is maximum but of opposite polarity on 090 and 270headings The approximate B coefficient is estimated by taking the mean of the deviations at 090 and 270 with the sign at270 reversed 2B 115 125 2B 240 B 120 120E 21 Similarly since C is the coefficient of semicircular cosine deviation its value is maximum but of opposite polarity on000 and 180 headings and the approximate C coefficient is estimated by taking the mean of the deviations at 000 and180 with the sign at 180 reversed 2C 105 55 2C 160 C 80 80 E D is the coefficient of quadrantal sine deviation having maximum but alternately opposite polarity on the Hence the approximate D coefficient is estimated by taking the mean of the four intercardinal deviations with thesigns at 135 and 315 reversed 40 200 12 80 68 40 200 D 50 50E E is the coefficient of quadrantal cosine deviation having maximum but alternately opposite polarity on the Therefore the approximate E coefficient is estimated by taking the mean of the four cardinal deviations with thesigns at 090 and 270 reversed 4E 105 115 55 125 4E 60 E 15 15E These approximate coefficients are estimated from deviations on compass headings rather than on magnetic headings Thearithmetic solution of such coefficients will automatically assign the proper polarity to each coefficient Summarizing theabove we find the approximate coefficients of the given deviation curve to be A 10E B 120E C 80E D 50E E 15E Each of these coefficients represents a component of deviation that can be plotted as shown in figure 501 The polarity ofeach component in the first quadrant must agree with the polarity of the coefficient A check on the components in figure 501will reveal that their summation equals the original curve This method of analysis is accurate only when the deviations areless than 20 The mathematical expression for the deviation on any heading using the approximate coefficients is Deviation A B sin C cos D sin 2 E cos 2 where represents compass heading The directions given above for calculating coefficients A and B are not based upon accepted theoretical methods ofestimation Some cases may exist where appreciable differences may occur in the coefficients as calculated by the abovemethod and the accepted theoretical method The proper calculation of coefficients B and C is as followsLetting D1 D2 D8 be the eight deviation data then 2 1 B D2 D4 D6 D8 D3 D7 8 4 2 1 C D2 D4 D6 D8 D1 D5 8 4 22Substituting deviation data algebraically east being plus and west minus 2 1 B 200 12 80 68 115 125 8 4 B 12 2 1 C 200 12 80 68 105 55 8 4 C 8502 Reasons for analysis This method of estimating approximate coefficients is convenient for 1 Analyzing an original deviation curve in order to anticipate necessary corrections 2 Analyzing a final deviation curve for the determination of additional refinements 3 Simplifying the actual adjustment procedure by anticipating effects of certain corrector changes on the deviations at all other headings503 Approximate and exact coefficients It is emphasized that the above estimations are for the approximate not for exact coefficients Approximate coefficients are in terms of angular deviations that are caused by certain magneticforces and as stated before some of these deviations are subject to change with changes in the directive force H The are expressions of magnetic forces dealing with a arrangements of soft iron b components of fields c components of the earths magnetic field and d the shielding factor Thus the exact coefficients areexpressions of magnetic force which produce the deviations expressed by the approximate coefficients The exact for mathematical considerations while the approximate coefficients are more practical for adjustment purposes For thisreason the exact coefficients and the associated mathematics are not expanded further in this text504 Compass heading and magnetic heading When deviations are large there is an appreciable difference in the deviationcurve if it is plotted on crosssection paper against compass headings or against magnetic headings of the ship Not only isthere a difference in the shape of the curves but if only one curve is available navigators will find it difficult in when converting from magnetic heading to compass heading and vice versa When deviations are small noconversion is necessary Figure 504 illustrates the differences mentioned above by presenting the deviation values used infigure 501 as plotted against magnetic headings as well as against compass headings Figure 504 Comparison of deviation curves magnetic heading vs compass heading 23 CHAPTER VI CORRECTOR BETWEEN CORRECTORS601 Until now the principles of compass adjustment have been considered from a qualitative point of view In general this isquite sufficient since the correctors need merely be moved until the desired amount of correction is obtained However it isoften valuable to know the quantitative effects of different correctors as well as their qualitative effects Furthermore as hasbeen stated previously all the correctors are not completely independent of each other Interaction results from the proximityof the permanent magnet correctors to the soft iron correctors with appreciable induction effects in the latter shift in the relative position of the various correctors will change their interaction effects as well as their effects Additional inductions exist in the soft iron correctors from the magnetic needles of the compass itself Theadjuster should therefore be familiar with the nature of these interactions so as to evolve the best methods of adjustment602 Quandrantal sphere correction Figure 602 presents the approximate quadrantal correction available with differentsizes of spheres at various positions on the sphere brackets and with different magnetic moment compasses corrections apply whether the spheres are used as D E or combination D and E correctors Quadrantal correctionfrom spheres is due partially to the earths field induction and partially to compass needle induction Since compass does not change with magnetic latitude but earths field induction does the sphere correction is not constant for allmagnetic latitudes A reduction in the percentage of needle induction in the spheres to the earths field induction in thespheres will improve the constancy of sphere correction over all magnetic latitudes Such a reduction in the percentage ofneedle induction may be obtained by 1 Utilizing a low magnetic moment compass article 613 2 Utilizing special correctors placed with their major axes perpendicular to their axis of position 3 Using larger spheres farther away from the compass Figure 602603 Slewing of spheres Figure 603a is a convenient chart of determining the proper slewed position for spheres The totalvalues of the D and E quadrantal coefficients are used on the chart to locate a point of intersection This point directly locatesthe angle and direction of slew for the spheres on the illustrated binnacle This point will also indicate on the radial scale theresultant amount of quadrantal correction required from the spheres in the new slewed position to correct for both D and Ecoefficients The total D and E coefficients may be calculated by an analysis of deviations on the uncorrected binnacle or bysummarizing the uncorrected coefficients with those already corrected The data in figures 602 and 603b will be useful ineither procedure 24 Figure 603a Slewing of quadrantal spheres Figure 603b Quandrantal error from Standard Navy Flinders barExample A ship having a Navy Standard binnacle with 7 spheres at 13 position athwartship and a 12 Flinders bar forward is being swung for adjustment It is observed that 4E D error and 6E E error exist with the spheres in position Since the spheres are athwartship the total E coefficient for the ship is 6E as observed Figure 602 indicates that the spheres in their present position are correcting 6E D error hence the total D coefficient of the ship and Flinders bar is 10E Figure 603a indicates that 6E E and 10E D coefficients require slewing the spheres 155 clockwise from their present athwartship position The resultant quadrantal error is indicated as 117 Figure 602 indicates that the 7 spheres should then be moved to the 11 position after slewing 155 clockwise so as to correct both the D and E errors Use of this chart will eliminate mathematical or trialanderror methods of adjustment for quadrantal errors as well as quickly provide information for physically moving the spheres 25604 Corrector magnet inductions in spheres Should a ship have spheres and many permanent B and C magnet to the compass there will be a condition of induction existing between these correctors which will require someshuttling back and forth between headings while making adjustments This situation can be improved by using larger spheresfurther out and by approximately setting the spheres before starting adjustments as well as by using more magnets furtherfrom the spheres and compass Magnetized spheres as well as magnetized Flinders bar will not only cause some adjustment but might introduce an unstable deviation curve if they should undergo a shakedown or change ofmagnetic condition605 Quadrantal error from Finders bar Figure 603b presents the approximate quadrantal error introduced by thepresence of the Standard Navy Flinders bar Since the Flinders bar is generally placed in the forward or aft position it acts asa small minus D corrector as well as a corrector for vertical induced effects This means that upon inserting the Flinders barin such a position the regular spheres should be moved closer to correct for the increased plus D error or vice versa if theFlinders bar is removed This D error in the Flinders bar is due mostly to compass needle induction since the bar is small incrosssection and is close to the compass Since such needle induction is practically constant the deviation effects on thecompass will change with magnetic latitudes because the directive force H changes However when balanced by this is advantageous because it tends to cancel out the variable part of the sphere correction which is due to thecompass needle induction606 Flinders bar adjustment As has previously been stated in Chapter II it is generally impossible to place the correctamount of Flinders bar without reliable data obtained in two widely separated magnetic latitudes The placing of Flinders barby the use of an empirical amount or by an inspection of the ships structures is merely an approximation method andrefinements will usually be necessary when data is obtained There are several methods of acquiring and utilizing suchlatitude data in order to determine the proper amount of Flinders bar hence an elaboration on the following items 1 The data necessary for calculation of Flinders bar length and the conditions under which this data should be acquired 2 The best method of utilizing such data to determine the proper length of the Flinders bar607 Data required for Flinders bar adjustment The data required for correct Flinders bar adjustment consists of accuratetables of deviations with details of corrector conditions at two different magnetic latitudes the farther apart the better Seefigure 418 for an example of how such data is recorded on NAVSEA Form 31204 Should it be impossible to swing ship fora complete table of deviations the deviations on east and west magnetic headings would be helpful On many occasionsships log data is available but is of little use for Flinders bar calculation because it is not reliable The following be observed when such data is to be taken in order to assure that observed deviation changes are due only to changesin the H and Z components of the earths field 1 Degaussing should be secured by a reversal process if necessary at both latitudes before data is taken 2 If the ship has been docked or steaming on one heading for several days prior to the taking of these data the resulting temporary magnetism Gaussin error would create erroneous deviations A shakedown on other headings prior to taking data would reduce such errors 3 Deperming structural changes heavy gunfire magnetic cargoes etc subsequent to the first set of data will make the comparative results meaningless 4 Inasmuch as the data will not be reliable if the ships permanent magnetism changes between the two latitudes it will likewise be unreliable if any of the binnacle correctors are changed including the heeling magnetIn the event that an intelligent approximation as to Flinders bar length cannot be made then the deviations at the two be taken with no Flinders bar in the holder This procedure would also simplify the resulting Methods of determining Finders bar length 1 Having obtained reliable deviation data at two different magnetic latitudes the changes in the deviations if any mayjustifiably be attributed to an incorrect Flinders bar adjustment EW and NS deviations are the ones that are subject to majorchanges from such an incorrect adjustment If there is no change in any of these deviations the Flinders bar adjustment isprobably correct A change in the EW deviations indicates an asymmetrical arrangement of vertical iron forward or aft of thecompass which requires correction by the Flinders bar forward or aft of the compass A change in the NS an asymmetrical arrangement of vertical iron to port or starboard of the compass which requires correction by theFlinders bar to port or starboard of the compass This latter case is very rare but can be corrected as indicated in article 613 26 Determine the B deviations on magnetic eastwest headings at both latitudes The constant c may then be calculated fromthe following formula c H1 tan B1 H2 tan B2 Z1 Z2where shielding factor 07 to 10 average H1 earths field H at 1st latitude B1 degrees B deviation at 1st latitude magnetic headings Z1 earths field Z at 1st latitude H2 earths field H at 2nd latitude B2 degrees B deviation at 2nd latitude magnetic headings Z2 earths field Z at 2nd latitude This constant c represents a resultant mass of vertical iron in the ship that requires Flinders bar correction If Flinders bar ispresent at the time of calculations it must be remembered that it is already correcting an amount of c in the ship see figure603b which must be added to the uncorrected c calculated by the above formula This total value of c is used in figure 603b to indicate directly the necessary total amount of Flinders bar If this total c is negative Flinders bar isrequired on the forward side of the binnacle and if it is positive a Flinders bar is required on the aft side of the binnacle Theiron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top Woodenspacers are used at the bottom of the tube to achieve such spacing It will be noted that the B deviations used in this formulaare based on data on EW magnetic headings rather than on compass headings as with the approximate coefficients 2 Should the exact amount of correction required for vertical induction in the ship at some particular magnetic dip beknown figure 608 will directly indicate the correct amount of Flinders bar to be placed at the top of the holder The exactamount of correction would be known when one of the latitudes is the magnetic equator and the deviations there arenegligible Then the B deviation in degrees on magnetic headings at the other latitude is the exact amount to correct by meansof curves in figure 608 Figure 608 Dip deviation curves for Flinders bar 3 Lord Kelvins rule for improving the Flinders bar setting is Correct the deviations observed on east or west courses bythe use of foreandaft B magnets when the ship has arrived at places of weaker vertical magnetic field and by the use ofFlinders bar when she has arrived at places of stronger vertical magnetic field whether in the Northern or 27609 After determining the correct amount of Flinders bar by either method 1 or 2 above the bar should then be inserted atthe top of the holder and the foreandaft B magnets readjusted to correct the remaining B error Sphere adjustments shouldlikewise be refined It is quite possible that on inserting the Flinders bar no visible deflection of the compass will beobserved even on an east or west heading This should cause no concern because certain additional induction effects exist inthe bar from 1 The heeling magnet 2 The existing foreandaft magnets 3 The vertical component of the ships permanent magnetic field610 Heeling magnet induction in Finders bar Figure 610 presents typical induction effects in the Flinders bar for of heeling magnet An adjuster familiar with the nature of these effects will appreciate the advantages ofestablishing the Flinders bar and heeling magnet combination before leaving dockside Deviations must also be checked afteradjusting the heeling magnet if Flinders bar is present Figure 610 Induction effects in Flinders bar due to heeling magnet611 Slewing of Flinders bar The need for slewing the Flinders bar is much more rare than that for slewing spheres Alsothe data necessary for slewing the Flinders bar cannot be obtained on a single latitude adjustment as with the spheresSlewing the bar to some intermediate position is in effect merely utilizing one bar to do the work of two one forward or aftand the other port or starboard Article 608 explains that a change of the EW deviations with changes in latitude indicates the need for Flinders barforward or aft of the compass and a change of the NS deviations with changes in latitude indicates the need for Flinders barto port or starboard of the compass A change of the B deviations on magnetic EW headings is used as explained in article 608 to determine the properamount of Flinders bar forward or aft of the compass by calculating the constant c If there is a change of the C deviations onmagnetic NS headings a similar analysis may be made to determine the proper amount of Flinders bar to port or starboard ofthe compass by calculating the constant f from f H1 tan C1 H2 tan C2 Z1 Z2when shielding factor 07 to 10 average H1 earths field H at 1st latitude C1 degrees C deviation at 1st latitude magnetic headings Z1 earths field Z at 1st latitude H2 earths field H at 2nd latitude C2 degrees C deviation at 2nd latitude magnetic headings Z2 earths field Z at 2nd latitude Any value of this f constant indicates the need for Flinders bar adjustment athwartship of the compass just as a value of thec constant indicates the need for Flinders bar adjustment forward or aft of the compass The f constant curve in figure 608b isused for the determination of this Flinders bar length If f is negative Flinders bar is required on the starboard side of thebinnacle 28612 Should both c and f exist on a ship the angular position for a Flinders bar to correct the resultant vertical may be found by f f tan or tan1 c c is the angle to slew the Flinders bar from the foreandaft axis If c and f are negative the bar will be slewed clockwisefrom the forward position if c is negative and f is positive the bar will be slewed from the aft position After so determining the angle to slew the Flinders bar from the foreandaft line the total amount of Flinders barnecessary to correct the resultant vertical induction effects in this position is found by r c2 f 2 The constant r is then used on the c or f constant curve in figure 603b to determine the total amount of Flinders barnecessary in the slewed position613 Compasses Compasses themselves play a very important part in compass adjustment although it is common belief thatthe compass is only an indicating instrument aligning itself in the resultant magnetic field This would be essentially true ifthe magnetic fields were uniform about the compass but unfortunately magnetism close to the compass imposes across the needles In other words adjustment and compensation sometimes employ nonuniform fields to correctuniform fields Figure 613a indicates the difference between uniform and nonuniform field effects on a compass Figure 613a Magnetic fields across compass needle arrays Such unbalanced torques arising from nonuniform magnetic fields create deviations of the compass which have Compass designs include many combinations of different length needles different numbers ofneedles and different spacings and arrangements of needlesall designed to minimize the higher order deviations resultingfrom such nonuniform magnetic fields Although compass design is rather successful in minimizing such deviations it isobvious that different compasses will be affected differently by the same magnetic fields It is further stressed that even withproper compass design it is the responsibility of all adjusters to exercise care in applying correctors in order to create themost uniform magnetic field possible This is the basis for the rule that requires the use of strong correctors as far away from the compass as possible instead of weak correctors very close to the compass In general it isbetter to use larger spheres placed at the extremities of the brackets equally distant from the center of the compass B and Cpermanent magnet correctors should always be placed so as to have an equal number of magnets on both sides of thecompass where possible They should also be centered as indicated in figure 613b if regular tray arrangements are notavailable The desire for symmetrical magnetic fields is one reason for maintaining a sphere of specified radius commonlycalled the magnetic circle about the magnetic compass location This circle is kept free of any magnetic or 29 Figure 613b Arrangements of corrector magnets The magnetic moment of the compass needle array is another factor in compass design that ranks in importance with theproper arrangement of needles This magnetic moment controls the needle induction in the soft iron correctors as discussedin articles 602 and 605 and hence governs the constancy of those corrector effects with changes in magnetic latitude The7 Navy No 1 alcoholwater compass has a magnetic moment of approximately 4000 cgs units whereas the 7 Navy No1 oil compass has a magnetic moment of approximately 1650 cgs units The lower magnetic moment compass less change in quadrantal correction although the periods are essentially comparable because of the differencein the compass fluid Other factors that must be considered in compass design are period fluid swirl vibration illumination tilt pivot frictionfluid expansion etc These factors however are less important from an adjusters point of view than the magnetic momentand arrangement of needles and are therefore not discussed further in this text 30 CHAPTER VII SHIPS HEADING701 Ships heading Ships heading is the angle expressed in degrees clockwise from north of the ships foreandaft linewith respect to the true meridian or the magnetic meridian When this angle is referred to the true meridian it is called a trueheading When this angle is referred to the magnetic meridian it is called a magnetic heading Heading as indicated on aparticular compass is termed the ships compass heading by that compass It is always essential to specify heading as trueheading magnetic heading or compass heading In order to obtain the heading of a ship it is essential that the line throughthe pivot and the forward lubbers line of the compass be parallel to the foreandaft line of the ship This applies also to thepeloruses and gyro repeaters which are used for observational purposes702 Variation Variation at any place is the angle between the magnetic meridian and the true meridian If the northerly partof the magnetic meridian lies to the right of the true meridian the variation is easterly and if this part is to the left of the truemeridian the variation is westerly The local variation and its small annual change are noted on the compass rose of charts Thus the true and magnetic headings of a ship differ by the local variation703 Deviation As previously explained a ships magnetic influence will generally cause the compass needle to deflect fromthe magnetic meridian This angle of deflection is called deviation If the north end of the needle points east of the the deviation is easterly if it points west of the magnetic meridian the deviation is westerly704 Heading relationships A summary of heading relationships follows 1 Deviation is the difference between the compass heading and the magnetic heading 2 Variation is the difference between the magnetic heading and the true heading 3 The algebraic sum of deviation and variation is the compass error The following simple rules will assist in naming errors and in converting from one heading expression to another 1 Compass least less than magnetic heading deviation east Compass best greater than magnetic heading deviation west 2 When correcting going from compass to magnetic to true apply the sign algebraically East West When uncorrecting going from true to magnetic to compass reverse the sign East West 3 When correcting easterly errors are additive This single rule can be used to recall all four cases When correcting easterly errors are additive westerly errors are subtractive When uncorrecting easterly errors are subtractive westerly errors are additive Formed from the first letter of each key word in the correcting process Compass Deviation Magnetic Variation Truethe sentence Can Dead Men Vote Twice is useful in making conversions of heading data Although the can be used for uncorrecting going from right to left in the statement as written the first letters of the key wordsin the uncorrecting process are also used to develop a memory aid for uncorrecting Complete facility with such conversion of heading data is essential for expeditious compass adjustment heading relationships are tabulated below Compass heading 358 magnetic heading 003 deviation 5E Compass heading 181 magnetic heading 179 deviation 2W Compass heading 040 deviation 3E magnetic heading 043 Compass heading 273 deviation 2W magnetic heading 271 Magnetic heading 010 deviation 2E compass course 008 Magnetic heading 270 deviation 4W compass course 274 Magnetic heading 358 variation 6E true heading 004 Magnetic heading 270 variation 6W true heading 264 True heading 000 variation 5E magnetic heading 355 True heading 083 variation 7W magnetic heading 090 31705 Use of compass heading and magnetic heading for adjustment The primary object of adjusting compasses is toreduce deviations to make the magnetic heading and the compass heading identical or as nearly so as possible The twomethods of accomplishing this are as follows 1 Place the ship on the desired magnetic heading and correct the compass so that it reads the same as this magnetic heading This is the preferred method 2 Place the ship on the desired compass heading and determine the corresponding magnetic heading of the ship and correct the compass so that it reads the same as this known magnetic heading This method is used whenever it is impractical to place the ship on a steady magnetic heading for direct correction In using the magnetic heading method the deviations of the compass are easily observed as the difference between thecompass reading and the known magnetic heading of the ship The difficulty in using this method lies in placing the ship onthe desired magnetic heading and holding the ship steady on that heading while adjustments are being made When using the compass heading method the ship can easily be brought to any desired compass heading but the difficultyis in the determination of deviations Further difficulty arises from the fact that the steersman is steering by an whose deviations are changing as the necessary adjustments are being made Therefore as each adjustment is beingmade the steersman should attempt to hold the ship steady on that heading by some means other than the compass beingcorrected Adjustments by this method are made as a series of approximations for example Place the ship on any desired compass course and correct the compass to read the corresponding magnetic heading Thiswill probably leave the ship on a course other than the desirable cardinal and intercardinal headings for compass adjustmentFor accurate results the above procedure should be repeated If the compass has no appreciable deviations the deviations taken on compass headings will closely approximate thosetaken on magnetic headings However as the magnitude of errors increases there will be a marked difference between thedeviations taken on compass headings and those taken on magnetic headings706 Methods of placing ship on magnetic headings A ship may be brought on a magnetic heading by reference to agyrocompass The magnetic variation is applied to true heading to determine the gyro course which must be steered in orderto place the ship on the desired magnetic heading If the gyrocompass has any error it must be taken into consideration It iswell to calculate all such problems through true headings since shortcuts on this procedure frequently lead to errors Examples of such relationships are tabulated below To steer Heading per magnetic With variation True course With gyro error gyro compass course 000 6W 354 0 354 180 10E 190 0 190 270 4W 266 1E 265 315 6E 321 2E 319 225 17W 208 2W 210 358 0 358 3W 001 The difference between gyro heading and magnetic heading will be constant on all headings as long as the is constant and the variation does not change This gyrocompass error may be determined by a comparison of thecalculated true azimuth of the sun and the azimuth as observed on a synchronized repeater It is to be remembered that gyrocompasses have certain errors resulting from latitude and speed change as well as turningerrors and that these errors are not always constant on all headings For these reasons the gyro error must be especially if the gyro is being used to obtain data for determining residual deviation curves of the A ship may be placed on a magnetic heading by aligning the vanes of an azimuth circle with the sun over the topsidecompass The sun is a distant object whose azimuth tangle from the north may be computed for any given time Methods ofcalculating suns azimuths are discussed in Chapter VIII By setting the line of sight of the vanes at an angle to the right orleft of the foreandaft line of the ship equal to the difference between the computed magnetic azimuth and the heading of the ship and then swinging the ship until the sun is aligned with the vanes the ship will be on thedesired magnetic heading Simple diagrams as in figure 707 with the ship and sun drawn in their relative positions will aidgreatly in the visualization of each problem The azimuth circle must always be kept level while making of celestial bodies 32 Figure 707 Azimuth circle setups708 A distant object 10 or more miles away may be use in conjunction with the azimuth circle for placing the ship onmagnetic headings provided the ship stays within a small area This procedure is similar to that used with the sun except thatthe magnetic beading of the object is constant With an object 114 nautical miles distant a change in position of 400 yards atright angles to the line of sight introduces an error of 1709 A pelorus may be used to place a ship on a magnetic heading using the suns azimuth in much the same manner as withthe azimuth circle Use of the pelorus has the further advantage in that the magnetic heading of the ship can be as the ship swings Such a procedure would be as follows The forward sight vane is clamped to the dial at the value of the suns magnetic azimuth and the sight vanes are then trained so the sun is reflected in the mirror As the ship turns the magnetic heading is always observed under the forward lubbers line if the vanes are kept on the sun and this will serve as a guide for bringing the ship onto any desired magnetic heading As the desired magnetic course is approached the compass can be read and corrected even before that magnetic course is actually obtained and a final check can be made when the ship is on the exact course The pelorus must always be kept in a level position while making observations particularly of celestial bodies710 A distant object can be used in conjunction with the pelorus as with the azimuth circle in order to place the ship onmagnetic heading provided the ship stays within a small area See article 708711 Methods of determining deviations on a compass heading The deviations on compass headings may be obtained by acomparison of the calculated magnetic azimuth of the sun and the azimuth as observed on the compass by use of an azimuthcircle Methods of calculating suns azimuth are discussed in Chapter VIII The ship is placed on the desired compass headingand an azimuth of the sun taken of the face of the compass card The difference in degrees between the observed azimuth andthe calculated magnetic azimuth of the sun is the deviation on that compass course712 The pelorus may also be used in conjunction with the suns azimuth to obtain deviations on compass headings The shipis brought to the desired compass heading and the forward sight vane is set on the calculated value of the suns The sight vanes are then trained on the sun and the magnetic heading of the ship is indicated under the forwardlubbers line of the pelorus The difference in degrees between the compass heading and magnetic heading of the shipindicated by the pelorus is the deviation on that compass course713 The azimuth circle or pelorus can be used in conjunction with ranges or a distant object to obtain deviations on compasscourses The procedure is similar to that used with the sun A range consists of any two objects or markers one in theforeground and the other in the background which establishes a line of sight having a known magnetic bearing The truebearing of such a range is determined from a local chart this true bearing is converted to the magnetic bearing by applyingthe variation corrected for annual change as given on the chart Multiple ranges consist of several markers in the backgroundand a single marker in the foreground or vice versa The ship is brought to the desired compass course and at the instant ofcrossing the line of sight of the range a bearing is taken with the azimuth circle or pelorus With the azimuth circle thedifference in degrees between the observed bearing of the range on the face of the compass and the known magnetic bearingof the range is the deviation on that compass course If using a pelorus the forward sight vanes are set to the magnetic 33bearing of the range and the magnetic heading of the ship is read under the forward lubbers line of the pelorus at the instantof taking a sight on the range The deviation is the difference in degrees between the compass heading of the ship and theknown magnetic heading of the ship as indicated by pelorus714 Deviations on compass courses may be obtained by the use of reciprocal bearings A pelorus is set up on shore and thesouth end of the dial is aligned with magnetic north A ship can then sight the pelorus on shore using an azimuth circle orpelorus at the same instant the observer on shore sights the ship The ships bearing from shore on the reversed pelorus is themagnetic bearing of the shore position from the ship Continuous communication between ship and shore is necessary andmust be so arranged as to provide simultaneous observations Two methods of such communication are by flashing lightsand preferably by short range twoway voice radio Additional methods of determining deviations are by the use of azimuths of the moon stars and planets For informationas to the calculation of azimuths of these celestial bodies refer to Pub No 9 The American Practical Navigator 34 CHAPTER VIII AZIMUTHS801 Azimuths of the sun Since accurate compass bearings of the sun can readily be observed for comparison with the sunscalculated true bearing or azimuth for time date and place of the observer to obtain the compass error the sun is a point for compass adjustment The azimuths of other celestial bodies can similarly be determined but are lesspractical for compass work because of the poor visibility of stars and the more variable time rates and declinations of themoon and planets Hence subsequent explanations concern themselves only with the sun and its azimuths802 Astronomical triangle The azimuth of the sun at any instant and place of the observer is determined by solving triangle for azimuth angle Z using the observers latitude and longitude and the celestial coordinates of the sunfor the time of the observation as taken from the Nautical Almanac to effect the solution The astronomical triangle is formed on the celestial sphere by 1 the elevated pole of the observer the radial projection of the geographic pole of the observer according to whether his latitude is north or south 2 the zenith of the observer the radial projection of the observers position on earth and 3 the celestial body803 Local hour angle LHA The Greenwich hour angle GHA of the sun as taken from the Nautical Almanac for the timeand date of the observation is combined with the observers longitude to obtain the local hour angle the angle at the elevatedpole between the local celestial meridian the observers meridian and the hour circle of the sun always measured westwardfrom 0 to 360 west LHA GHA longitude east Meridian angle t is sometimes used instead of local hour angle to express the angle at the elevated pole between the localcelestial meridian and the hour circle of the sun The meridian angle t of the sun is the angle at the elevated pole measuredfrom the meridian of the observer to the hour circle of the sun eastward or westward from 0 to 180 Thus t denotes thesuns position east or west of the local celestial meridian When the sun is west of the meridian t is equal to LHA when eastt is equal to 360 minus LHA804 Declination d Also taken from the Nautical Almanac for the time and date of the observation declination d of the sunis used with local hour angle LHA and the latitude L of the observer to calculate the azimuth angle Z805 Azimuth angle Z The azimuth angle of the sun is the angle at the zenith between the principal vertical with the local celestial meridian and the vertical circle through the sun It is measured from 0 at the north orsouth reference direction clockwise or through 180 It is labeled with the reference direction direction ofelevated pole of observer as a prefix and direction of measurement from the reference direction as a suffix Thus azimuthangle S144W is the angle between the principal vertical circle of an observer in the Southern Hemisphere and circle 144 westward Azimuth angle is converted to azimuth by use of the following rules 1 For north latitudes a Zn Z if the sun is east of the meridian b Zn 360 Z if the sun is west of the meridian 2 For south latitudes a Zn 180 Z if the sun is east of the meridian b Zn 180 Z if the sun is west of the meridian It must be remembered that in order to obtain magnetic azimuths from true azimuths the appropriate variation must beapplied to the true azimuths806 Azimuth by tables One of the more frequent applications of sight reduction tables is their use in computing theazimuth of a celestial body for comparison with an observed azimuth in order to determine the error of the compass Incomputing the azimuth of a celestial body for the time and place of observation it is normally necessary to interpolate thetabular azimuth angle as extracted from the tables for the differences between the table arguments and the actual values of 35declination latitude and local hour angle The required of the azimuth angle using Pub No 229 SightReduction Tables for Marine Navigation is effected as follows 1 Refer to figure 806a The main tables are entered with the nearest integral values of declination latitude and local hour angle For these arguments a base azimuth angle is extracted Figure 806a Extracts from Pub No 229 2 The tables are reentered with the same latitude and LHA arguments but with the declination argument 10 greater or less than the base declination argument depending upon whether the actual declination is greater or less than the base argument The difference between the respondent azimuth angle and the base azimuth angle establishes the azimuth angle difference Z Diff for the increment of declination 3 The tables are reentered with the base declination and LHA arguments but with the latitude argument 10 greater or less than the base latitude argument depending upon whether the actual usually DR latitude is greater or less than the base argument to find the Z Diff for the increment of latitude 4 The tables are reentered with the base declination and latitude arguments but with the LHA argument 10 greater or less than the base LHA argument depending upon whether the actual LHA is greater or less than the base argument to find the Z Diff for the increment of LHA 5 The correction to the base azimuth angle for each increment is Inc Z Diff x 60 The auxiliary interpolation table can normally be used for computing this value because the successive azimuth are less than 100 for altitudes less than 84 Example In DR lat 33240N the azimuth of the sun is observed as 0965 pgc At the time of the observation the declination of the sun is 20138N the local hour angle of the sun is 316412 Required The gyro error Solution By Pub No 229 The error of the gyrocompass is found as shown in figure 806b Base Correction Actual Arguments Base Z Tab Z Z Diff Increments Z Diff x Inc60 Dec 20138N 20 978 964 14 138 03 DR L 33240N 33 same 978 989 11 240 04 LHA 316412 317 978 971 07 188 02 Base Z 978 Total corr 01 Corr 01 Z N977E Zn 0977 Zn pgc 0965 Gyro error 12E 36807 Azimuth by calculator When calculators are used to compute the azimuth tedious triple interpolation is can be effected by several formulas The azimuth angle Z can be calculated using the altitude azimuth formula if the altitude is known The formula stated interms of the inverse trigonometric function is sin d sin L sin Hc Z cos1 cos L cos Hc If the altitude is unknown or a solution independent of altitude is required the azimuth angle can be calculated using thetime azimuth formula The formula stated in terms of the inverse trigonometric function is sin LHA Z tan1 cos L tan d sin L cos LHA The sign conventions used in the calculations of both azimuth formulas are as follows 1 If latitude and declination are ofcontrary name declination is treated as a negative quantity 2 If the local hour angle is greater than 180 it is treated as anegative quantity If the azimuth angle as calculated is negative it is necessary to add 180 to obtain the desired valueExample In DR lat 41259S the azimuth of the sun is observed as 0160 pgc At the time of the observation thedeclination of the sun is 22196N the local hour angle of the sun is 342376Required The gyro error by calculation of sin LHA Z tan1 cos L tan d sin L cos LHAPreliminary Convert each known quantity to decimal degrees Latitude 41259S 41432 Declination 22196N 22327 LHA 342376 342627 Prepare form on which to record results obtained in the several procedural steps of the Procedure varies according to calculator design and the degree to which the user employs the features of the design enabling more expeditious solutions In this example only the initial step of substituting the given quantities in the formula in accordance with the sign conventions is given before the azimuth angle is obtained by the calculator is stated sin 342627 Z tan1 cos 41432 x tan 22327 sin 41432 x cos 342627 Z 176 Since Z as calculated is a negative angle 176 180 is added to obtain the desired azimuth angle 1624 Z S1624E Zn 0176 Zn pgc 0160Answer Gyro error 16E808 Curve of magnetic azimuths During the course of compass adjustment and swinging ship a magnetic direction isneeded many times either to place the vessel on desired magnetic headings or to determine the deviation of the compassbeing adjusted If a celestial body is used to provide the magnetic reference the azimuth is continually changing as the earthrotates on its axis Frequent and numerous computations can be avoided by preparing in advance a table or curve ofmagnetic azimuths True azimuths at frequent intervals are computed The variation at the center of the maneuvering area isthen applied to determine the equivalent magnetic azimuths These are plotted on crosssection paper with time as the otherargument using any convenient scale A curve is then faired through the points Points at intervals of half an hour with a minimum of three are usually sufficient unless the body is near the and relatively high in the sky when additional points are needed If the body crosses the celestial meridian thedirection of curvature of the line reverses Unless extreme accuracy is required the Greenwich hour angle and declination can be determined for the the same value of declination used for all computations and the Greenwich hour angle considered to increase 15per hour 37 An illustration of a curve of magnetic azimuths of the sun is shown in figure 808 This curve is for the period 07000900zone time on May 31 1975 at latitude 23095N longitude 82241W The variation in this area is 247E At the midtimethe meridian angle of the sun is 664723 and the declination is 21523N Azimuths were computed at halfhour intervalsas follows Zone time Meridian angle Declination Latitude Magnetic azimuth 0700 81471E 5h 271m E 219N 232N 06939 0730 74171E 4h 571m E 219N 232N 07157 0800 66472E 4h 271m E 219N 232N 07406 0830 59172E 3h 571m E 219N 232N 07608 0900 51472E 3h 271m E 219N 232N 07807 This curve was constructed on the assumption that the vessel would remain in approximately the same location during theperiod of adjustment and swing If the position changes materially this should be considered in the computation Figure 808 Curve of magnetic azimuths Extreme care must be exercised when using the sun between 1100 and 1300 LMT since the azimuth changes very rapidlyduring this time and the sun is generally at a high altitude 38 CHAPTER IX COMPASS RECORDS AND REPORTS901 OPNAV Instruction 312032 Standard Organization and Regulations of the US Navy of 30 July 1974 requires thenavigator to make frequent checks of the magnetic compass to determine its error and to make frequent comparisons with thegyrocompass while the ship is underway Specific information relative to compass observations records and reports isoutlined below 1 The Magnetic Compass Record NAVSEA 31203 is part of the official record of a ship and is maintained as an adjunct to the Deck Log aboard every US Naval ship in commission It is a complete history of each magnetic compass on board 2 Each volume of the Magnetic Compass Record contains a sufficient number of Compass Check Log forms for 3 months continuous entries based upon halfhourly observations Observations may be made at shorter than halfhourly periods if desired In those cases where the ship is not operating continuously the book will be usable for a more extended period Provision is made in this book for accommodating the record of both the standard and steering magnetic compasses The latitude and longitude columns of the check log may be left blank when this information would make the record confidential 3 Whenever a magnetic compass is adjusted or the deviations on all cardinal and intercardinal headings are observed the results are recorded on a Magnetic Compass Table NAVSEA 31204 A copy of the latest completed table should be kept in the envelope attached inside the back cover of the Magnetic Compass Record On ships equipped with degaussing circuits and compass compensation coils the residual deviations are recorded with DGOFF and DGON A copy of NAVSEA 31204 should be posted near the compass so as to be readily accessible to the navigator and other personnel concerned with the navigation of the ship Each time a new NAVSEA 31204 is prepared as a result of the adjustment of the compass a duplicate copy of the completed form should be forwarded to the Naval Ship Engineering Center A transmittal letter is not required Special attention should be given to completing all the information requested on the back of the NAVSEA 31204 form so that the changes in deviation with latitude may be correctly evaluated in terms of Flinders bar requirements 4 A NAVSEA 895041 should be filed with the compass manuals at the rear of the Degaussing Folder One copy of the form should be forwarded to Naval Ship Engineering Center at the time of initial compensation and upon any subsequent compensations made as a result of adding additional compensating equipment or of changing the type of this equipment In the case of changing or adding equipment this form will normally be made out by the installing activity However if this activity does not perform the compensation the form should be submitted by the ship 39 CHAPTER X TRANSIENT DEVIATIONS OF THE MAGNETIC COMPASS1001 Stability The general treatise on compass adjustment concerns itself only with the principles of ie the effects of permanent and induced magnetism and their appropriate correctors This knowledgealong with the ability to handle suns azimuth and ships heading is the backbone of compass adjustment However acorrection may be very carefully and accurately made and still prove disastrous to the ship for example a compassmay have a perfect deviation curve but when a nearby gun is trained the magnetic effects on the compass are a compass adjuster cannot place correctors on the binnacle for such variable effects it is definitely his duty torecognize and handle them in the best possible manner If it is impossible to eliminate the source of trouble orimpractical to relocate the binnacle the details of alignment or excitation of the sources of error should be specified onthe deviation card With such information the navigator would know when or when not to rely on his In other words a good adjuster should not only provide a good deviation curve which is reliable stated conditions but also point out and record probable causes of unreliability which cannot be eliminated1002 Sources of transient error The magnetic circle about the magnetic compass is intended to reduce such but there still are many items both electrical and magnetic which cause erratic effects on the compass Thefollowing list is presented to assist in the detection of such items If in doubt a test can be made by swinging anymovable object or energizing any electrical unit while observing the compass for deviations This would best be triedon two different headings 90 apart since the compass might possibly be affected on one heading and not on the other 1 Some magnetic items which cause variable deviations if placed too close to the compass are as follows a Guns on movable mounts b Ready ammunition boxes c Variable quantities of ammunition in ready boxes d Magnetic cargo e Hoisting booms f Cable reels g Metal doors in wheelhouse h Chart table drawers i Movable gyro repeater j Windows and ports k Signal pistols racked near compass l Sound powered telephones m Magnetic wheel or rudder mechanism n Knives or ash trays near binnacle o Watches wrist bands spectacle frames p Hat grommets belt buckles metal pencils q Heating of smoke stack or exhaust pipes r Landing boats 2 Some electrical items which cause variable deviations if placed too close to the compass are as follows a Electric motors b Magnetic controllers c Gyro repeaters d Unmarried conductors e Loudspeakers f Electric indicators g Electric welding h Large power circuits i Searchlights j Electrical control panels or switches k Telephone headsets l Windshield wipers m Rudder position indicators solenoid type n Minesweeping power circuits 40 o Engine order telegraphs p Radar equipment q Magnetically controlled switches r Radio transmitters s Radio receivers t Voltage regulators1003 There is another source of transient deviation trouble known as the retentive error This error results from thetendency of a ships structure to retain some of the induced magnetic effects for short periods of time For example aship traveling north for several days especially if pounding in heavy seas will tend to retain some hammered in under these conditions of induction Although this effect is not too large and generally decayswithin a few hours it may cause incorrect observations or adjustments if neglected This same type of error occurswhen ships are docked on one heading for long periods of time A short shakedown with the ship on other headings willtend to remove such errors A similar sort of residual magnetism is left in many ships if the degaussing circuits are notsecured by the reversal sequence A source of transient deviation trouble of shorter duration than retentive error is known as Gaussin error This error iscaused by eddy currents set up by a changing number of magnetic lines of force through soft iron as the ship changesheading Due to these eddy currents the induced magnetism on a given heading does not arrive at its normal value untilabout 2 minutes after change to the heading1004 Deperming and other magnetic treatment will change the magnetic condition of the vessel and readjustment of the compass The decaying effects of deperming are sometimes very rapid therefore it isbest to delay readjustment for several days after such treatment Since the magnetic fields used for such treatments aresometimes rather large at the compass locations the Finders bar compass and such related equipment is from the ship during these operations 41 CHAPTER XI USE OF THE DIP NEEDLE FOR HEELING As indicated in Chapter III the heeling effects of both the permanent and induced magnetism are corrected byadjusting the position of the vertical permanent heeling magnet This adjustment can be made in either of two ways 1 With the ship on an even keel and as close to the east or west magnetic heading as possible adjust the heeling magnet until a dip needle inserted in the compass position is balanced at some predetermined position article 1103 2 Adjust the heeling magnet while the ship is rolling on north and south headings until the oscillations of the compass card have been reduced to an average minimum Inasmuch as it is desirable to establish the condition of induction between the heeling magnet and Flinders bar and toreduce the heeling oscillations to a minimum before making the adjustments at sea the heeling magnet is usually set atdockside by the first method above Further it would be difficult to correct the heeling error by rolling at sea before makingthe other adjustments because the spheres and Flinders bar produce a certain measure of heeling correction and hence they should be positioned at least approximately before making the heeling adjustments by either method1102 The fact that the heeling magnet corrects for induced effects as well as permanent effects requires that it be readjustedwith radical magnetic latitude changes of the ship Movement of the heeling magnet with Flinders bar in the holder willchange the induction effects in the Flinders bar and consequently change the compass deviations See article 610 Thus thenavigator is responsible for 1 Moving the heeling magnet up or down invert when necessary as the ship changes magnetic latitude so as to maintain a good heeling adjustment for all latitudes 2 Maintaining a check on his deviations and noting changes resulting from movements of the heeling magnet when Flinders bar is in the holder Any deviation changes should be either recorded or readjusted by means of the fore andaft B magnets1103 To elaborate on the details of the dip needle method of adjustment it is pointed out that there are two types of dipneedles one of which assumes the angle of inclination or dip for its particular location and one on which the magnetictorque is balanced by a movable weight The latter is a nullifying type instrument which renders the final position of theneedle more independent of the horizontal component of magnetic fields and hence is more useful on For ships which introduce no shielding to the earths field at the compass the procedure for adjusting the heeling magnet isquite simple Take the dip needle into a nearby area where there is no local magnetic attraction level the instrument and setthe weight so as to balance the needle under those conditions of earths magnetic field It is preferable to align the instrumentsuch that the north seeking end of the needle is pointing north Next level the instrument in the compass position on boardship place the spheres in their approximate position and adjust the heeling magnet until the needle assumes the This presumes that all the effects of the ship are canceled leaving only the effect of the vertical earths field Thedegaussing circuits are secured during this adjustment In the case of ships which have shielding effects on the earths field at the compass as in metal enclosed wheelhouses theprocedure is essentially the same as above except that the weight on the dip needle should be moved toward the pivot so asto balance against some lesser value of earths field The new position of the weight expressed in centimeters from the pivotcan be approximately determined by multiplying the value of lambda for the compass location by the original distance ofthe weight from the pivot in centimeters Should for the compass location be unknown it may generally be considered asabout 08 for steering compass locations and 09 for standard compass locations By either method the weight on the dipneedle should be moved in to its new position Next level the instrument in the compass position on board ship and adjust theheeling magnet until the needle assumes the balanced condition Theoretically these methods of adjusting the heeling magnet by means of a dip needle should be employed only with theship on east or west magnetic headings so as to avoid heeling errors resulting from asymmetrical foreandaft If it is impractical to place the ship on such a heading approximations may be made on any heading andrefinements made when convenient1104 In the final analysis a successful heeling magnet adjustment is one whereby the objectionable oscillations due torolling of the ship maximum effects on north and south compass headings are minimized Therefore the rolling method is avisual method of adjusting the heeling magnet or checking the accuracy of the last heeling magnet adjustment Generally theoscillation effects due to roll on both the north and south compass headings will be the same However some of foreandaft soft iron will introduce different oscillation effects on these two headings Such effects cannotbe entirely eliminated on both headings with one setting of the heeling magnet and the heeling magnet is generally set for theaverage minimum oscillation condition 42 CHAPTER XII USE OF THE HORIZONTAL FORCE INSTRUMENT1201 Occasionally it will be necessary to determine the actual strength of the magnetic field at some compass location Thisproblem may arise for one of the following reasons 1 It may be desired to determine accurately the horizontal shielding factor lambda for a A complete mathematical analysis b Accurate Flinders bar adjustment c Accurate heeling adjustment d Calculations on a dockside magnetic adjustment e Determining the best compass location on board ship 2 It may be desired to make a dockside magnetic adjustment and hence determine the existing directive force at the magnetic compass both for its magnitude and direction Lambda is the horizontal shielding factor or ratio of the reduced earths directive force H on the compass to thehorizontal earths field H as H H From this it is apparent that may easily be determined for a compass location by making a measurement of the reducedearths directive force H On a corrected compass this value H may be measured with the ship on any heading since thisreduced earths directive force is the only force acting on the compass If the compass is not corrected for the shipsmagnetism and the deviations are large H is determined from the several resultant directive forces observed with equallyspaced headings of the ship as indicated later Lambda should be determined for every compass location on every ship1202 The actual measurement of such magnetic fields may be made by use of a suitable magnetometer or by the use of ahorizontal force instrument The magnetometer method is a direct reading method which needs no calculation The is by far the simpler form of equipment hence the force instrument method is discussed below The horizontal force instrument is simply a magnetized needle pivoted in a horizontal plane much the same as a compassIt will settle in some position that will indicate the direction of the resultant magnetic field The method used to determine thestrength of this resultant field is by comparing it with a known field If the force needle is started swinging it will be dampeddown with a certain period of oscillation dependent upon the strength of the magnetic field The stronger the magnetic fieldthe shorter the period of time for each cycle of swing in fact the ratio is such that the squares of the period of vibration areinversely proportional to the strengths of the magnetic fields as H T2 H T 2 In the above formula let H represent the strength of the earths horizontal field in gauss and T represent the time in secondsfor 10 cycles of needle vibration in that earths field Should it be desired to find the strength of an unknown magnetic fieldH a comparative measurement of time in seconds T for 10 cycles of vibration of the same needle in the unknown field willenable calculation of H Since is the ratio of two magnetic field strengths it may be found directly by the inverse ratio of the squares of theperiods of vibration for the same horizontal force instrument in the two different magnetic fields by the same formulawithout bothering about the values of H and H H T2 H T 2 The above may be used on one heading of the ship if the compass deviations are less than 4 To obtain the value of more precisely and where deviations of the compass exceeds 4 the following equation should beused T2 cos d n cos d e cos d s cos d w 4 Tn2 Te2 Ts2 Tw2 where T is the time period for the field H Tn is the time period for the resultant field with ship on a north heading etc cos d n is the cos of the deviation on the north heading etc 43 CHAPTER XIII INTRODUCTION TO DEGAUSSING1301 Degaussing A steel vessel has a certain amount of permanent magnetism in its hard iron and induced magnetism inits soft iron Whenever two or more magnetic fields occupy the same space the total field is the vector sum of theindividual fields Thus within the effective region of the field of a vessel the total field is the combined total of the earthsfield and that due to the vessel Consequently the field due to earths magnetism alone is altered or distorted due to the fieldof the vessel Certain mines and other explosive devices are designed to be triggered by the magnetic influence of a vessel passing nearthem It is therefore desirable to reduce to a practical minimum the magnetic field of a vessel One method of doing this is toneutralize each component by means of an field produced by direct current of electricity in electric so as to form coils around the vessel A unit sometimes used for measuring the strength of a magnetic field is thegauss The reduction of the strength of a magnetic field decreases the number of gauss in that field Hence the process is oneof degaussing the vessel When a vessels degaussing coils are energized the magnetic field of the vessel is completely altered This introduces in the magnetic compasses This is removed as nearly as practicable by introducing at each compass an equal andopposite force of the same type one caused by direct current in a coil for each component of the field due to thedegaussing currents This is called compass compensation When there is a possibility of confusion with compass adjustmentto neutralize the effects of the natural magnetism of the vessel the expression degaussing compensation is used Since may not be perfect a small amount of deviation due to degaussing may remain on certain headings This is thereason for swinging ship twice once with degaussing off and once with it on and having two separate columns in thedeviation table If a vessel passes over a device for detecting and recording the strength of the magnetic field a certain pattern is tracedSince the magnetic field of each vessel is different each has a distinctive trace known as its magnetic signature Several degaussing stations have been established to determine magnetic signatures and recommend the currents needed inthe various degaussing coils Since a vessels induced magnetism varies with heading and magnetic latitude the of the coils which neutralize induced magnetism need to be changed to suit the conditions A degaussing folder isprovided each vessel to indicate the changes and to give other pertinent information A vessels permanent magnetism changes somewhat with time and the magnetic history of the vessel Therefore theinformation given in the degaussing folder should be checked from time to time by a return to the magnetic station1302 Degaussing coils For degaussing purposes the total field of the vessel is divided into three components l vertical 2 horizontal foreandaft 3 horizontal athwartships Each component is opposed by a separate degaussing field just strong enough to neutralize it Ideally when this has beendone the earths field passes through the vessel smoothly and without distortion The opposing degaussing fields areproduced by direct current flowing in coils of wire Each of the degaussing coils is placed so that the field it produces isdirected to oppose one component of the ships field The number of coils installed depends upon the magnetic of the vessel and the degree of safety desired Theships permanent and induced magnetism may be neutralized separately so that control of induced magnetism can be varied asheading and latitude change without disturbing the fields opposing the vessels permanent field 44 CHAPTER XIV DEGAUSSING COMPASS Degaussing effects The degaussing of ships for protection against magnetic mines has created additional effects uponmagnetic compasses that are somewhat different from the permanent and induced magnetic effects usually effects may be considered as effects that depend upon 1 Number and type of degaussing coils installed 2 Magnetic strength and polarity of the degaussing coils 3 Relative location of the different degaussing coils with respect to the binnacle 4 Presence of masses of steel which would tend to concentrate or distort magnetic fields in the vicinity of the binnacle 5 The fact that degaussing coils are operated intermittently with variable current values and with different polarities as dictated by necessary degaussing conditions1402 The magnetic fields at the binnacle must be considered separately for each degaussing coil The magnetic field fromany individual degaussing coil will vary with the excitation of the coil and its direction will with changes in the coil polarity Uncompensated degaussing coil effects create deviations of the compass card and conditions of sluggishness which are similar to and generally larger than the effects of normal ships magnetism on the magnetic compass1403 Degaussing compensation The fundamental principle of compass compensation is to create magnetic fields at thecompass that are at all times equal and opposite to the magnetic effects of the degaussing system To accomplish this it isnecessary to arrange coils about the binnacle so they create opposing effects for each degaussing circuit that affects thecompass These opposing effects can be created directly or by a combination of component parts In most cases it is best tocreate the compensating field by a combination of three vectors along mutually perpendicular axes rather than by one vectoradjusted to the proper angle Figure 1403 illustrates the concept of the resultant magnetic field established by three perpendicular components Figure 1403 Resultant degaussing field and its equivalent three vector components1404 The various standard compass coil installations utilize a threecoil arrangement of one type or another to by the threecomponent method Such a group of coils are so interconnected that they can be and each group is so connected to its associated degaussing coil that its compensating effect will with changes in the degaussing coil effect1405 Degaussing compass coil compensation consists of regulating the current delivered to the coils so that no change in themagnetic field occurs at the center of the binnacle when the degaussing coils are energized or the degaussing currents arevaried This regulation is accomplished in a control box by means of control resistors for each degaussing circuit When theseresistors have once been set their settings need not be altered with current changes in the degaussing circuits It is best to check coil installations electrically and compensate at dockside before the ship leaves the yard of compensation is impaired by welding adjacent ships and moving cranes time and trouble are still saved for theship during final compensation at sea All this results from the fact that is the greater part of 45


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