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Downwind Tacking


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Downwind Tacking Foredeck hands are at the ready as a pair of Cal20s split tacks in downwind racing tactics By Arvel Gentry SEA and Pacific Motor Boat February 1970 Arvel Gentry is a member of Seal Beach YC and an active However this information doesnt tell the complete racing skipper in his Cal 20 No 1177 He started sailing in story since the racing skipper is not interested in just a Lido 14 and also crews on a Newport 20 and a Cal 40 If breaking even He has to get to the finish line in the the tone of the article seems scholarly in its development it shortest time possible to win With years of experience he may be because Gentry has applied his professional as well might learn by trial and error what tacking angle to use for as sailing talents to the material He is an aerodynamics his boat in different wind and sea conditions Most sailing engineer and works as Chief of Applied research literature seems to discuss only the breakeven aspect of Aerodynamics Section Douglas Aircraft Co At Seas request Gentry listed his Cal 20 racing achievements and the problem The purpose of this article is to expand this as a side remark noted In the other racesIve just tried information and to show a skipper how he can easily find to avoid coming in last Hopefully his comments on the best downwind tacking procedure for his own boat Optimum Downwind Tacking will help other skippers in without suffering through years of trial and error doing the same In downwind sailing the sails are primarily drag producing devices with the drag forces giving the push Every racing sailor has heard of downwind tacking He that makes the boat go Unfortunately we usually havemay even carry in his wallet a business card from his our sails set so that the mainsail tends to block or distort thesailboat dealer with some downwind tacking information air flow over the jib or spinnaker As we change the on the back such as that shown below This position of the sails with respect to the wind direction byinformation may be a tabulation of the speed increase altering our course we change the flow patterns aroundrequired to break even against the increased distance the sails so that the headsail gets more air and the boattraveled when deviating from the deaddownwind speeds up It is this increase in speed that leads to the ideaheading of downwind tacking We intentionally deviate from the short straight downwind course and hope that the increase in speed will more than make up for the greater distance traveled To get the most out of your boat when sailing downwind you must have some idea as to how its speed changes as you deviate from the deaddownwind course An exact determination of this speed trend would require more sophisticated than the average sailor has available However with the careful use of a masthead wind vane a compass and a good knotmeter in several afternoons of test sailing you can accomplish about the same thing The results for the skipper will be an Figure 1increased confidence that he is sailing the best downwind The important thing to note is that the amount of speedcourse regardless of what the other boats might be doing gained by deviating from the deaddownwind course and Before we start out test sailing we should first take a thus the variation of the speed factor with heading willlook at the various factors involved so we understand vary a great deal between boats of different classes Eachwhat we are looking for Figure 1 contains a diagram boat will therefore have a different optimum these factors This diagram is for the general tacking angle It should also be noted that on a single boatcase where the downwind mark is not deaddownwind the optimum downwind tacking angle will depend upon dbut is at an angle delta theta u from the true dead the wind and sea conditions involved as if the racingdownwind direction Similar diagrams shown in sailing skipper didnt have enough variables to contend withliterature are usually made only for the condition where d However with a good testing procedure and a littlethe lee mark is deaddownwind u 0 In the following mathematics a skipper will be able to determine thediscussions and in the analysis of our test data we will at influences of these variablesfirst also make this simplifying assumption However later The first step in this process is easy go sailing To makein this article we will look at the condition where the lee the first test sailing runs easy try to pick a day withmark is not deaddownwind medium winds of about 10 knots Stay behind the For the mathematically inclined and for convenience in breakwater if possible where the water is smooth and staythe analysis of our test data three important parameters away from obstructions that block or deflect the wind Thehave been defined the distance factor the speed factor more stable the wind conditions the more consistent theand the downwind tacking factor The distance factor D test results will be In brief the procedure to be useddepends just upon the geometry of the path taken and is involves taking a number of speed readings at differentequal to one divided by the cosine of the downwind d downwind headings and then returning to the club bar totacking angle when u 0 The speed factor S is formed by complete the analysis of the datadividing the speed at a given tacking angle by the speed More likely however youll do your data analysis atthat you would get by sailing deaddownwind The home in spite of the competitive skipper who is offering todownwind tacking factor DS is formed by dividing the pay for the drinks in exchange for a look at your factor by the speed factor The shortest time is To gather the sailing data first set the sails for deadobtained when the downwind tacking factor DS is the downwind sailing with the jib on a whisker pole or thesmallest spinnaker flying It is important throughout all the 2measurements to do the best job that you can in setting you have a light displacement boat Under theseand adjusting the sails for best speed at each heading test conditions downwind tacking measurements becomepoint Now change the heading of the boat until the hopeless so just hang on and enjoy the ridemasthead wind vane indicates that the boat is headed After the first day of test sailing you will be anxious todeaddownwind Write down this indicated dead see just how all these speed and heading compass reading can improve your downwind sailing For this we must The mainsail distorts the flow somewhat about the turn from the actual sailing to the data analysis After youwind vane so that this reading may not represent the true get home review your test log sheets carefully Mark heading To check this quickly jibe onto all those test points that have notes indicating any windthe other tack and repeat the indicated deaddownwind speed or direction changes Number all the procedure The difference between the two good test points consecutively in the order that they wereheadings divided by two will be the correction to be taken Calculate the indicated downwind tacking angle forapplied to an indicated deaddownwind heading to obtain each point This is found by taking the difference betweenthe true deaddownwind heading Now you can start the compass heading at each data point and the the actual speed readings deaddownwind heading Next find the true downwind Adjust the heading of the boat until the wind vane is tacking angle for each point by adding the directly aft Record this compass reading the previously determined to account for the deaddownwind heading and the knotmeter between the wind vane indicated to the nearest tenth of a knot Since the knotmeter heading and the true deaddownwind headingneedle will usually be oscillating slightly because of the Plot the points on graph paper and include the numberrocking of the boat try to record the average reading over a for each test point so that you can see the order in whichperiod of several seconds Now apply the indicatedto the test points were taken Typical plots for two differenttrue correction to the compass heading and adjust the boat wind conditions are shown in Figure 2to this new heading Record this heading and boat speedThis will be the true deaddownwind condition and will beused in the subsequent data analysis Now using the compass head up into the wind about10 degrees above the deaddownwind heading Adjust thesails and after the speed has stabilized record the newheading and knotmeter reading Repeat this headingsweep at 20 30 and 40 degrees off of the original heading each time recording theheading and speed Now return to the heading where thewind vane is again pointing directly aft If the compass andspeed readings are close to the original then you probably have a good set of datapoints with consistent wind speed and direction Repeat this test procedure several times to data to establish consistent If any repeat check point is not close to the originaltest point reading then make a note on your test log sheet Figure 2indicating this and then repeat the entire procedure again The lower speed set of data were taken under veryIf as you take a test point you sense an increase in wind smooth sea conditions and with a steady wind The upperspeed or apparent shift in direction then note this fact on set of data were for a higher wind and rougher seathe test log sheet abandon the test sweep and return to the condition Note the increase in data scatter and the deaddownwind starting condition Continue number of data points used to obtain a useful data trendrepeating the above test procedure until you have several The smooth lines drawn through each set of test pointssets of data that seem to have been taken under consistent represents a smoothed average of the data taken and iswind and sea conditions used in subsequent calculations to find the optimum The next time out sailing try to pick a day with more tasking angle for these wind and sea conditionswind and repeat the data gathering procedure As the The shortest time to the mark is obtained where thewind and sea conditions increase you will have more downwind tacking factor DS is at a minimum To find in the knotmeter needle your measurements condition we use the smooth curves drawn through thewill be less accurate and you will have more scatter in the test data and a few simple calculations Samplefinal plotted results For these conditions more data points calculations for the higher speed test data shown in Figurewill be required to obtain good data trends As the wind 2 are shown in Table Iand sea conditions continue to increase the boatspeed will The calculations are made at 5 degree increments in thebegin to fluctuate wildly as surfing conditions develop if downwind tacking angle u The distance factor D varies 3 Table I different sail designs The boat used for the measurements shown in Figure 3 had a hull speed of about 6 knots so little is gained from downwind tacking when the deaddownwind boatspeed approaches 6 knots Of course this chart does not apply when the wind is so high that surfing conditions exist Note from Figure 3 that as the wind and boatspeed decreases the amount to be gained by downwind tacking increases For this boat the optimum downwind tacking angle also increases as the wind speed goes down Before we get into the more practical aspects of using this information we might first answer another question How much is gained in tacking downwind when the leewith the downwind tacking angle and does not change mark is not deaddownwind but is at some angle off the dfrom boat to boat For these calculations the lee mark is deaddownwind This angle is shown as u in Figure 1 dassumed to be deaddownwind u 0 The speed as read The only change in the analysis procedure is that thefrom the smoothed data curve Su the speed factor S and distance factor is now given by one divided by the cosine of dthe downwind tacking factor DS however are all a the tacking angle and multiplied by the cosine of u function of the boat and sail As shown in Tabulated results for a course where the lee mark is at a 10Table I the speed factor is obtained by dividing the speed degree angle off of the true deaddownwind are shown inat each tacking angle by the speed at zero tacking angle Table IIThe downwind tacking factor DS is found by dividing Table IIthe distance factor by the speed factor The results of are shown in Figure 3 along with data forseveral other wind and sea conditions The downwind tacking factor results of these calculations are presented in Figure 4 together with the previously determined line where the lee mark was dead d downwind Note that the curve for the u 10 has exactly d the same shape as the u 0 curve except that it is shifted Figure 3 up so that it passes through the DS10 point at a The numbers for S 0 on each curve give the downwind tacking angle of 10 degrees The boatspeed on a true deaddownwind downwind tacking angle will therefore remain the sameheading The lowest point on each curve gives the at 22 degrees However the reduction in time from a nooptimum downwind tacking angle The percent scale on downwind tacking course is only about 3 percentthe right side of the plot gives the percent reduction in timefrom a deaddownwind course For the 46 knot curve theoptimum tacking angle is 22 degrees and the time is 6percent shorter than a nontacking course If this boatsailed at a downwind tacking angle of 15 degrees insteadof the 22 degree optimum then the time saved would bejust a little less than 5 percent Using these data for a onemile leg the boat sailing the optimum 22 degree course willgain 3 boat lengths over a boat sailing at the 15 degreetacking angle assuming a boat length of 20 feet The boatsailing the deaddownwind course for these conditionswill be about 18 lengths behind the optimum tacking boat Figure 4 Of course such large differences may not be seen in an We will now turn to the practical considerations ofactual race because the lee mark is seldom dead actually applying this knowledge on downwind tacking indownwind Even direct comparisons between boats of the actual racing conditions First lets take the armchairsame class is difficult because of different boat speeds due mathematical approach and after that well see whatto skipper ability different boat bottom conditions and happens out on the race course 4 Figure 5 Lets assume that we are approaching a windward understanding of how his sails should look and in whatmark to be followed by a run downhill and we are trying to direction the masthead wind vane should point for thedecide on the best downwind tacking procedure First we optimum downwind conditionsmust determine the direction of the wind true dead You may find it helpful to fix a reference pointer at thedownwind heading Note the compass heading as you masthead to help judge when the wind vane is pointing insail close hauled toward the weather mark From this the right direction for optimum downwind and knowing how close your boat sails to the true This ability to look up at the wind vane and know that youwind usually about 45 degrees you will be able to find are on the optimum course is the most important singlethe true wind direction Having previously measured all piece of knowledge to be gained from all these speedcourse headings on the night before the race you can measurements and calculations and represents the finalquickly calculate the difference between the straight line approach attained by the expert skipper with his years ofcourse to the lee mark and the deaddownwind direction experienceYou now consult your chart of downwind tacking factor There is yet another way of handling the downwindDS versus tacking angle u and select the curve that best tacking procedure and that is to make your the wind and sea conditions Knowledge of how and analysis during actual race conditions Most of thethe deaddownwind speed of the boat varies with close proceeding discussions have dealt with round the buoyhauled boatspeed will help in selecting the DS vs u curve racing However for the long distance racer the speedto use in these preliminary calculations An example of versusheading measurements and optimum coursethese calculations is given in Figure 5 calculations may be best made under the actual race itself So much for the armchair sailing Few racing skippers In a long race the time required to make the speedwill be willing to follow this detailed mathematical heading measurements and calculations shown in Table Iprocedure in actual racing conditions Then what do we will be well worthwhiledo One answer which follows the above procedure in The previous discussions have neglected allprincipal is to use one of the small plastic circular sailing considerations of what the rest of the fleet is doing Thecomputers now available I have found the Command first problem of this type occurs as you approach the markComputer excellent for this purpose A quick setting and with a number of other boats before starting the runreading of this device by the skipper or crew gives a very Having decided upon your downwind tactics in advancerapid solution and allows the skipper to anticipate the round the mark set the spinnaker or put the jib on theconditions that he should expect even before he starts on whisker pole and above all search for clear air in thethe downwind leg direction that you have established Once you have clear The other possible solution is to develop the air away from the boats behind you may want to take aexperienced skippers feel for the optimum downwind few seconds to verify your original estimates as to thecourse But how do we do this without the years of optimum tacking under our hat Again a few practice sailing Using your masthead wind vane bear away to thesessions may do the trick Take the time to use the detailed indicated deaddownwind direction If the wind directionhand calculation procedure shown in Figure 5 when you checks with your original estimate then pull back up toare not under the pressure of racing conditions Repeat the your optimum downwind tacking course If your several times in a single practice session These estimate was wrong or the wind has shifted use the newexercises will begin to give the skipper a better wind direction and recalculate your optimum downwind 5tacking procedure The primary purpose of this is to From Robert that you are starting off on the right tack I too own a Cal 20 and have just started racing and have Throughout the rest of the downwind leg sail by found the downwind run quite perplexing when racing withwatching the masthead wind vane to keep the boat at the experienced sailors I also had some difficulty understanding your reference to a means of estimating the masthead windoptimum angle relative to the wind at all times Boats with indicators position relative to some fixed reference Do yourelative wind direction gauges will find them particularly mean you have some fixed indicating device attached to theuseful in maintaining the optimum angle relative to the mast head beneath the plane of rotation of the wind indicatorwind Without such electronic gadgetry you may find ithelpful to fix a reference pointer at the masthead under the Gentrys Responsewind vane to help judge when the wind vane is pointing in Yes I do have a reference indicating device fixed to thethe right direction for optimum downwind performance masthead just below the wind vane A small block of wood is If you have a good lead with clear air or in varying attached to the masthead fitting with epoxy and the reference card is held on with a screw into the wood block Thewind conditions you may want to jibe several times to stay indicator card itself is a 90 degree plastic pieshaped sectionin the general path being taken by the other boats since that I cut out of the bottom of a childs sand pail The indicatoryou at least want to get all the strong puffs of air that they card has sawtoothed notches cut into it at appropriate anglesget While ahead you wouldnt want to tack away from the from the boat centerline The installation is simple light andfleet and then get caught in a spot with no wind However cheapif you are in the lead they may just follow you regardlessof what course you are sailing There is one remaining question When do you jibe onto the other tack to lay the lee mark You may have noticedin Figures 1 and 5 that the relative bearing from the boat ofthe lee mark at the moment of the jibe is twice the tacking angle The proper jibe point is difficultto judge so it may be wise to resort to the use of angularmarks on the cabin top or a pelorus in deciding when youhave reached the jibe point With the relative bearing ofthe mark for the jibe known you can make over the cabin top marks or the pelorus todetermine when the jibe should be made One last point to remember Different boats havedifferent optimum downwind tacking angles Differentsail combinations on a given boat will also give tacking angles And of course the wind and seaconditions always have their effect Dont try to rely on the numbers given in the samples inthis article Go out and determine the optimum tackingangles for your boat and stay ahead of the pack Ifhowever they have read this article and are usingoptimum downwind tacking procedures they will be rightbehind you In that case keep your air clear and good luck 6


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