The Double Head Rig
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The Double Head Rig Arvel Gentry explores the benefits of s staysail By Arvel Gentry SAIL Magazine October 1973 The double head rig is experiencing a revival One area would be reachedin particular is its use on level class or ton boats to improve If you were going to add a staysail between this jibtheir beating and close reaching performance in light and topsail and the mainsail just where would you put it Letsmedium winds To keep their rating down these boats first assume the staysail has its luffDecember at point A in Figure 1999 1frequently are measured with a 150 genoa instead of a and that it is sheeted so that it has a shape indicated by the170 to 180 headsail The 150 is great for mediumto line AB You can see this shape lies exactly along anstrong winds but when the wind drops many of the boats existing streamline in the flow between the topsail and thesuffer from a lack of sail area The double head rig with a mainsailhigh cut jib topsail and genoa staysail is a popular way of Therefore even though this staysail will hold its curvedtrying to get back some of the lost sail area shape like the other airfoils the pressures will be the same I have applied aerodynamic analysis techniques to the on both sides of the airfoil and it will not contribute any liftdouble head rig and hopefully the conclusions presented at all to the sail combination Similarly it will not have anyhere will shed some new light If nothing else you will see effect either on the topsail or the mainsail You can see thiswhy the staysail is such a sensitive sail to handle situation aboard a boat when the staysail appears very To understand the effect of adding a staysail we must soft in shape and obviously does not seem to be carryingmove in for a closeup and study in detail the flow much load In some cases the whole staysail may seem between the genoa and the main An accurate but usually it is just a part of the sail that is drawing is essential to obtain a true Therefore if the staysail is going to contribute any lift of this sail interaction problem all it will have to be placed at an angle of attack to the local Figure 1 shows an accurately calculated set of curved flow field created between the topsail and thestreamlines about a jib topsail and mainsail combination main In Figure 1 the line CD fits this requirement Thisunder beating conditions without the staysail This is the sail is placed at an angle to the local curved flow andsame basic sail combination I have used in earlier parts of should therefore cause the flow field picture to change Asthis series except that the mainsail is sheeted in 25 closer a result it will have a pressure difference between the twoto the centerline and the genoa is let out 5 I did study the sides of the sail and it will hold its shape and not luffflow with the genoa out only 25 but the conclusions did Finally it may contribute to the total driving force of thenot vary from those presented here Other sail angles and sails Note that I have said may for as we will see theshapes might give differing amounts of sail interaction but staysail has some strong effects on both the topsail and theI rather doubt that significantly different conclusions mainsail Figure 1 Figure 2 An accurate flow diagram for this threesail The primary thing a staysail does do is to cause a is shown in Figure 2 If you compare Figures 1 redistribution of the slot air by taking some of the airand 2 very carefully you will see some interesting things flowing near the lee side of the mainsail and shifting itsFirst the streamlines downstream of the leech of the flow path so that it is closer to the windward side of the jibtopsail and the mainsail are almost exactly the same in both topsail The streamline on the forwardlee side of thediagrams Only the streamline passing right through the mainsail becomes further away from the sail while thearea of the staysail shows any change at all Apparently streamline on the windward side of the topsail becomesthis staysail has not affected the downstream flow either of closer to the sailthe topsail or mainsail If we remember Bernoullis principle we realize that Next you will observe that the stagnation streamline the forwardleeside pressures on the mainsail will befor the mainsail Sm is a little straighter as it approaches higher and the leeside pressures on the topsail will bethe mainsail In Figure 2 the dotted line is the mainsail lower which means that the theoretical lift contributed bystagnation streamline when the staysail is not present The both the topsail and mainsail will go down This is what weaddition of a staysail seems to have reduced the upwash mean when we talk about sail coming into the mainsail however it is important to All these effects are clearly shown in the pressurenote that the level of the mainsail stagnation streamline distribution plots shown in Figures 3 4 and 5 In each plotwell out in front of the sail is about the same In other the solid line shows the pressures with the staysail set andwords the staysail apparently has not had a great effect on the dashed line shows the pressures without the staysailthe total amount of air that is made to flow on the lee side of In Figure 3 note that the leeside suction pressuresthe mainsail including the lee side of the topsail negative pressures have not been affected by the staysail In other words this means that the total lift obtained but the jib topsail windwardside pressures have beenfrom the three airfoil combination is about the same as it is strongly affected by the staysail The lifting force on anywith just the topsail and mainsail We have added sail area part of the sail is represented by the difference between thebut the lift does not increase leeside and windwardside pressures and the total lift is This conclusion is also verified by a comparison of the represented by the area between the lee and streamline for the topsail in Figures 1 and 2 pressure curves This jib topsail suffers a large loss in liftThey are almost exactly the same The staysail itself has not because of the presence of the the upwash of the topsail stagnation streamline Mainsail pressures are shown in Figure 4 The staysailBut dont throwaway your staysail quite yet has caused a significant reduction in suction pressures Despite the fact that the staysail has little effect on the over the forwardlee part of the sail and has slowed downflow in front of and downstream of the sails it does have a the air in this area This produces a loss in the influence on the streamlines between the sails theoretical liftThe general effect is that the staysail just plays with the air Pressures for the staysail are shown in Figure 5 Againthat is flowing between the jib tip and mainsail but does the lift of the staysail is determined by the area inside of thenot in general change the actual amount of air flowing lee and windward side curves In this example the lift ofbetween these two sails the staysail is just enough to offset the lift losses from the 2 Figure 3 Figure 4 Figure 5interference on the topsail and the main However there change muchare some strong positive effects the staysail has as we will You might well ask the following question at this pointsee later If a mainsail causes an increased upwash of the flow into I do not mean to imply from this analysis that no the jib then why doesnt the staysail do the same thing forstaysail will contribute additional lift The airfoil shapes I the jib topsail instead of actually reducing it The answerused for this study were rather arbitrary in shape and seems to lie in the fact that the staysail leech is ahead of thewere selected only to illustrate the types of effects that can leech of the topsail and does not significantly affect theoccur between the three sails Other airfoil shapes and flow conditions at the topsail leech If the staysail werepositions would give differing amounts of interference going to affect the leeside velocities of the jib topsail itbut the types of effects that occur would probably not would have to be through some change in the leech flow of 3the topsail The staysail is not positioned to do this the topsail The jib topsail is cut with a high clew which In a previous article SAIL Aug 1973 I explained how increases its overlap on the mainsail This helps thethe leech of a jib was in a high speed flow region created by mainsail leeside flow in the upper part of the sailthe mainsail and how this creates increased velocities However with the high clew the foot of the topsailreduced pressures and more lift all along the lee side of the loses the end plate benefits earned by a low deck sweeperjib I call this the bootstrap effect and its effect is present genoa This is where the genoa staysail apparently fits inin both a twosail and a threesail combination again Because it is tacked low with a low clew it makes use Look at the leech pressure in Figure 4 point G This of some of this air Additionally the foot of the staysail doespressure level near zero pressure coefficient represents a not have much topsail area to interfere with for the leech velocity that is near freestream as is is high cut in this arearequired by the Kutta condition by which air leaves the In this discussion I have not distinguished between theairfoil at the leech smoothly with the same speed and different types of staysails that could be used It is on both sides The leech pressure of the topsail that when a short hoist genoa staysail is used it will havepoint E in Fig 3 is negative about 1 which means the its maximum interference with the topsail and itsvelocity of air coming off the topsail leech is much higher suppression of peak velocities on the mainsail will be onlythan freestream This is beneficial because the velocities all in the lower portion of this sailalong the lee side of the topsail will also be higher Up high above the staysail we will have to rely on the Note in Figure 5 that the staysail also has a high leech large overlap and close proximity of the topsail leech tovelocity point F about the same level as for the topsail help the mainsail leeside flow remain attachedThis occurs because the leeches of both sails are in the same When bearing off from close hauled to a reachinghigh speed region of flow created by the mainsail course the topsail is let way out and it is no longer able toHowever if the topsail were much smaller and positioned keep the tightly trimmed upper part of the main fromso that its leech was located at about the maximum camber stalling To remedy this you must let out the mainsail andpoint of the staysail as with a cutter rig then we would get allow the upper section to twist off more Anothera double bootstrap effect The mainsail would help the approach would be to change to a tallboy type of staysail Ifstaysail and the staysail in turn would help the topsail the tall staysail goes near the top of the main it will helpinstead of hindering it This combination would have less suppress the peak velocities on the mainsail and keep thesail area but I wonder what its resulting performance upper part from separating All these arguments ofwould be course assume the wind is not high enough to create Before you cut up your staysail for a duffel bag I should either excessive heel angle or weather helmtell you of some of the positive benefits of the staysail After A closereaching condition is where the threesailall we do know the double head rig can be an effective sail combination really comes into its own The staysail First in these illustrations the mainsail was control the separation on the mast and mainsail it alsosheeted in rather tightly In this position it gave a strong carries a high lift because of the bootstrap effect and theupwash flow field for the topsail and created large topsail now is sheeted out so far that staysail in the topsail leeside suction pressures But this with it is at a minimummeant the leeside pressures at the leading edge of the This all leads me to one important conclusion Themainsail had very high negative values about 3 in Fig 4 staysail is a very tricky sail to handle If the mainsail is notpoint H This suction peak was followed by a rapid trimmed so that it can benefit from the interaction in pressure to the positive side The boundary created by the staysail then the staysail may not increaselayer probably would not like this rapid increase in the total driving force of the sail combination at all evenpressure it would separate and the mainsail would be though considerable sail area is addedstalled The staysail does reduce the efficiency of the topsail if However with the staysail present the suction peak is they are too close together However the side benefits ofnot so high the pressure rise not so steep and the the staysail I have already mentioned do make it a usefulboundary layer is able to withstand this change sail but it must be used with great it remains attached and does not stall Thestaysail has suppressed the high velocities around themast which allows the main to be trimmed tighter The flow field created by this unstalled mainsailgives an increased upwash into the topsail and furnishes ahigher velocity flow region that favorably influences theleech and leeside velocities of the topsail It is the staysailthat permits the main to do all of this From practical experience we also know there are effects that help compensate for the interference with the windwardside pressures of 4
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