More On The Slot Effect
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More on the Slot Effect Arvel Gentry continues his discussion of jibmainsail interaction By Arvel Gentry SAIL Magazine August 1973 Last month we first studied the air flow around the This diversion of air to the lee side of the jib has a verymain alone then we added a jib and this gave us a good important effect We know that the more air that flows onpicture of how the jib affects the mainsail This month we the lee side of a sail the greater its lifting force Because wewill do just the opposite First we will look at the airflow December 1999 have more air flowing to lee of the jib it will travel at higherabout the jib alone and then add the main By doing this speeds and the leeside pressures will be lowerwe will see that the main actually helps the jib to become With lower leeside pressures we have a larger pressurethe very efficient sail that it is difference across the sail and more jib lift This also can be If you have been following this series you should be an achieved by a higher angle of attack or it can beexpert at reading streamline drawings and pressure accomplished at lower angles with the help of the plots so lets jump right in Figure 1 shows the All this is illustrated in the jib pressure coefficient about a typical jibmainsail combination with in Figure 2 The negative pressure coefficients representthe solid lines the streamlines when both sails are used pressures lower than freestream suction pressures andand the dotted lines the streamlines when the jib is used high velocities and the positive coefficients are higheralone than freestream lower speeds First note that the dotted stagnation streamline for the Note that the negative pressures on the jib are muchjib alone Sjo goes right into the leading edge of the jib more negative when both the jib and mainsail are usedHowever when the main is also used the stagnation The jib develops much more lift when it is operating in thestreamline shifts to the position Sj so that it starts much flow field influence of the mainsail We would expect this tolower further to windward and comes into the jib luff happen however for the jib is operating in the upwashslightly on the windward edge In this example the jib field of the main In an actual situation afloat we can makecould be pointed a little closer to the wind without luffing use of this by sailing a bit closer to the wind But this fact From this we see that the mainsail shifts the jib alone does not account for the great efficiency attributed tostagnation streamline to windward a lifting wind shift the jib in the presence of the mainand allows the boat to sail closer to the wind This If we examine pressures near the leech in Figure 2 weincreased upwash on the jib is caused by the fact that the will see a second reason why the jib is such an efficient fields for the two sails SAIL July 1973 add Note that pressures near the leech are slightly on thetogether to become stronger in the area in front of the jib positive side A with a jib alone This means the jib leechThis is not a new fact but at least now we see exactly how velocity without a main is near the freestream valuethis jib wind shift occurs required to meet the Kutta condition on a single sail Another result is that the mainsail causes more air to be However the jib leech pressures with a main around to the lee side of the jib You can see this by actually are negative which indicates leech the levels of the two jib stagnation streamlines higher than freestream point B In this case the velocitywell out in front of the sails The stagnation streamline of at the leech of the jib is about 30 higher than freestreamthe jib when both sails are set SJ is much further to speed How has this happened I have mentioned before that airspeeds must be thewindward lower than it was without the main Sjo The same on both sides of a single sail at the leech to satisfy thedistance between these two lines L represents additional Kutta condition For a single sail the leech speed turns outair that the mainsail causes to flow on the lee side of the jib to be close to freestream airspeed When we have two sailsWithout the mainsail this chunk of air would pass on thewindward side of the jib Figure 1 Figure 2jib and mainsail the airspeed at the leech of the last sail inthe line the mainsail also will satisfy this Kutta conditionand be near freestream values Yet the leech of the jib is in a high speed region of flowcreated on the lee side of the mainsail and show that the air flowing around the jibadjusts itself so that the Kutta condition is satisfied not atfreestream conditions but at a speed that blends with thehigh speed flow created by the mainsail in the region of thejib leech This high speed region would be there even if the jib 60was not present providing the flow on the main is notseparated for airfoil shapes and angles used in myresearch show that jib leech airspeed would be about thesame if the mainsail were used alone The Kutta condition on the jib must be satisfied in a 30highvelocity region created by the combined flows of thejib and mainsail point B in Figure 2 The net result of thisis that the entire velocity distribution on the lee side of the jibis increased by a considerable amount These mean lower leesurface pressures and a resulting 25increase in jib lift In other words the mainsail actually helps the jib notonly by giving it a lifting wind shift but also by causing it to Figure 3have much higher velocities on its lee side because of the 3B the jib has been moved five degrees closer to theKutta condition requirements at the jib leech While the aft mainsail and as you can see by the number at the left thiswindward side of the jib does lose some of its positive causes a 60 reduction in the amount of air that flowspressure this is a small price to pay for the large increases through the slotin the suction pressure on the lee side The stagnation streamline for the jib now comes into This phenomenon will be referred to as the dumping the sail on the lower or windward surface and thevelocity or bootstrap effect The name bootstrap indicates stagnation streamline for the main comes into the sail on itsthat the main is actually helping the jib in this unusual upper or lee side This would cause higher pressures onmanner the lee side of the main than on the windward side and the What if a third sail were added forward and to lee of the main would luff carry a large bubble As soon as the sailjib with its leech in the increased velocity region of the jib changes its shape our nicely calculated third sail would have an even higher leech velocity become invalid for the entire flow field changes a bit inand higher leeside velocity distribution because of what response to the new shape of the sailsthe main is doing to the jib and what the jib in turn does But in Figure 3B we see what happens when we sheetto the third sail the jib in too close The slotflow air is reduced the The higher velocity flow that is forced to the lee side of pressures on the forwardlee side of the main becomethe jib by the main has another important effect These higher as the air flow becomes slower and the mainsailhigher velocities all along the lee side of the jib mean that loses more and more of its driving potential until we reachthe boundary layer will be able to withstand more rapid the point where it luffs Even in this luffing in pressure stronger adverse pressure gradients however the main serves a useful purpose by causing anwithout separating The boat can be pointed at a higher upwash in front of the jib And it still with the jib stagnation streamline coming in slightly something though less to the bootstrap effecton the windward side without the whole jib separating This of course assumes that only the forward part ofand stalling the main is carrying a bubble and that the aft part of the sail Now lets examine the effects of four different jib and is loaded upmainsail angles Figure 3 shows four streamline drawings In Figure 3C both the main and the jib are sheetedwith only the stagnation streamlines appearing so that you closer to the centerline of the boat by five degrees Thiscan see clearly what happens to the slot flow as the sail causes a 30 reduction in the amount of slot air comparedangles are changed Rather large sail angle changes of five to the sails with basic settings in Figure 3A However indegrees were selected so that the overall effects would be Figure 3C sheeting both the main and the jib in closereasier to illustrate causes even more upwash on the jib and the stagnation Figure 3A has the sails at the same setting used in the streamline comes into the jib even further back on thejibmain flow study we already have discussed In Figure windward side Here the air is treated rather roughly as it 2makes the sharp turn to get on the lee side and will reduction in velocities over the forwardlee part of theprobably separate and cause the jib to stall unless the boat mainsail rather than by a speedup in the flow which isis pointed closer to the wind Both sails in short have been the popular theorytrimmed in too tightly for the boat angle being sailed 5 Much less air goes between the headstay and the mast In Figure 3D the jib is at its original basic setting but the when the jib is placed in the flow with the main Themain has been sheeted in five degrees to open up the slot circulations of the main and the jib tend to oppose andThis causes a 20 increase in the amount of air that flows cancel each other in the area between the two sails andthrough the slot and the stagnation streamlines for both more air is therefore forced over the lee side of the jibthe main and the jib have moved slightly around to the 6 As the jib is sheeted in closer to the main there is awindward leading edge of the sails continuing decrease in suction pressure on the lee side These comments do not pretend to show how sails of the main When pressures both to the windward andshould be trimmed for maximum speed they are included leeward side of the mainsail become equal there noonly to show the general effects as the sail angles are longer is the pressure difference across the sailchanged All these results however clearly indicate that necessary to maintain the airfoil shape and the sailthe amount of air that flows through the slot between the begins to luffjib and main will vary depending upon the angles of the two sails The angles also have a The major effects of the mainsail on the jib aredirect influence over the way in which the stagnation 1 The upwash flow ahead of the mainsail causes thestreamline comes into the sails And this of course affects stagnation point on the jib to be shifted around towardthe pressure distributions and determines whether or not the windward side of the sail and the boat can bethe leeside boundary layers will separate and the sails pointed closer to the wind without the jib stalling orstall luffing All my comments to date have primarily applied to a 2 The leech of the jib is in a highspeed flow region combination having considerable sail overlap by the mainsail The leech velocity on the jib is thereforeA similar situation would have occurred if the sails had less higher than if the jib alone were usedoverlap The only difference would have been in the 3 Because of the higher leech velocity velocities along therelative magnitude of the observed effect With less entire lee surface of the jib are greatly increased whenoverlap a jib would not have such a strong effect on the both the jib and main are used and this contributes tomainsail and the mainsail leeside suction pressures the high practical efficiency of a jibwould not have been reduced as much 4 The higher leesurface velocities on the jib mean the jib The downwash effect of the jib on the main also would can be operated at higher angles of attack before the jibhave been smaller With less overlap the leech velocity on leeside flow will separate and stallthe jib would have been a bit lower and the bootstrap effect 5 Because of all this proper trim and shape of the mainsailnot quite so strong significantly affect the efficiency of the overlapping jib Of course all these comments also would apply to a Anything that causes a velocity reduction in the regionsingle sail combination with varying sail shape and of the leech of the jib such as some separation on the aftoverlap from the deck to the top of the mast Nevertheless part of the main results in a lower driving forcethe basic conclusions still would be the same contributed by the jib I now would like to classify the major jibmainsail 6 The trim of the main significantly affects the effects Although they are for the most part ability of the boat for it directly influences the I will segregate them into the effect of the that approaches the luff of the jibjib on the mainsail and the effect of the mainsail on the jib 7 The mast in front of the mainsail always has been blamed for making the main less efficient than a jib From myThe major effects of a jib on the mainsail are studies I believe this is only part of the answer Another1 The jib causes the stagnation point on the mainsail to and probably equally important factor is the increased shift around toward the leading edge of the mast the velocity on the jib and the fact that its Kutta condition header effect must be satisfied in a local high speed flow region that is2 As a result the peak suction velocities on the forward created by the mainsail leeside of the main are greatly reduced Since the peak suction velocities are reduced the recovery adverse pressure gradients also are reduced3 Because of reduced pressure gradients on the mainsail the possibility of the boundary layer separating and the airfoil stalling is reduced4 A mainsail can be operated efficiently at higher angles of attack without flow separation and stalling than would be the case with just a mainsail alone This is caused by a 3
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