Why The Save Leeward Works
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Why the Safe Leeward Works Arvel Gentry analyzes the principles behind this tactic By Arvel Gentry SAIL Magazine September 1973 Interaction between sails on a boat has been confusing leeward position and the sloteffect of two sails on theto many sailors Not surprisingly this confusion continues same boat Everything is in fact consistent if properwhen one discusses what happens between sails located streamlines and flow solutions are usedon two separate boats one boat in the safeleeward December 1999 Lets look at the complete flow field around a singleposition The old explanation of the slot effect between two boat Figure 1 The dotted lines show the on the same boat and what we experience when the about the boat and spaced along these lines are pairs ofsails are on two different boats seemed to create a paradox numbers The top number is the local wind velocity at that When the sails are on the same boat the old theory had point in the flow All these numbers are what I call relativethe jib forming a venturi of high speed air that helped the values for they include both the true wind velocity and themainsail But when the sails are on different boats in a close velocity created by the actual movement of the boatsafe leeward situation somehow the boat behind and to through the waterwindward like the mainsail is hurt by the boat to leeward The bottom number is the local flow angle The plusHow can such sail interaction help in one case and be a angle number represents an upwash flow direction fromhindrance in the other the freestream direction and a negative number is the Previous articles in this series have totally disproved local downwash In this example the freestreamthe old venturi sloteffect explanation and we have seen conditions out in front of the boat were taken at 10 knotshow the jib influences the main and how the main in turn and zero flow angle apparent wind speed and angle Weactually helps the jib Exactly the same type of thing would have to go several boat lengths upstream beforehappens between the sails on two different boats and this these conditions are reachedarticle will explore this situation The air is significantly influenced for a considerable Obviously there is no paradox between the safe distance all around the boat to windward to leeward and Figure 1downstream The effects are strongest near the boat anddecrease as you move away Because small differences inlocal wind speed or flow angle can make a in the performance of a boat it comes as nosurprise that the safelee situation is in fact a dynamicexample of sail interaction In Figure 1 the favored safelee position is the flow fieldarea marked by the letter A The flow conditions at thispoint are a wind speed of 110 knots and a flow angle of57 This means a boat in the safelee position has a windspeed higher than freestream and it sails in a to a lift increased upwash all created bythe boat to windward The safelee boat creates a downwash flow field thatproduces both a heading windshift and a reduction inwind speed on the aft boat the flow field area marked BHere the wind speed is down to only 88 knots and the flow Figure 3 Aft windward boatangle is a header of 47 Obviously this is not a good placeto be A complete flow field about the two boats is shown inFigure 2 Both boats have been placed at the same angle tothe freestream wind so we can judge the between the two sets of sails Comparing thesingleboat flow field in Figure 1 with the flow field aboutthe two boats we see that the windward boat W has muchless upwash than it does in Figure 1 The reason is that it issuffering from the downwash created by boat L The shape of the streamlines downstream of boat Whave not changed very much but boat L has a far greaterupwash in front of it than was the case in Figure 1 It isexperiencing a lifting wind shift that is created by boat W Figure 4 Safelee boat that with the safelee boat present the aft windward boat has much lower leeside suction pressures and therefore much less lift We see that both the jib and mainsail of the aft windward boat are seriously hurt by the safelee boat Equally important however are the effects the aft windward boat has on the safelee boat The first effect we have already seen the aft boat creates increased wind speed and a favorable wind shift that benefits the safelee Figure 2 boat You might ask whether the aftwindward boat creates a higher speed wind flow region on the windward How do these changes in the flow fields affect the side of the safelee boat and if it does shouldnt thispressures on boat Ws sails The answer to this lies in the actually hurt the safelee boatpressure distribution plots shown in Figure 3 The dashes It is true that the effect does tend slightly to the pressures on the sails when the aft flow speeds on the windward side of the safelee boat overwindward boat is sailing alone and the solid lines show what they would be if it were sailing all by itself Thethe pressures when the safelee boat is present windward side of the safelee boat does therefore suffer a The negative pressure coefficients represent pressures slight reduction in contributing to the drive of the sails onless than the freestream value suction pressures and the the safelee boat Why then does the safelee boat stilllarger the number the higher the suction pressure on the leave the aftwindward boat behindlee side of the sail and the higher the lift Note however The explanation lies in what happens to velocities on 2the lee side of the sails of the safelee boat First letsassume the safelee boat has only one sail and its leech is inthe highspeed flow region created by the Because of this the leech speeds on the safe lee boatdo not have to return to freestream conditions as theywould have to with the mainsail of the aft windward boatThis is the same situation that was discussed last monthwhen a jib came under the influence of the mainsail The airflow on the lee side of the sail of the safelee boatdoes not return to freestream speed at the leech Instead itis at a much higher velocity This higher velocity on the leeside occurs because the Kutta condition on the safeleeboat must be satisfied in a highspeed region created by theaftwindward boat This means the entire on the leeside of the sail of the safelee boat ishigher than it would be without the aftwindward boat These increased velocities and therefore on the lee side of the sail of the safelee boat willmore than offset the loss in drive on the windward side ofthe sail This is the or bootstrap last month Only this time the sails are on different boats and theaftwindward boat actually helps the safe lee boat byincreasing its air velocity and angle and by velocities all along the lee surface of the safeleeboats sails If the safelee boat has a mainsail and jib there is adouble bootstrap effect The aftwindward boat helps themainsail of the safelee boat This in turn helps the jib of Figure 5the safelee boat even more The proof of all this is shown made through the sails of both boats at the same heightin the pressure distribution plot in Figure 4 On a real boat there are different airfoil shapes and sizes These plots show that the pressures at the leech of both that run from the deck to the highest point on the sailsthe main and the jib are more negative higher airspeeds This means the relative influence of the effects justwhen the aft boat is present and the lee sides of both sails discussed will vary at different heights above the deck Thehave higher negative pressures higher velocities all along effect is still there at every level its just the relativetheir surfaces There also is a slight reduction in windward magnitude that changesside positive pressures However this loss in lift is more Look back at Figure 1 You will see that although therethan compensated for by significant increases in the lee are large differences in speed and flow angle in the flowsuction pressures field about the boat there are no sudden changes in short How does this situation actually affect the two boats distances If this is so why do we sometimes experience aFirst a boat to windward and aft of the safelee boat has a sudden change in our own boat when we cross behindlower windspeed than the safelee boat and it also sails in another One explanation may be that the sails on thea header Because he will have to bear off from his original other boat have large regions of separated flow whichcourse to keep his sails at the same angle to the local wind cause a wide unstable wake behind the boat This ishis speedmadegood to windward goes down particularly noticeable when you pass a boat on a reach The safelee boat however can point higher because of Another reason lies in the fact that a sail is threethe increased upwash caused by the aft windward boat dimensional It has a foot and a head that create a trailingand a higher local wind speed Although he does suffer a tipvortex system just as airplanes do The loss in lift contributed by the windward side of his system is illustrated in Figure 5 though I want tosails this is more than compensated for by increased emphasize that this figure is just a sketch of what is really avelocities and suction forces on the leeside of his sails The very complex phenomenon The sketch shows a swirlingresult is that he can point higher and go faster than the aft vortex of air that is shed off the top and bottom of the sailwindward boat and in fact points higher and goes faster Each sail has its own tip vortex system and those flowingthan he would if he were merely sailing alone from the jib will interact and possibly merge with those All my analysis assumes the sails on both boats are from the maintrimmed to perfection and have no flow separation The Each vortex is caused by high pressure air on theanalysis uses twodimensional airfoils that is a cut is windward side of the sail either at the top or bottom 3trying to flow around and on to the lower pressure leeside Although this twisting is present to a certain extent allalong the leech its major influence is felt at the ends of thesail If a boat passes nearby it will experience a sudden flowchange as its sails pass through these trailing vortex in the center part of the sail flow speeds andangles are not affected too much by these vortex systemsand remain about as shown in Figures 1 and 2 4
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