Sail Trim

十一月 14, 2010 at 15:52 | 張貼於sail trim | 發表留言

Sails
Sails provide power by creating lift and drag. We can alter the sail’s lift-to-drag ratio through sail controls.

Light air
In light air the goal is to maximize lift and minimize drag. Simply put, deeper sails tend to create lift faster then than drag, but if they get too deep the leeward aft edges start to stall. We monitor this stall with leech tell tails and visual clues such as distance off the spreader, and top batten angle. Increasing the angle of attack such as raising the traveler or sheeting the Genoa increases lift faster than drag, but if you go too far, the wind will strike the leading edge of the sail at too wide an angle and the leading edge will start to stall. When the leading edge stalls, the entire leeward side of the sail is vulnerable to stall. That is why most sailing handbooks suggest sailing with the windward tell-tail just lifting. In light-to-moderate air, the best sail trimmers are very good at keeping their sails at the verge of stall.

Heel
Lift generated by upwind sails is not directly oppostite of the lift generated by the keel. The degree to which the lift is not directly opposed is vectored into forward motion. As the wind increases, there comes a point where the opposition of these forces create heel. Too much heel is disastrous to performance for a multitude of reasons, but let it suffice that too much heel increases drag and reduces lift in both keels and sails. It’s nature’s way of equalizing the forces.

Reducing Heel
Centerboard boats can reduce heel by reducing their exposed underwater surfaces. Keelboats do not have such flexibility. Heel has to be controlled through minimizing the lift and drag that the sails produce. As it turns out, flat sails reduce lift and drag fairly effectively. Decreasing the angle of attack reduces both lift and drag. A flogging sail, however, creates almost no lift at all, but it generates an large amount of drag. Great sail trimmers are able to flatten their sails and maintain an appropriate angle of attack, but they rarely if ever let their sails flog. Their goal is to minimize drag and maintain the appropriate amount of lift suitable to the boat’s ballast. It is better to have a little too much heel and keep it consistent than it is to over heel and then over flatten over and over again

 

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