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Racing sailboats can be easily described as hours of boredom mixed with moments of terror. One of the things I missed most after "Bent course Bob", God Rest His Soul, lost his boat to Fran and soon after his own life, was the "discussions" of sail trim during bobbing sessions. A good source for those addicted to trying to make a sea slug "fast" is a book by Dick Kenny "Looking at Sails." Although very technical, it does give a deeper look into why you yank on that rope when the boat does this or that...
We all know about how a wing works. Air over a curve moves faster creating low pressure and thus lift. What is often overlooked is "separation." Separation occurs when flow tries to follow too sharp a curve. On a main sail, the leeward side of flow rushes up the curve and pressure falls creating "lift". What is forgotten is after reaching the top of the curve, pressure rises and the flow slows down. If the curve is too sharp, the flow will misbehave and "separate" from the fluid flow. The sharper the curve, the farther back up the curve the separation occurs, ultimately stalling the sail. This is why in stronger winds you pull on the outhaul to flatten the bottom of the sail, increase mast bend to flatten out the top of the main (balance top to bottom) and take out the slack in the luff induced by the mast bend with the cunningham. With the sail "flatter" or more precisely with less of a curve, laminar flow is preserved over the sail. But that only goes so far... As the flow rushes past the sail and meets, a vortex is formed creating induced drag. Induced drag is made worse by the assymetrical design of the main. That is why a full batten main with a large leech roach has less induced drag and can be trimmed easier. Induced drag kills about 75% of the power in the main. But the sail can only be flattened to the point where the force of the induced drag and resistance on the hull outweighs the force of lift. The only alternative to keep going in stronger winds is to reduce the plan of sail. It is often overlooked how important it is to keep the sail flat while doing so to preserve laminar flow to keep the main from stalling.
That's only the beginning. Mast bend works against the main and thus, twist is induced with the main sheet to counter-act that. But that's another story... Balance, balance, balance. Something to think about: if you think you need to reef, make sure you're not carrying too much headsail first. Ol' Bob would carry the 120 too long and we would fall behind. As soon as we switched to a smaller headsail, the boat would ride higher and faster and he would always remind us to kick him in the ass later for it. No one ever dared of course. "Sailbad the Sinner"
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