Sail twist
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Sail twist is a phenomenon in sailing where the head of the sail is at a different point of sail from the foot of the sail. This means that the top of the sail is further out than the bottom of the sail. This happens because the sail is made of cloth and doesn't hold its shape by itself. When the sail is filled by the wind yet trimmed in only at the foot, it becomes curved or twisted.
The three sail shape controls which control sail twist are the mainsheet, the boom vang and the traveler. The mainsheet pulls the boom (and therefore the foot of the sail) primarily inwards at lower points of sail and downwards at higher points of sail. Manipulation of the traveler can counteract this because it always pulls the boom laterally. The vang on the otherhand does the opposite, it always changes the height of the boom.
Twist is also essential for stablizing the wing that is a sail; it reduces drag and helps avoid "stalling".
Computer cut sails can produce any amount of twist one desires and in different conditions, different amounts of twist may be desirable. Less twist is generally desirable in light air coditions and slackening the luff tension will tighten the leech of the sail. This moves the center of effort on the sail toward the trailing edge and reduces twist thus making the sail more powerful but vulnerable to stalling.
