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For maximum efficiency a sail should have a curve, or draft.
The larger the draft or curvature of the sail usually creates a more powerful
sail. In moderate wind conditions, the draft should be approximately
45% of the way back from the luff (front edge of the sail), much like a wing
of an airplane.
In stronger winds it may be desirable to flatten the mainsail. Flattening
the curvature of the sail will make it less aerodynamic and reduce the "pull"
on the leeward side of the sail. This can be done by tightening the
backstay (flattens the upper 2/3 of the sail), and by tightening the outhaul
of the mainsail (flattens the lower 1/3 of the sail).
Tightening the backstay will bend the top of the mast backward and the
mid-portion of the mast forward. This will flatten the upper two thirds
of the mainsail and depower the sail. However, this also moves the
draft or maximum curvature of the sail aft (toward the
stern). Setting the Cunningham will tighten the sail's
luff (front edge of the sail) and move the draft forward and back to its
correct position approximately 45% of the way back from the luff (front edge
of the sail). MENU
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