Sail Course®
Vocabulary
Aspect Ratio. In general usage, the aspect ratio of a sail is taken to be the length of the luff to the length of the foot.
Attached Flow. The movement of particles along a surface, such as the flow of air particles along the
leeward side of a sail.
Backwinded. A sail is backwinded when another sail to leeward of it is trimmed so closely as to cause
increased pressure on the lee side of the windward sail thus causing the windward sail to bulge to windward.
Lift. A force generated on the leeward side of a sail by air passing over the airfoil shape. It is the force
that drives a sailboat. Also, when sailing close-hauled, a wind that shifts aft to allow a change of course toward the wind direction.
Separation. Detachment of air in a smooth laminar flow from the leeward side of a sail. See also “Stall.”
Stall. The condition that arises when the airfoil (sail) or hydrofoil (rudder) is turned too far off and the flow of air or water separates from the foil; there is no longer an attached flow.