Watersail
Encyclopedia
A watersail is a sail hung below the boom. It is used on gaff rig
Gaff rig
Gaff rig is a sailing rig in which the sail is four-cornered, fore-and-aft rigged, controlled at its peak and, usually, its entire head by a spar called the gaff...

boats for extra downwind performance when racing. Often a watersail will be improvised from an unused foresail. Its psychological effects may be more effective than its aerodynamic ones.
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