BMW Oracle Racing USA-17 from the 2010 America's Cup, with a rigid mainsail wingsail, and a conventional jib at the fore
The top of the wing of an Oracle AC45 racing catamaran
A wingsail is a built-up mechanical structure, much like an airplane wing, that is fitted to a marine vessel in place of conventional sails. The geometry of wingsails provides more lift, and a better lift-to-drag ratio, than traditional sails.
Wingsails are often composed of one or more elements on the leading edge with an 'adjustable flap' element on the trailing edge; the angle of incidence of both front and rear elements determines the overall lift. The trailing element can be divided along the height of the wing into several flaps in order to provide twist. Wingsails are typically fitted on racing craft such as multihulls and need to be lightweight. They are typically manufactured from carbon fiber frames and have a retractable skin.