An outrigger describes any contraposing float rigging beyond the side (gunwale/ˈɡʌnəl/) of a boat to improve the vessel's stability. If a single outrigger is used it is usually but not always windward.
In an outrigger canoe and in sailboats such as the proa, an outrigger is a thin, long, solid, hull used to stabilise an inherently unstable main hull. The outrigger is positioned rigidly and parallel to the main hull so that the main hull is less likely to capsize. If only one outrigger is used on a vessel, its weight reduces the tendency to capsize in one direction and its buoyancy reduces the tendency in the other direction.
In a rowing boat or galley, an outrigger (or just rigger) is a triangular frame that holds the rowlock (into which the oar is slotted) away from the saxboard (gunwale for gig rowing) to optimize leverage. Wooden outriggers appear on the new trireme around the 7th or 6th centuries BC and later on Italian galleys around AD 1300 while Harry Clasper (1812–1870), a British professional rower, popularised the use of the modern metal version and the top rowing events accepted the physiological and ergonomic advantages so acceded to its use in competitions. Wing-riggers are made by some manufacturers of racing shells which are reinforced arcs or form a single projection akin to aircraft wings instead of conventional thin metal triangular structures.