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 gunwale 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.