Sponsons are projections from the sides of a watercraft, for protection, stability, or the mounting of equipment such as armaments or lifeboats, etc. They extend a hull dimension at or below the waterline and serve to increase flotation or add lift when underway.
Sponsons are commonly used on jetskis and other personal watercraft such as canoes to provide either additional buoyancy and thus stability against capsize, or hydrodynamic forces to resist capsize. They can often be easily attached to an existing craft in order to improve its stability.
They are far less common on ships than such stabilizing means as pontoons, outriggers, and dual hulls due to their comparatively poor performance in stabilizing large hulls. Sponsons are sometimes added to improve stability when ships are modified.
Sponsons are used on the fuselages of flying boats. They take the form of a short wing which when travelling through the water provides hydrodynamic stability during take off and landing.
They are often used in larger helicopters where the internal space of the sponson can be used for fuel or to house landing gear without reducing cargo or passenger space in the fuselage as, for example, with the Sikorsky S-92.
By extension, the term "sponson" is sometimes used to describe similar projections from the sides of land vehicles, such as the World War I Mark I tank. In the case of the Bradley Fighting Vehicle, which doubles as a troop transport and armament platform, the sponson refers to the aspect of the vehicle body directly over the tracks and includes layers of hardened, bullet-proof materials to protect the occupants. It is also used as a storage space for both vehicle equipment and components, and either ammunition or crew belongings.