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A model of a Pan Am Boeing 314 flying boat, with its left sponson visible
MS Princess of Scandinavia, a cruiseferry with a side sponson.
Sponsons could contain weapons in military craft such as pre-dreadnoughts and some of the first tanks.

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.

The term is also used to describe a gun platform standing out from the side of a warship, or from the side of a military tank, principally those fitted to British tanks during World War I.

Sponsons are commonly used on jetskis and other personal watercraft such as canoes to provide either additional buoyancy and thus stability against capsizing, or hydrodynamic forces to resist capsizing. 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, as pioneered by German aerospace engineer Claudius Dornier during World War I. 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 and the Bell 222.

By extension, the term "sponson" is sometimes used to describe similar projections from the sides of land vehicles, such as World War I British tanks. 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.

Another (somewhat exotic) use of the term "sponson" is on some types of aircraft, such as on the Yakovlev Yak-141, whose tail-surfaces are mounted on two "sponsons" extending behind either side of its single main jet engine's nozzle.

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