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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 floatation 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.

On ships, they are far less common than such stabilizing means as pontoons, outriggers and dual hulls, due to their comparatively poor performance in stabilizing large hulls. However, 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.

Sponsons on helicopters are short wings used to provide additional lift when in forward flight. 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, for example the Sikorsky S-92.

The term "sponson" for armament mounting is sometimes used for land vehicles as well, such as the WWI 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 / Components and either ammunition or crew belongings.