thumb|Model of a Pan Am [[Boeing 314 flying boat with its left sponson visible bottom center of the photo ]]
thumb|[[MS Moby Otta|MS Princess of Scandinavia, a cruise-ferry with a side sponson (the brighter part at middle right labelled DFDS Seaways)]]
Sponsons are projections extending from the sides of land vehicles, aircraft or watercraft to provide protection, stability, storage locations, mounting points for weapons or other devices, or equipment housing.
Watercraft
On watercraft, a sponson is a projection that extends outward (usually from the hull, but sometimes other parts of the vessel) to improve stability while floating, or to act as a securing point for other equipment. Vessels with unstable body shapes or unevenly distributed weight are likely to feature sponsons to help prevent capsizing or other instabilities. On many vessels, these projections from the main body of the vessel can be attached and removed quickly and fairly easily.
Canoes and kayaks sometimes feature sponson attachments as well, for stability in rough waters. These differ from outriggers, which extend a significant distance away from the body of the craft, and are employed on craft designed for open waters. A sponson's terminus is close to the craft, thereby allowing the boat to maneuver through narrower spaces, and so is more likely to be used on smaller craft that still require maneuverability through narrow passages.
There are essentially three types of sponsons for watercraft available on the market todaythe basic block type, the hooked or winged type, and the paddle or rudder type.a sponson can help extend the hull higher in the water, reducing drag and providing additional lift when the plane is taking off.
Helicopters may also feature one or more sponsons, and though most helicopters are not designed to land in or take off from water, these safety features are important should the aircraft crash land in water. Sponsons on a helicopter can also store fuel or landing gear,