Sponson

Sponson (pronounced spon-suhn)

(1) In naval architecture, a structure projecting from the side or main deck of a vessel to support a gun or the outer edge of a paddle box.

(2) In nautical design, (1) a buoyant appendage at the gunwale of a canoe to resist capsizing, (2) a structural projection from the side of a paddle steamer for supporting a paddle wheel and (3) a float or flotation chamber along the gunwale of a boat or ship

(3) In aeronautics, (1) a protuberance at the side of a flying-boat hull, designed to increase lateral stability in the water or (2) a structural unit attached to a helicopter fuselage by fixed struts, housing the main landing gear and inflatable flotation bags.

(4) A semi-circular gun turret on the side of a tank.

1825–1835: Origin unknown but thought a variant of expansion, most likely a form of imperfect echoic related to the regional accents of workers in ship-building yards.  The first sponsons were the platforms on each side of a steamer's paddle wheels.  Sponson is a noun (and curiously so is sponsing because it's an alternative spelling), sponsoning & sponsoned are verbs.  All subsequent derivations are based on the original nautical form. 

Boeing 314 Clipper cutaway.

Re-using some of an earlier design for a bomber which failed to meet the military’s performance criteria, between 1938-1941, Boeing built twelve 314 Clippers, long-range flying boats with the range to cross both the Atlantic and Pacific oceans.  Although used by the military during World War II, most of their service was with the two commercial operators Pan Am (Pan-American Airways) and BOAC (British Overseas Airways Corporation).  Very much a machine of the pre-war age, the last Clippers were retired from service between 1946-1948, the advances in aviation and ground infrastructure built during war-time rendering them obsolete and too expensive to maintain.

Passengers boarding Boeing 314 Clipper via port-side sponson.

The sponsons built into the hull structure at the waterline were multi-functional.  They provided (1) a gangway for passengers and crew boarding and departing, (2) a stabilizing platform for the craft while moored or at anchor, (3) were an integral part of the aerodynamics, providing additional lift and thus were a kind of mini-wing al la the biplane and (4) served as auxiliary fuel tanks, the craft carrying some 4,500 gallons (20,460 litres) of aviation spirit.

Lindsay Lohan ascending ladder attached to a yacht's sponson while off the Sardinian coast, July 2016.  Because of the proximity to the water's surface, sponsons are often used for purposes such as ladders and mooring points for dinghies.

On watercraft, a sponson is an architectural feature extending from the hull or other part of the superstructure to aid in stability while floating or as a securing point for equipment.  Sponsons add stability when underway or at rest but some designs, notably those on high-performance craft, are there to make possible sharper changes of direction as they “dig in” (which is probably not the best phrase to use) to the water on the inside of the turn.  On some vessels, sponsons can even be essential to ensure seaworthiness because they can be used to provide additional buoyancy.  In some specialized applications (notably those designed for canals or other internal waterways) there are hull designs which actually wouldn’t float unless fitted with sponsons.  Sponsons can be designed to act as a protective barrier, shielding main hull from damage.  Essentially, this is a structural version of the car tyres often seen strung over the sides of vessels, a useful precaution to prevent damage which might be caused during low-speed docking manoeuvres such as docking.  It may sound an extreme approach but it’s almost always easier & cheaper to repair or replace a sponson than a hull.  When moored, large sponsons can also be used as an ad-hoc addition to deck space and it’s not unusual for them to be used as diving platforms or places from which to fish.