sponson
noun
spon·son ˈspän(t)-sən 
1
a
: a projection (such as a gun platform) from the side of a ship or a tank
b
: an air chamber along a watercraft (such as a canoe) to increase stability and buoyancy
2
: a light air-filled structure or a winglike part protruding from the hull of a seaplane to steady it on water
Example Sentences
Recent Examples on the Web That will allow salvage crews to gradually lift the ship one deck at a time, as water is pumped out and compressed air pumped in to some 30 sponsons attached to its side. 
Barbara Peterson, Popular Mechanics, 14 July 2014 Jones, a veteran boat racer himself, points out that the only original part of the boat are the sponsons. Nathan Joyce, The Seattle Times, 5 Aug. 2017 The boats came in all shapes and sizes — like the 10-horsepower shovel nose and the pickle fork with two sponsons jutting out front. Hasan Dudar, Detroit Free Press, 15 July 2017 Runabouts are boats with a hull that resembles that of a rowboat, and hydroplanes are faster and have sponsons that reduce surface area, according to show organizer Ed Zaleski, who was displaying his 1969 runabout named Challenger. Hasan Dudar, Detroit Free Press, 15 July 2017
Barbara Peterson, Popular Mechanics, 14 July 2014 Jones, a veteran boat racer himself, points out that the only original part of the boat are the sponsons. Nathan Joyce, The Seattle Times, 5 Aug. 2017 The boats came in all shapes and sizes — like the 10-horsepower shovel nose and the pickle fork with two sponsons jutting out front. Hasan Dudar, Detroit Free Press, 15 July 2017 Runabouts are boats with a hull that resembles that of a rowboat, and hydroplanes are faster and have sponsons that reduce surface area, according to show organizer Ed Zaleski, who was displaying his 1969 runabout named Challenger. Hasan Dudar, Detroit Free Press, 15 July 2017 Word History
Etymology
perhaps by shortening & alteration from expansion
First Known Use
1835, in the meaning defined at sense 1a