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1. Any of several structures that project from the side of a boat or ship, especially a gun platform.
2. A short, curved, air-filled projection on the hull of a seaplane, imparting stability in the water.

[Perhaps alteration of expansion.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


1. (Nautical Terms) navy an outboard support for a gun enabling it to fire fore and aft
2. (Firearms, Gunnery, Ordnance & Artillery) a semicircular gun turret on the side of a tank
3. (Nautical Terms) a float or flotation chamber along the gunwale of a boat or ship
4. (Nautical Terms) a structural projection from the side of a paddle steamer for supporting a paddle wheel
5. (Aeronautics) a structural unit attached to a helicopter fuselage by fixed struts, housing the main landing gear and inflatable flotation bags
[C19: perhaps from expansion]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˈspɒn sən)

1. a structure projecting from the side or main deck of a vessel, as one supporting a gun.
2. a buoyant appendage at the gunwale of a canoe to resist capsizing.
[1830–40; earlier also sponsing, sponcing, of uncertain orig.]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
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But it was another disappointing outing for teammate and former four-time world champion Alex Carella, who was forced back into the pits on Lap 18 with a holed left sponson.
Caption: Final sponson lift--these flight deck extensions give aircraft carriers their unique profile and create large deck area.
When the airplane arrived at about 1230 on the day of the accident, the pilot requested assistance because the airplane was taking on water in the left wing sponson. Boats assisted the airplane to the dock, and both the sponson and an integral fuel tank were emptied.
He wrote: "I got out of right sponson door but I heard bullets hitting the tank and saw some Boche about 30 yds off firing at me.
The NBC protection system has been moved in the rear left sponson, the right one being used to accommodate equipment.
The crew reported that the NBC OVERHEAT SPONSON OUT light didn't provide any warning.
Its Progressive Stepped Draft V-Hull and Performance Sponson design mean that whether you are carrying one, two or three passengers, the handling and straight-line tracking are outstanding.
The 75-millimeter gun mounted on the right sponson was removed, the hole was sealed, and the space was allotted for internal storage of the 425 gallons of fuel required to operate the flamethrower.
"As he went by his starboard sponson (a projection on the side of the boat) lifted out of the water then dropped back as he continued at an unbelievable speed, and almost immediately Bluebird took off, somersaulted and crashed."
He said: "Llandudno inshore lifeboat took a portable pump, and put a crewman and the pump on board the trimaran to keep the water level on the damaged sponson down until we could make the boat safe."
Screw wash from the port screw during the port-twist maneuver had caused water to flow over the RHIB's sponson, and the craft capsized.