An airplane designed to take off and land on water. This include floatplanes and flying boats.
A seaplane is an airplane designed to make takeoffs and landings on bodies of water, rather than needing to use conventional runways. Contrary to what the name implies, the body of water does not have to be the sea (although it often is), but can be any body of water of sufficient size; many lakes (especially in places with a lot of lakes and not a lot of fixed infrastructure, like Alaska, Siberia, or northern Canada) and sufficiently-whitewater-free rivers are frequented by seaplanes, and a few airports on land have dedicated water runways (long, runway-size trenches dug in the ground and filled with water) to allow seaplanes to operate to and from them.
There are two main types of seaplanes:
- Floatplanes are mostly shaped like a regular aircraft, but have large floats mounted under the sides of the fuselage and/or extending downward from the wings (some floatplanes have additional "stub wings" - mounted lower down on the fuselage than the main wings - whose main purpose is to carry floats); when on the water, the floats float on the water, and the airplane sits on top of them. Most smaller airplanes can be retrofitted with floats with a bit of hangarwork and paperwork, but larger airplanes would require prohibitively-long, -heavy, and -draggy floats to keep them from tipping over on the water.
- Instead of using external floats, a flying boat uses the airplane's entire fuselage as one giant float; as the name indicates, a flying boat has a boat-hulled fuselage which rests in the water when not airborne. Flying boats can be made very large; before World War II, all of the largest and longest-range airliners were flying boats, as land-based airports with long enough runways for a large airplane were rarities, whereas nearly any harbour city could accommodate a flying boat (the fact that a midocean forced-landing in a flying boat is considerably more survivable than attempting a ditching in a landplane can't have hurt).
Either type can also be made as an amphibious (or amphibian) airplane, with retractable landing-gear extending from the floats of a floatplane or the hull of a flying boat; this allows the airplane to take off and/or land from a land-based airport if necessary and/or desired.
Both types of seaplane suffer from increased drag - a floatplane due to its large floats hanging around in the airstream, and a flying boat because of the big fat boat hull which is shaped to keep the airplane afloat and stable in the water, and not so much to be streamlined and efficient in flight.
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