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Any vehicle used in or on water as well as underwater, including boats, ships, yachts, hovercraft and submarines, is a watercraft (: watercraft), also known as a water vessel or waterborne vessel. A watercraft usually has a propulsive capability (whether by sail, oar, paddle, or engine) and hence is distinct from a stationary device, such as a pontoon, that merely floats. Most watercraft may be described as either a ship or a boat. However, numerous items, including surfboards, underwater robots, seaplanes and torpedoes, may be considered neither ships nor boats. Although ships are typically larger than boats, the distinction between those two categories is not one of size per se. Ships are typically large ocean-going vessels; whereas boats are smaller, and typically travel most often on inland or coastal waters. A rule of thumb says "a boat can fit on a ship, but a ship can't fit on a boat", and a ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. Local law and regulation may define the exact size (or the number of masts) that distinguishes a ship from a boat. Traditionally, submarines were called "boats", perhaps reflecting their cramped conditions: small size reduces the need for power, and thus the need to surface or snorkel for a supply of the air that running marine diesel engines requires; whereas, in contrast, nuclear-powered submarines' reactors supply power without consuming air, and such craft are large, much roomier, and classed as ships in some navies. A merchant ship is any floating craft that transports cargo for the purpose of earning revenue. In this context, a passenger ship's "cargo" is its passengers. The term "watercraft" (unlike such terms as aircraft or spacecraft) is rarely used to describe any individual object: rather the term serves to unify the category that ranges from jet skis to aircraft carriers. Such a vessel may be used in saltwater and freshwater; for pleasure, recreation, physical exercise, commerce, transport or military missions.

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Marine propulsion

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