For some applications, notably ferries, electric motors are used to allow greater maneuverability. Steam turbines having 1,000 shaft horsepower and more are used for the most powerful ships. Diesel engines may supply power for vessels ranging in size from small boats to medium-size ships requiring as much as 40,000 total horsepower. Gas turbines have been used experimentally in merchant ships and naval patrol boats. Some submarines, merchant ships, and icebreakers have nuclear power plants in which a nuclear reactor replaces the boiler of a stream turbine plant.
Conventional submarines have a diesel-electric drive and run on batteries when submerged.
Small boats usually have gasoline outboard engines that clamp on the stern or inboard engines to drive propeller shafts. Shallow-draft boats for use in swamps have aircraft engines and air propellers. A few small boats are propelled by a pumped jet of water. The Columbia Encyclopedia, Sixth Edition.