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Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electric motor or engine turning a propeller, or less frequently, in pump-jets, an impeller. Marine engineering is the discipline concerned with the engineering design process of marine propulsion systems. Manpower, in the form of paddles, and sail were the first forms of marine propulsion.
Rowed galleys, some equipped with sail, also played an important early role. The first advanced mechanical means of marine propulsion was the marine steam engine, introduced in the early 19th century. During the 20th century it was replaced by two-stroke or four-stroke diesel engines, outboard motors, and gas turbine engines on faster ships. Electric motors using electric battery storage have been used for propulsion on submarines and electric boats and have been proposed for energy-efficient propulsion.