Several innovations in propulsion became important during the late 20th century. One is propulsion by a high-speed jet of water ejected from the stern by a pump within the boat hull. The pump is driven by an engine that is identical to the one that might drive the conventional propeller. Water-jet propulsion is most suitable for high-speed boats, since for good propulsion efficiency the forward speed of the boat must be close in magnitude to the afterward speed of the jet. Regardless of boat speed, water-jet propulsion is often used for craft that must operate in shallow or rocky waters, where a conventional propeller might be damaged.
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