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This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (February 2008) Typical 'jet ski' pump jet Rear view of pump-jet on a Mark 50 torpedo A pump-jet, hydrojet, or water jet, is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle. The first functioning man-made pump-jet engine was created by New Zealand inventor Sir William Hamilton in 1954. In the Ordovician period the first known cephalopods swam by a natural built-in reciprocating hydrojet.citation needed Contents 1 Advantages 2 Disadvantages 3 Sources 4 See also // Advantages Pump jets have some advantages over bare propellers for certain applications, usually relate to requirements for high-speed or shallow-draft operations. These include: Increasing the speed before the onset of cavitation, because of the raised internal dynamic pressure High power density (with respect to volume) of both the propulsor and the prime mover (because a smaller, higher-speed unit can be used) Protection of the rotating element, making operation safer around swimmers and aquatic life Improved shallow-water operations, because only the inlet needs to be submerged Increased maneuverability, by adding a steerable nozzle to create vectored thrust Noise reduction, resulting in a low sonar signature; this particular system has little in common with other pump-jet propulsors and is also known as "shrouded propeller configuration"[1]; applications: submarines, for example the Royal Navy Trafalgar-class and Astute-class, the US Navy Seawolf-class, the French Navy Triomphant class, and the Russian Navy Borei class. modern torpedoes, such as the Spearfish, the Mk 48 and Mk 50 weapons. Disadvantages Can be less efficient than a propeller at low speed More expensive Higher weight in the boat because of entrained water Will not perform well if the boat is heavier than the jet is sized to propel Can suffer more easily from cavitation than a conventional propeller Can become clogged with debris; e.g., seaweed[2] Sources Charles Dawson, "The Early History of the Water-jet Engine", "Industrial Heritage", Vol. 30, No 3, 2004, page 36. ^ FAS Military Analysis Network: MK-48 Torpedo ^ http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html See also Propulsor Personal water craft Jet ski Jetboat Wetbike Kitchen rudder Water rocket