Pump-jet

A pump-jet 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 pump-jet engine was created by New Zealand inventor, Sir William Hamilton in 1954.

Advantages

Pump jets have some advantages over bare propellers for certain applications, usually related to requirements for high-speed or shallow-draft operations. These include:
* Increasing the speed before the onset of cavitation, due to the raised internal dynamic pressure
* High power density (with respect to volume) of both the propulsor and the prime mover (since a smaller, higher-speed unit can be used)
* Protection of the rotating element and making operation safer around swimmers and aquatic life
* Improved shallow-water operations, since 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" [http://www.fas.org/man/dod-101/sys/ship/weaps/mk-48.htm FAS Military Analysis Network: MK-48 Torpedo] ] ; applications:
** submarines, for example the Royal Navy "Trafalgar"-class, the US Navy "Seawolf"-class, the French Navy "Le Triomphant" class, and the Russian Navy "Borei" class.
** torpedoes, such as the Spearfish and Mk 48 weapons.

In these situations, the benefits outweigh the somewhat reduced efficiency (especially marked at low forward speeds) caused by the increased wetted surface and higher rotational speeds than an open propeller. One disadvantage is that pump jets (especially when fitted to outboard engines) are prone to becoming clogged with debris such as seaweed. The operator must clear the impeller by stopping the craft and briefly revving the engine in reverse.

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ee also

*Propulsor
*Personal water craft
*Jet ski
*Jetboat
*Wetbike
*Kitchen rudder
*Water rocket