Pump-jet
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A pump-jet or more commonly a 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.
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[edit] 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"[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.
- torpedoes, such as the Spearfish and Mk 48 weapons.
[edit] Disadvantages
- Can be less efficient than a propeller at low speed
- More expensive
- Higher weight in boat due to entrained water
- Will not perform well if boat is heavier than the jet is sized for
- Can become clogged with debris e.g. seaweed[2]
[edit] Sources
- ^ FAS Military Analysis Network: MK-48 Torpedo
- ^ http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html
