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Typical 'jet ski' pump jet

Rear view of pump-jet on a Mark 50 torpedo

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.

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.

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]

Sources

1. ^ FAS Military Analysis Network: MK-48 Torpedo
2. ^ 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