Pump-jet

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.

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:

• Higher speed before the onset of cavitation, because of the raised internal static 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.

The principles of air thrust vectoring have been adapted to advanced sea applications in the form of fast water-jet steering [pump jets] that provide sea super-agility. Examples are the fast patrol boat Dvora Mk-III craft, the HAMINA Stealth Attack craft and the new U.S. Littoral Combat Ships [LCS][3].

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]

See also

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

Sources

• Charles Dawson, "The Early History of the Water-jet Engine", "Industrial Heritage", Vol. 30, No 3, 2004, page 36.

1. ^ FAS Military Analysis Network: MK-48 Torpedo
2. ^ http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html