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.

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 for 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
• Improved shallow-water operations, since only the inlet needs to be submerged
• Increased maneuverability, by adding a steerable nozzle to create vectored thrust
• Ultra-quiet designs for submarines; for example the Trafalgar class
• Propulsion of torpedoes - for example the Spearfish or Mark 48

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.

See also

• Propulsor
• Personal water craft
• Jet ski
• Jetboat
• Kitchen rudder a low speed precursor

Marine propulsion | Jet engines

Wasserstrahlantrieb | Waterjet | ウォータージェット推進 | Pędnik wodnoodrzutowy | Vattenjet