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.
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 48In 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.
*
Propulsor*
Personal water craft*
Jet ski*
Jetboat*
Kitchen rudder a low speed precursor
