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:
Higher speed before the onset of cavitation, because of the raised internal dynamic 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”;[2] applications:
submarines, for example the Royal Navy Trafalgar-class and Astute-class, the US Navy Seawolf-class, Virginia-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.
Disadvantages
Can be less efficient than a propeller at low speed
More expensive
The intake grill can become clogged with debris; e.g., seaweed. The effects of this can be mitigated by having a reversing gearbox between the engine and the water jet.