Pump-jet

Typical 'jet ski' pump jet

Rear view of pump-jet on a Mark 50 torpedo

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.

Pump-jets used to be limited to high-speed pleasure ships (such as jet skis and jet boats) and other small craft, but since 2000-2010 the desire for high-speed vessels has increased and thus the pump-jet is gaining popularity on larger craft, military vessels and ferries in particular. On these larger craft they can be powered by diesel engines or gas turbines. Speeds of up to 40 knots can be archieved with this configuration, even with a conventional hull.

Another prime advantage for pumpjet powered ships is maneuvrebility. Pumpjet powered ships are well known to be very maneuvreble. Examples of ships using pumpjets are the fast patrol boat Dvora Mk-III craft, the HAMINA Stealth Attack craft, the Stena HSS 1500 ferries and the United States Littoral Combat Ships [LCS][3].

Workings

This image illustrates the workings of a reversing bucket. 1: Forward thrust, reversing bucket disengaged 2: Reverse thrust, reversing bucket pushes the thrust flow backwards

A pump-jet works by having an intake (usally at the bottom of the hull) that allows water to pass underneath the vessel into the engines. Water enters the pump through this inlet. The pump can be of a centrifugal design for high speeds, an inducer for low speeds, or an axial flow pump for medium speeds. The water pressure inside the inlet is increased by the pump and forced backwards through a nozzle. With the use of a reversing bucket, reverse thrust can also be archieved for faring backwards, quickly and without the need to change gear or adjust engine thrust. The reversing bucket can also be used to help slow the ship down when braking. This feature is the main reason pump jets are so maneuvreble.

The nozzle also provides the steering of the pump-jets. Plates, simmilar to rudders, can be attached to the nozzle in order to redirect the water flow port and starboard. In a way, this is simmilar to the principles of air thrust vectoring, a technique which has for long been used in military jet powered aircraft. Unlike aircraft though pumpjets only need to be actuated in two directions. This provides pumpjet powered ships with superagility at sea. Another advantage is that when faring backwards by using the inversion bucket, steering is not inversed, as opposed to propellor-powered ships.

Pump-jet powered watercraft do suffer from the Coandă effect, which must be taken into account when making changes in heading. The heading needs to be adjusted two degrees further than what would normally be required because of this effect.

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"; 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.

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

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.