A pump-jet, hydrojet, or water jet is a

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

system that produces a jet of water for propulsion. The mechanical arrangement may be a

ducted propeller A ducted propeller, also known as a Kort nozzle, is a marine propeller fitted with a non-rotating nozzle. It is used to improve the efficiency of the propeller and is especially used on heavily loaded propellers or propellers with limited dia ...

(

axial-flow pump An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted ...

), a

centrifugal pump Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic ...

, or a mixed flow pump which is a combination of both centrifugal and axial designs. The design also incorporates an intake to provide water to the pump and a nozzle to direct the flow of water out of the pump.http://www.hamiltonmarine.co.nz/includes/files_cms/file/JetTorque%2008.pdf

Design

A pump-jet works by having an intake (usually at the bottom of the

hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affi ...

) 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 Centrifugal (a key concept in rotating systems) may refer to: *Centrifugal casting (industrial), Centrifugal casting (silversmithing), and Spin casting (centrifugal rubber mold casting), forms of centrifigual casting *Centrifugal clutch *Centrifu ...

design for high speeds, or an

axial flow pump An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted ...

for low to 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 achieved 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 maneuverable. The nozzle also provides the steering of the pump-jets. Plates, similar to rudders, can be attached to the nozzle in order to redirect the water flow port and starboard. In a way, this is similar to the principles of air '' thrust vectoring'', a technique which has long been used in launch vehicles (rockets and missiles) then later in military jet-powered aircraft. This provides pumpjet-powered ships with superior agility at sea. Another advantage is that when faring backwards by using the reversing bucket, steering is not inverted, as opposed to propeller-powered ships.

Axial flow

An axial-flow waterjet's pressure is increased by diffusing the flow as it passes through the impeller blades and stator vanes. The pump nozzle then converts this pressure energy into velocity, thus producing thrust. Axial-flow waterjets produce high volumes at lower velocity, making them well suited to larger low to medium speed craft, the exception being

personal water craft A personal watercraft (PWC), also called water scooter or jet ski, is a recreational watercraft that a rider sits or stands on, not within, as in a boat. PWCs have two style categories, first and most popular being a runabout or "sit down" whe ...

, where the high water volumes create tremendous thrust and acceleration as well as high top speeds. But these craft also have high power-to-weight ratios compared to most marine craft. Axial-flow waterjets are by far the most common type of pump.

Mixed flow

Mixed-flow waterjet designs incorporate aspects of both axial flow and centrifugal flow pumps. Pressure is developed by both diffusion and radial outflow. Mixed flow designs produce lower volumes of water at high velocity making them suited for small to moderate craft sizes and higher speeds. Common uses include high speed pleasure craft and waterjets for shallow water river racing (see River Marathon).

Centrifugal flow

Centrifugal-flow waterjet designs make use of radial flow to create water pressure. Centrifugal designs are no longer commonly used except on outboard

sterndrive A sterndrive or inboard/outboard drive (I/O) is a form of marine propulsion which combines inboard power with outboard drive. The engine sits just forward of the transom while the drive unit (outdrive or drive leg) lies outside the hull. Op ...

Advantages

Pump jets have some advantages over bare propellers for certain applications, usually related to requirements for high-speed or shallow-

draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vesse ...

operations. These include: * Higher speed before the onset of

cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...

, 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 Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the ...

* 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: ** Warships designed for low observability, for example the Swedish. ** Submarines, for example the

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against ...

and , the US Navy and , the French Navy and ''Barracuda'' class, and the Russian Navy . ** Modern torpedoes, such as the

Spearfish Spearfish may refer to: Places *Spearfish, South Dakota, United States * North Spearfish, South Dakota, United States * Spearfish Formation, a geologic formation in the United States Biology * ''Tetrapturus'', a genus of marlin containing spe ...

, the Mk 48 and Mk 50 weapons.

History

The water jet principle in shipping industry can be traced back to 1661 when Toogood and Hayes produced a description of a ship having a central water channel in which either a plunger or centrifugal pump was installed to provide the motive power. On December 3, 1787, inventor

James Rumsey James Rumsey (1743 – December 21, 1792) was an American mechanical engineer chiefly known for exhibiting a boat propelled by machinery in 1787 on the Potomac River at Shepherdstown in present-day West Virginia before a crowd of local notables ...

demonstrated a water-jet propelled boat using a steam-powered pump to drive a stream of water from the stern. This occurred on the Potomac River at Shepherdstown, Virginia (now West Virginia) before a crowd of witnesses including General Horatio Gates. The 50-foot long boat traveled about one-half mile upriver before returning to the dock. The boat was reported to reach a speed of four mph moving upstream. In April 1932, Italian engineer

Secondo Campini Secondo Campini (August 28, 1904 – February 7, 1980) was an Italian engineer and one of the pioneers of the jet engine. Campini was born at Bologna, Emilia-Romagna. In 1931 he wrote a proposal for the Italian Air Ministry on the value of jet pr ...

demonstrated a pump-jet propelled boat in Venice, Italy. The boat achieved a top speed of , a speed comparable to a boat with a conventional engine of similar output. The Italian Navy, who had funded the development of the boat, placed no orders but did veto the sale of the design outside of Italy. The first modern jetboat was developed by New Zealand engineer Sir William Hamilton in the mid 1950s.

Uses

Pump-jets were once limited to high-speed pleasure craft (such as jet skis and jetboats) and other small vessels, but since 2000 the desire for high-speed vessels has increased and thus the pump-jet is gaining popularity on larger craft, military vessels and

ferries A ferry is a ship, watercraft or amphibious vehicle used to carry passengers, and sometimes vehicles and cargo, across a body of water. A passenger ferry with many stops, such as in Venice, Italy, is sometimes called a water bus or water t ...

. On these larger craft, they can be powered by diesel engines or gas turbines. Speeds of up to can be achieved with this configuration, even with a

displacement hull A hull is the watertight body of a ship, boat, or flying boat. The hull may open at the top (such as a dinghy), or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, de ...

.The Information page of the Stena HSS 1500
Pump-jet powered ships are very maneuverable. Examples of ships using pumpjets are the s, the s, s, the Stena

high-speed sea service High-speed Sea Service or Stena HSS was a class of high-speed craft developed by and originally operated by Stena Line on European international ferry routes. The HSS 1500 had an in-service speed of 40 knots (75 km/h). Several paten ...

ferries, the United States ''Seawolf''-class and ''Virginia''-class, as well as the Russian ''Borei''-class submarines and the United States littoral combat ships.

Notes

References

* Charles Dawson, "The Early History of the Water-jet Engine", "Industrial Heritage", Vol. 30, No 3, 2004, page 36. * David S. Yetman, "Without A Prop", DogEar Publishers, 2010 {{refend Marine propulsion Jet engines