A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

. The mechanical arrangement may be a

ducted propeller A ducted propeller, also known as a Kort nozzle, is a marine propeller shrouded 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 d ...

(

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 mounte ...

), a

centrifugal pump Centrifugal pumps are used to transport fluids by the Energy transformation, 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 ...

, 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.

Design

A pump-jet works by having an intake (usually at the bottom of the hull) that allows water to pass underneath the vessel into the engines. Water enters the

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

through this inlet. The pump can be of a centrifugal 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 Electric motor, motor in the pipe or by electric motor or petrol/diesel ...

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 A gear or gearwheel is a rotating machine part typically used to transmit rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or ...

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 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 Aircraft flight control system, control the Spacecra ...

'', 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 Jet Ski or water scooter, is a primarily recreational watercraft that is designed to hold only a small number of occupants, who sit or stand on top of the craft, not within the craft as in a boat. P ...

, 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.

Centrifugal flow

Centrifugal-flow waterjet designs make use of radial flow to create water pressure. Examples of centrifugal designs are the Schottel Pump-Jet and 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. Ope ...

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).

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 v ...

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

cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...

, because of the raised internal

dynamic pressure In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5 :q = \frac\rho\, u^2 where (in SI units): * is the dynamic pressure in pascals ...

* 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 Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects o ...

signature. This kind of system is also known as a "shrouded propeller configuration" because the pump can be understood as a conformal shroud and a set of stator blades added onto a propeller – a step beyond the

. In this view, the stator serves to "recycle" the rotational energy of the outflow into thrust. – describes a submarine pump-jet design with 7 stator blades and 9 rotor blades Applications include: **

Warships A warship or combatant ship is a naval ship that is used for naval warfare. Usually they belong to the navy branch of the armed forces of a nation, though they have also been operated by individuals, cooperatives and corporations. As well as ...

designed for low observability, for example the Swedish . **

Submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

s, for example the

Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...

and , the

US Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...

and , the

French Navy The French Navy (, , ), informally (, ), is the Navy, maritime arm of the French Armed Forces and one of the four military service branches of History of France, France. It is among the largest and most powerful List of navies, naval forces i ...

and ''Barracuda'' class, and the

Russian Navy The Russian Navy is the Navy, naval arm of the Russian Armed Forces. It has existed in various forms since 1696. Its present iteration was formed in January 1992 when it succeeded the Navy of the Commonwealth of Independent States (which had i ...

. ** Modern

torpedoes A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, such ...

, 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 with shorter ...

, the Mk 48 and Mk 50 weapons. *** The Mk 48 has also been described as using

contra-rotating marine propellers Contra-rotating propellers have benefits when providing thrust in marine applications. Contra-rotating propellers are used on torpedoes due to the natural torque compensation and are also used in some motor boats. The cost of boring out the outer ...

. However, it does have a fully-enclosing, conformal shroud. Computer-generated images released by the US Navy in 2011 appear to suggest only one set of rotating blades.

History

The water jet principle in shipping industry can be traced back to 1661 when Togood 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. On December 21, 1833, Irish engineer John Howard Kyan received a UK patent for propelling ships by a jet of water ejected from the stern. 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 Ministry of Defence (Italy), Italian Air Min ...

demonstrated a pump-jet propelled boat in

Venice Venice ( ; ; , formerly ) is a city in northeastern Italy and the capital of the Veneto Regions of Italy, region. It is built on a group of 118 islands that are separated by expanses of open water and by canals; portions of the city are li ...

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

. 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 A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses an external propeller in the water below or behind the boat, a jetboat draws the water from under the boat through a ...

was developed by

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

engineer Sir William Hamilton in the mid 1950s.

Uses

Pump-jets were once limited to high-speed pleasure craft (such as jet skis and

s) 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 A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. Militaries are typically authorized and maintained by a sovereign state, with their members identifiable by a d ...

vessels and

ferries A ferry is a boat or ship that transports passengers, and occasionally vehicles and cargo, across a body of water. A small passenger ferry with multiple stops, like those in Venice, Italy, is sometimes referred to as a water taxi or water bus. ...

. On these larger craft, they can be powered by

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s or

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

s. 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, submarine, 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 ...

. Pump-jet powered ships are very maneuverable. Examples of ships using pumpjets are the s, the s, s, the Stena high-speed sea service ferries, the Royal Navy ''Swiftsure'', ''Trafalgar'' and ''Astute''-class submarines, as well as the United States ''Seawolf'' and ''Virginia''-classes, and the Russian ''Borei''-class submarines. They are also used by the United States

littoral combat ship A littoral combat ship (LCS) is either of two classes of relatively small surface vessels designed for littoral warfare in near-shore operations by the United States Navy. It was "envisioned to be a networked, agile, stealthy surface comba ...

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