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A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for propulsion. The mechanical arrangement may be a ducted propeller (axial-flow pump), a centrifugal pump, 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 pump-jet, hydrojet, water jet, or jet drive uses a ducted propeller (axial-flow pump), centrifugal pump, or mixed flow pump to create a jet of water for propulsion. These incorporate an intake for source water and a nozzle to direct its flow out, generating momentum, and in most cases, employing thrust vectoring to steer the craft.
A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition may include rocket , water jet , and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet , turbofan ...
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 an intake and into a pump-jet inside the boat, before expelling it through a nozzle at the stern.
In 1802, American lawyer and inventor John Stevens built a 25-foot (7.6 m) boat with a rotary steam engine coupled to a four-bladed propeller. The craft achieved a speed of 4 mph (6.4 km/h), but Stevens abandoned propellers due to the inherent danger in using the high-pressure steam engines.
The word is commonly used in the marine vernacular, and implies a mechanical assembly that is more complicated than a propeller. The Kort nozzle, pump-jet and rim-driven thruster are examples. An example propulsor is shown in the accompanying picture. It has a shroud which cuts down on blade-tip cavitation and radiated noise.
Jet propulsion is produced by some reaction engines or animals when thrust is generated by a fast moving jet of fluid in accordance with Newton's laws of motion.It is most effective when the Reynolds number is high—that is, the object being propelled is relatively large and passing through a low-viscosity medium.
[clarification needed] The choice between using thrusters or traditional propellers to propel marine vessels is a compromise between versatility and efficiency. Propellers are designed to work in-line with a propulsion plant and produce one-directional thrust while thrusters are more customizable and have a more versatile application.