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A pump-jet is a propulsion system for watercraft, and is known by several other names such as water jet, hydrojet, and water rocket. While most common on smaller boats and especially personal watercraft, some larger craft are outfitted with these propulsion systems, including some very large vessels, such as submarines. The main difference between a pump-jet and a traditional propulsion system is that the former draws water into the propulsion assembly and then forcibly ejects it out through a nozzle rather than turning a screw or propeller that propels the boat by pushing on the water. In many ways, the relationship between the two is much like that between propeller-driven aircraft and jet aircraft.
A typical pump-jet has four main components, which are generally mostly, or completely, contained within the hull of the craft itself, although some designs have external thruster nozzles or assemblies. The intake is the opening through which the pump-jet draws water into itself. An impeller, much like a jet turbine, is driven by a traditional engine, which can be diesel-, gas-, or even nuclear-powered, as on some submarines. As the water passes through the impeller, it is accelerated and then ejected out through the the nozzle, or thruster.
On many watercraft, the nozzle is capable of movement, allowing the craft to be steered by simply turning the nozzle. Some pump-jet propulsion systems have external pods for the impeller and the nozzle that allow a greater range of motion, although this counteracts one of the advantages of some pump-jet powered craft, which is an increased ability to operate in shallow water due to the absence of a protruding screw or propeller assembly. Other pump-jet powered craft may use a system of rudders placed in the flow of the expelled water from a fixed nozzle.
Compared to vessels with traditional screw assemblies, pump-jet powered craft tend to be able to operate in shallower water and can be extremely fast. They are generally safer in recreational settings, as there is no danger of injury from an exposed propeller. Pump-jet driven vessels can also exhibit a higher degree of maneuverability compared to traditional propeller driven craft. Pump-jet systems on large vessels such as submarines can be much quieter than other systems.
The main drawback for these engines is that they are often more expensive than others. Maintenance can be more difficult compared to the upkeep of a typical outboard, small craft engine, as many of the components are integral to the hull. This can mean having to dry dock a pump-jet vessel for repairs or maintenance. The efficiency of a pump-jet engine can be reduced if the intake becomes fouled or clogged with debris or seaweed, and at low speeds, they are often not as efficient as traditional propeller systems.