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