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General information about water jet propulsion units

The Alamarin-Jet water jet propulsion unit* is a high-quality propulsion unit for boats. It is a single-stage mixed flow pump which produces a high-volume flow rate and thrust with high efficiency. The jet is located at the back of the boat and receives pump water through an intake at the bottom of the boat (figure 1, section A). The intake duct (figure 1, section B) leads the water onto the impeller (figure 1, section C) with minimal loss in efficiency and the impeller raises the pressure of the water. This pressure is transformed into flow rate in the nozzle (figure 1, section E). The stator located before the nozzle (figure 1, section D) straightens the flow. The change in the flow rate creates a reactive force in the direction of the flow, which thrusts the boat forward.

*also referred to as a “jet”

Figure 1. Cross-section of the jet
The jet is extremely safe since the impeller rotates while protected inside the jet, and the grill in front of the water intake prevents large objects from entering the intake duct.

Figure 2. Steering and control system
A Steering device (Steering nozzle)
B Controlling device (reversing deflector)
C Steering lever
D Controlling device for the reversing deflector
E Coupling flange

By changing the direction of the jet flow, the boat can be steered in the desired direction. This occurs by means of the steering and control system. The jet flow can be directed to the left or right of the centre line with the steering nozzle, while the reversing deflector can be used to direct the jet flow to the bow of the boat, reversing propulsion and causing the boat to move in reverse. The reversing deflector can also be used to keep the boat stationary – even when the jet shaft is rotating and generating thrust (middle position). The various combinations of these settings give the jet a degree of control beyond compare. For example, the boat can be rotated round its vertical axis while stationary. Moreover, a boat equipped with two jets can move sideways without the aid of bow thrusters.

The impeller is usually powered by a high-speed diesel engine. Petrol engine installations are also possible. The engine is connected to the coupling flange with the intermediate shaft. Dedicated pressure bearings are not required, since the front bearing functions as a two-way pressure bearing.

Some engines require the use of a reduction gear that matches the power/ RPM coverage areas of the engine with those of the jet. Alamarin-Jet jets feature a conical impeller housing. Thanks to this feature, the power of the jet can be adjusted by changing the diameter of the impeller. This way, the jet can be precisely matched with the type of engine used. A reduction gear can also be used if you want a true neutral gear for the jet – i.e. a setting in which the shaft of the jet stops rotating.

A gearbox can also be used to flush the intake duct in reverse, cleaning the jet of litter and other clogging that reduces water flow (for example chunks of ice). The intake duct can also be cleaned through the inspection hatch. For installation and safety reasons, the inspection hatch is located outside the boat in all Alamarin-Jet propulsion units.

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