The Alamarin-Jet water jet propulsion unit is a high-quality propulsion unit for boats. It is 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. The intake duct leads the water onto the impeller with minimal loss in efficiency and the impeller raises the pressure of the water. This pressure is transformed into flow rate in the nozzle and the stator located before the nozzle straightens the flow.
The change in the flow rate creates a reactive force in the direction of the flow, which thrusts the boat forward.
The waterjet 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.
A conical impeller housing refers to the section of the jet which surrounds the impeller, the conical design allows the impeller to move backward and forward inside the tunnel to adjust the gap between blade tip and tunnel wall and also contributes to the pumps mixed flow characteristics.
When beaching often and operating in waters with a lot of sand and silt the resulting wear can cause the gap between impeller blade tip and housing to increase, this has a negative effect on performance and due to the straight impeller housing of other jets it means a ‘wear ring’ must be replaced or impellers must be replaced.
A simple adjustment is required with Alamarin-Jets with no requirement for special tools.
Alamarin-Jet estimates a boat’s speed based on the information provided, calculated resistance is plotted against jet thrust and estimated top speed can be predicted.
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In most cases, a jet can be used directly without a gearbox or with a 1:1 gearbox. However, if a suitable impeller is not available, a gearbox must be used to adjust the RPM of the engine to correspond with the jet.
Though fouling can damage a jet (particularly fishing line), all open propeller propulsion types can also be fouled by rope, fishing line, rocks, and logs. Therefore, waterjets are actually more protected against fouling since they have an intake grill that blocks ingress of larger flotsam; something conventional propellers lack.
Furthermore, removal of foreign objects can actually be easier with waterjets as most feature either an inboard or outboard inspection hatch to access inside the jet without have to enter the water or lift the vessel. Alamarin-Jet have paid particular attention to the designing much larger inspection hatches than comparable products to allow increased ease of use.
In highly polluted operational areas this can be an issue and therefore an installation with gearbox is recommended to enable back flushing of the jet. As the point above describes this is also a reason why Alamarin-Jet have paid attention to the inspection hatch allowing ease of access and operation for clearing. If no gearbox is installed then clearing while underway is possible by killing the engine while underway, this creates suction which often pulls the debris from the jet intake screen.
Though waterjets are usually more expensive than equivalent stern drive or outboard-equipped vessels when purchased, they are certainly more robust in terms of reliability. Over the long term, vessels equipped with an inboard and jet are usually, in terms of OPEX, far below the alternatives. When compared against conventional propellers, the costs of maintenance are usually comparable, since waterjets typically have few parts that need service.
Though the efficiency of waterjets exceeds the efficiency of other propulsion types at high speeds, they are less efficient at slow speeds. However, efficiency of the propulsion does not account for the advantages of far greater manoeuvrability of jet-equipped vessels at any speed. Indeed, a significant advantage of waterjet propulsion is the ability to navigate in any direction, precisely, when required. Vessels equipped with conventional propellers lack this manoeuvrability unless equipped with expensive thrusters and highly experienced crews to operate. Waterjet operation is the most manoeuvrable operation for any vessel, in particular when equipped with sophisticated electronic control systems such as Alamarin-Jet’s SIGMA Propulsion Control system.