A marine jet drive, or waterjet, is an installation often found on high-performance watercraft that allows for high amounts of propulsion to be generated. Through the pumping of large amounts of water through a closed duct, forward thrust may be generated for movement. This operation is assisted with components such as the impeller and nozzle, both of which ensure proper water direction and power creation. In this blog, we will discuss the waterjet, allowing you to better understand their use for certain marine vessels.
Unlike a propeller shaft assembly that functions through pressure differences and minimal velocity changes, the waterjet takes advantage of velocity changes for its propulsion capabilities. To do this, the device increases the velocity change between the inlet and outlet, generating thrust through adjustments to momentum. Though a large standard flow rate, thrust may be multiplied for the means of propulsion.
In some ways, a waterjet can be considered to be a type of pump that is situated within a short pipe. Without the limitations of propeller assemblies, the waterjet may obtain efficiencies upwards of 90%. Despite this, there are various aspects that may detract from the device’s overall efficiency, some examples including intake losses, changes in elevation, internal skin friction, outlet diffuser losses, and more.
The greatest determiner of efficiency is the velocity of the jet outlet to the engine, due to its effect on flow rates which directly influence efficiency. If water acceleration is low, little thrust will be able to be produced for movement. Meanwhile, a large amount of acceleration is not optimal as the rate of efficiency begins to drop fast as acceleration continues to rise after a certain threshold. In general, increased velocity ratios lead to an increase of thrust generation by jets, all while decreasing efficiency. Raising velocity ratios can help to a degree, lowering peak efficiency while increasing the range of efficiency.
While there are certain detractors from efficiency with waterjets, they are not held to some of the same drawbacks that are seen with propellers. For one, the waterjet is completely devoid of any propeller shaft or shaft brackets that would drag in the water during operations and cause a decrease in speed and efficiency. Additionally, the functionality of waterjets means that they are devoid of a rudder and its appendages. All of this allows the waterjet to excel in operations over a standard propeller assembly, and finding the right fit for a particular vessel often comes down to finding a fitting engine and speed.
When searching for various waterjets, it is important to remember that they typically show higher efficiency as speeds increase. This is due to their functionality of creating thrust through water flow rate changes. As such, waterjets may benefit a light patrol vessel more than it would a larger transport that moves much slower. If one is operating a vessel that travels under 30 knots, a typical propeller assembly may still prove best. Meanwhile, a watercraft traveling 30 knots or above will begin seeing efficiency improvements with the waterjet.
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