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Thread: How a Waterjet Works

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    How a Waterjet Works

    A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful fire hose. Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the crafts hull, propelling it forward.

    In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.

    The picture shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator (C), increases the pressure, or head, of the flow. This high pressure flow is discharged at the nozzle (D) as a high velocity jet stream. The driveshaft attaches at the coupling (E) to turn the impeller. (See picture below.)

    Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. Pointing the jet stream one way forces the stern of the boat in the opposite direction which puts the vessel into a turn.

    Reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.

    In recreational usage, a jet drive is usually coupled to a higher horsepower motor than would be used in a prop driven application to arrive at a similar speed. Jet drives can produce a fun ride, unlike with a prop which requires time to go from stop to full power (except maybe in the case of a changing-pitch propeller), with a jet you can apply the full power of the engine no matter what the boats speed.
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