A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton’s Third Law of Motion – “every action has an equal and opposite reaction”. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful firehose.
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 craft’s hull, propelling it forward (see diagram below).
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 below shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator ©, 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 (F) to turn the impeller.
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 (E) 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.