Water Jet Drives

The Jet propulsion unit, also known as a water-jet unit, is primarily fitted to exclusive high-speed motor yachts, ferries, racing boots and similar full-size vessels. In principle this power system consists of a pump which sucks in water though an opening in the bottom of the hull, accelerates it by means of an impeller, then forces it out again through an opening in the stern. Newton’s law of motion now applies: the jet of water expelled at the rear generates an equal and opposite reaction in the boat’s hull.

Boats with Jet propulsion units can operate in very shallow waters, as all the parts usually mounted below the hull, such as propeller, shaft, shaft bracket and rudder, are eliminated. Initial acceleration with a jet unit is very high, and this can be a distinct advantage in certain types of boat such as dragsters. The heeling moment produced by conventional boat propellers is completely absent. Since the water in a Jet propulsion system has to cover long distances in narrow ducts, there are inevitable friction losses which require rather more energy than for an orthodox power system.