Waterjet propulsion is often choosen instead of conventional propellers for vessels requiring high speeds, shallow draught, protected propulsion, high manoeuvrability at all speeds, low noise emissions for military applications and low vibration.
A typical waterjet system includes a flush-mounted inlet channel guiding the water to the rotating pump impeller, a stationary guide vane package, an outlet nozzle and a steering/reversing mechanism. The basic operating principle of waterjet propulsion is similar to that of a screw propeller system. Namely the propelling force is generated by adding momentum to the water by accelerating a certain flow of water in an astern direction.
The water from under the vessel is fed through an inlet duct to a precision inboard pump, usually mounted at the transom, adding head to the water. This head is then applied to increase the velocity when the water passes through an outlet nozzle into the ambient atmospheric pressure.
The steering and reversing forces are generated by deflecting the jet by a flat-bucket gear which is normally hydraulically operated. For speeds above 30 knots waterjets are more efficient than conventional propellers, they also include benefits like improved manoeuvrability and radical reduction in the ship draught.
An extensive reference list in the ropax fast ferry sector can be claimed by Wärtsilä for its wide range of Lips waterjets. Capabilities in the high power arena are demonstrated by four MEKO A-200 SAN corvettes handed over to the South African Navy by the German yards Blohm+Voss and HDV. A CODAG WARP (combined diesel and gas turbine waterjet and refined propellers) propulsion system embraces two diesel-driven trains with CP propellers and a centreline gas turbine with an output of 20MW driving a Lips 210E waterjet.
With a 2.8m-diameter six-bladed impeller and an intake duct of 2.1m, the waterjets for these projects were at the time the largest ever built and the first applied to naval vessels of this size (121m long/3500 tons displacement).
Further reading: Waterjets Product Guide, can de downloaded from;www.wartsila.com