Alternative propulsion. Don Mee Kite

Flushing out an age old principle...

Prototype proves hydrodynamic flushing technique...

By IBI Magazine/David Foxwell

August was expected to see further tests of a prototype vessel driven by a unique form of propulsion known as hydrodynamic flushing, a technique which its developer believes could provide displacement craft with a number of advantages.

Pioneered by Norwegian naval architect Arne Kristiansen, managing director of Tyvik, a company formed to pursue development of the technology, working closely with students at Vestfold University College, the concept is said to offer the potential of reduced construction costs, reduced operating and maintenance costs, and exceptional manoeuvrability.

Hydrodynamic flushing eliminates the need for a propeller, and uses instead a pressure difference created over the fore part of the hull by flushing water away from the bow. A craft using the technique is thus moved in the direction of the lower hydrodynamic pressure, which is obtained by fitting the rounded bow of the vessel with a number of nozzles from which water is expelled using a pump driven by a diesel engine.

The technique is based on that the fact that unequal streams of a fluid over opposite sides of a body lower the pressure unequally, producing a lifting or pushing force. Around a ship at rest, the water pressure is equal on all sides, but if the pressure forward is lowered whilst the pressure aft remains the same, the craft is moved forward, being, as it were, drawn towards the area of lower pressure, a technique that Kristiansen and his colleagues liken to the lifting force produced by an aerofoil or wing.

"A pressure difference over the hull gives a better propulsion at less cost. A lowered pressure fore will release the force of the pressure aft. The ship is moved towards the lowest pressure, which is obtained by flushing the rounded bow of the ship with water streams from a number of nozzles," he explained.

The ship has no screw propeller, or rudder, and the nature of the hydrodynamic flushing technique means that ships using it could be shorter, broader and, in some cases, deeper than slender-hulled propeller driven ships.

As Kristiansen explained, in its current form, the hydrodynamic flushing technique is suitable for use on displacement hullforms. He admits that the technique is unlikely to be suitable for faster types of craft, but could well be applicable to slower moving vessels, such as, for instance, riverboats or narrowboats, or other types of non-planing recreational craft.

Kristiansen says he believes that a craft using hydrodynamic flushing would be much quieter than conventional craft, more manoeuvrable, and the absence of a rudder, propeller-shaft and ancillary equipment will make maintenance easier and cheaper.

Manoeuvrability, would, he believes, be much better than a rudder-equipped ship. "The ship will turn at a small radius, keep the stern in the ship's course when turning, and turn on the spot; and it will be steerable at low speed or in a stream, and braking is efficient even in larger craft," said Kristiansen.

The 30 tonne, 14.5m (48ft) prototype shown here was used for initial trials in the Oslo area in June, trials that Kristiansen described as "satisfactory."

(23 August 2005)