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March 30, 2010

Stena prototype tests air cushion concept

Stena Teknik is using a 15m long ship prototype to evaluate the extent to which an air cushion can reduce the friction between a ship's hull and the water. Named Stena Airmax in ceremonies held in Gothenburg, Swden, today, the 15 m technology demonstrator is a prototype for a 182 m full scale ship.

Stena is investing some SEK 50 million in the air cushion project. The large scale model, which weighs around 25 tons, has been developed following very good results achieved in tests with small ship models.

Five years ago, Stena Teknik initiated a development project in order to find a method of reducing a ship's water resistance, thus radically reducing energy utilzsation and fuel consumption.

"The results of the tests carried out are very promising. Depending on the type of ship and speed, we expect energy savings of 20-30 percent. This will now be verified in tests with the newly built prototype Stena Airmax," says Ulf G. Ryder, President and CEO of Stena Bulk.

Stena Teknik has co-operated with Chalmers University of Technology and SSPA in Gothenburg in the development project and the construction of the P-MAXair model. The Stena Airmax will be tested in the Gullmars Fjord on the Swedish West Coast, during spring 2010 when extensive - test programs will be run. The model will be powered by electric motors during these simulations.

The concept involves reducing the "wet surface," that part of the hull that is in contact with the water, slowing down the ship.

The reduction in wet surface is achieved by means of a cavity filled with air in the bottom of the hull. Thus the water is in contact with air instead of steel plate, reducing friction.

Sounds simple? In fact, several phenomena complicate the picture. These include internal wave formation in the cavity, which reduces the positive effect. A balance must also be struck between optimizing the air pressure to achieve the greatest possible reduction in resistance and, at the same time, minimizing air leakage.

Testing ship models in test basins has a long history and the technique of scaling up the results from a model to an actual ship is well known. However, Stena's project is different in that air is also involved. There is no previous experience of the effect of the air when a model is scaled up to full scale. Building a full-size ship based on normal model tests of a 4 m long model would have been very risky.

Accordingly, Stena decided to build a 1:12 model, i.e. a 15 m long model, to verify that the results achieved also applied on a larger scale. This has never been done before. Testing the air regulation system will also be an important part of a future project.

Stena has applied for a patent for the design's wide and flat bulbous bow, which facilitates a favorable water flow below the hull.

Technical data for the demonstrator model Stena AirMAX on a 1:12 scale

Length 15 m

Breadth 3.3 m

Draft fully loaded" 0.9 m

Weight fully loaded 35 tons

Speed 5 knots

Propulsion 2 x 10 kW

Full scale:

Length 182 m

Breadth 40 m

Draft fully loaded 11.3 m

Weight fully loaded 65,000 tons

Speed 14 knots

Propulsion 2 x 8,000 kW

The model consists of a steel box, containing all the equipment, surrounded by a hull made of fiberglass reinforced plastic. The flat bottom has a cavity for an "air cushion" that is almost as wide and half as long as the ship.

The model is powered by two electric engines with electricity provided by a diesel generator.

The air cavity is fed with air by fans and the air is controlled so that the bottom of the air cushion is in line with the bottom of the hull.

The model will be manned by one to two persons. Test data is transmitted via a link to an tender boat. Personnel and boats from the Swedish Sea Rescue Society will assist during the tests.

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