Stena Airmax

Air cushion reduces the friction
The concept is all about reducing the “wet surface”, i.e. the part of the hull that is in contact with the water and slows down the vessel. This is achieved by means of an “air cushion” – a part of the flat bottom of the vessel is replaced by a cavity in the hull which is filled with air. Consequently, the water will meet an air surface instead of a steel plate, which means reduced frictional resistance. This might seem simple, but a number of arising phenomena complicate the picture. For example, waves will develop in the cavity and reduce the positive effect. This is also a question of balance between optimising the air pressure for maximum resistance reduction and, at the same time, minimising the air leakage.

Full-scale air influence
Trials of vessel models in testing pools have a long history and the technology for scaling up the results from model to vessel is well-known. But with the addition of air, Stena’s project differs from this. There is no experience available on how the air effect on model scale should be transferred to full scale. Building a vessel based on traditional model trials on a 4-metre long model would mean taking a huge risk. In order to verify that the obtained results would apply also in a larger scale, Stena decided to build a model on the scale of 1:12, which means a 15-metre long model. This had never been done before. The testing of the air-regulation system will also be an important issue for future projects.

Patented bulb
Stena has applied for a patent for the broad and flat bulb in the forebody that will make it easier for water to pass in under the vessel in a favourable manner.