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Airships of the sea

ceccioEconomist - If you blow a lot of air bubbles under a ship, and keep them coming, "good things will happen," says ME Professor Steven Ceccio, an expert on bubbles at the University of Michigan's mechanical-engineering department in Ann Arbor. When air is pumped rapidly out of small holes in a ship's hull, the swarming bubbles will quickly join together and coat the hull with a layer of air a centimetre or two thick. This reduces drag, because air offers far less resistance than water.

As the ship moves forward, the layer of air slides back and out from under the hull. But blowing more bubbles to replenish it does not require much energy, so fuel savings of 5-10% are within reach, says Dr Ceccio. He studies air-lubrication systems, as the field is known, for the American navy, even though warships generally have V-shaped hulls, which facilitate fast travel but are unfriendly to bubbles. Almost all cargo vessels, by contrast, have flat bottoms, which allow a larger volume to be kept buoyant for a given amount of hull metal. Bubbles work well on these and, since the cost of fuel is often half or more of a cargo ship's total operating expenses, the potential savings are enormous.

Bubbles are wont to slip past the edges of even flat hulls, but efforts to hold them in place are paying off, says Uwe Hollenbach of the Hamburg Ship Model Basin, a facility that tests new naval technologies. One trick is to trap the blanket of air between two ridges that protrude a few centimetres downward from the port and starboard edges of the hull. Another is to shape the vessel's stern in a way that stops air being sucked into the propeller, where it would reduce thrust by lessening the propeller's grip on the water. It is even possible to design hulls that include air-trapping recesses a couple of metres deep.

Damen Shipyards Group, a Dutch firm that builds more than 150 ships a year, has found that such cavities cut fuel consumption by about 15% on a 60-metre test ship that carries cargo on rivers. Tests at MARIN, a naval-engineering institute based in Wageningen, also in the Netherlands, suggest air cavities can cut a big ship's fuel costs by as much as 20%. Damen, meanwhile, estimates that the air-lubrication system will increase the cost of building a 110-metre cargo ship by only about 5%. It expects production to begin early next year, and plans to license designs to other shipbuilders soon thereafter.

As yet, few shipping companies have adopted air lubrication, in spite of results like these. That is partly because shipbuilding in general has slumped along with global trade. It takes years for a new ship to be designed and built, and investment in such a thin market can appear risky. Retrofitting ships with air-lubrication kits, rather than building them from scratch, is much cheaper, says Katia Kardash, boss of the DK Group, a naval-engineering company in Rotterdam. The firm's retrofit product, Air Cavity System (ACS), went on sale this year. It can be built into the bottom of a container ship in two weeks during regular dry-dock maintenance. Some tests indicate ACS could provide fuel savings of 15%. Ms Kardash says that even with DK's more conservative estimate, of 7-10%, an investment in ACS will be recouped in one and a half to two and a half years.

Another reason why air lubrication has not yet taken off, though, is the innate caution of sailors. The sea is a harsh mistress and some people fear that the cavities (which, for best results, should cover at least two-thirds of a ship's hull) would affect the vessel's handling.

They will indeed, and in some ways for the better. Air cavities serve as shock absorbers, reducing pitching and rolling. In rough seas, however, there could be a problem. If surging water flushed the air out of a cavity, the ship would grab at the water rather than sliding over it. That would give it a nasty shaking.

The way round this is to use radar and laser sensors to predict where and when a surge will hit, so that the cavities at particular risk of flushing can have a tremendous blast of air pumped into them to keep the water out. With that high-tech grace-note, a new sort of airship could soon safely ply the ocean blue.

The above story is reprinted (with editorial adaptations by the U-M Mechanical Engineering Department) from materials provided by the Economist. Read the original story at the Economist.

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