December 09, 2010

Air bubbles will reduce drag and improve fuel economy by 5-20% for ships

Economist - If you blow a lot of air bubbles under a ship, and keep them coming, “good things will happen”, says 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.

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 more than half of a cargo ship’s total operating expenses, the potential savings are huge.

Commercial shipping uses 9% of the worlds oil and causes 60,000 death per year from air pollution. US academic research which showed that pollution from the world's 90,000 cargo ships leads to 60,000 deaths a year in the US alone and costs up to $330bn per year in health costs from lung and heart diseases. One giant container ship can emit almost the same amount of cancer and asthma-causing chemicals as 50 million cars.

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 also 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 ship that carries cargo on rivers. Tests at MARIN, a naval-engineering institute based in Wageningen, also in the Netherlands, suggest air cavities can reduce 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.

DK Group, a naval-engineering company in Rotterdam, has a retrofit product the Air Cavity System (ACS). The retrofit product 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 18-30 months.

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