Slippery Ships That Float on Air

Air-carpet hulls could sail faster, save fuel and cut emissions

By Steven Ashley

For three days in September, an ordinary-looking cargo ship traveled up and down Norway's Oslo Fjord. Few casual observers would have guessed that the 272-foot (83-meter) -long vessel was gliding on a carpet of air.

Air pumped about 25 feet (less than 10 meters) below the waterline into subsurface cavities—broad, shallow recesses built into the underside of the ship's hull—creates buoyant pockets that help reduce drag, allowing the craft to slip more easily through the sea surface, according to Jørn Winkler, founder of DK Group, the small Rotterdam marine-engineering firm that developed the new system. Because less energy is required to propel the ship, less oil needs to be burned and emissions can be cut by as much as 15 percent, he says.

Winkler reports that his company's demo ACS (Air Cavity System (pdf)) reduced the ship's drag by up to 7 percent, a performance that confirmed DK Group's earlier results in tank tests on a smaller model. Greater efficiencies should be realized by bigger, standard-size ships, he says, "because larger hulls pitch less and are generally more stable, which helps maintain the air lubrication effect."

The recent sea trial could turn out to be significant. After all, the world's merchant fleet—50,000 ships that transport 90 percent of global trade goods—emit 800 million tons of carbon dioxide annually (about 5 percent of the planet's total), according to the International Maritime Organization. Anything that can green up the operating efficiencies of new shipping by double-digit percentages would be a notable contribution.

The technique would also help address the problem of polluted cargo ports. "Just the 40 or so ships that dock at the port of Los Angeles at Long Beach each day," Winkler says, "release six times as much sulfur and nitrogen oxides than are emitted daily by all the land transport in the entire state of California."

The DK Group's program is only the latest effort to study the use of air to lessen hull drag and improve energy efficiency. Investigations by specialists at laboratories such as the Maritime Research Institute Netherlands (MARIN), in Wageningen, Holland, as well as Russian marine-engineering academies indicate that a 20 percent drag reduction is theoretically within reach employing such air-assist techniques (although their tests have never achieved better than a 10 percent improvement). And a full-scale project to lubricate a ship hull with air, attempted three years ago by a team led by Yoshiaki Kodama at Japan's National Maritime Research Institute (NMRI) in Tokyo, yielded a net drop in drag of only 3 percent.

The greatest component of drag, and the main difficulty for ship designers, is frictional drag created from the interaction between the hull surface and the surrounding water. The region of water affected by the passage of a ship—known as the boundary layer—is a turbulent area where the presence of the solid surface slows general water flow. Injected air lubricates the boundary layer. Because air's viscosity—its resistance to flow—is only about 1 percent that of water, the ship moves through more efficiently. "Most of the action occurs only a millimeter or two away from the surface," says Steven Ceccio, a University of Michigan mechanical engineer leading a U.S. team's research of ship-hull drag. "One bubble diameter away is enough to halt the effect." Ceccio's work is supported by the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research.

Over the past eight years, the Michigan team has investigated a variety of techniques to cut friction drag. First it looked at injecting slippery polymers into the water at the boundary layer. "Near the injector, drag was reduced by 70 percent, but the polymer degrades in the turbulence and just diffuses away," Ceccio says, "which means it needs constant replenishment, so we turned elsewhere."

The researchers next shot bubbles—a millimeter or less in diameter—into the boundary layer. They got an 80 percent drag decrease for six feet (two meters) or so, but again, no satisfaction; the bubbles refused to cling to the hull surface long enough to have a significant effect on overall efficiency. If one injects enough gas, however, the bubbles eventually coalesce into a buoyant film that can sit (at least for awhile) between the horizontal hull and the water, which is what Ceccio's team is working on now—air layer drag reduction. In this concept, the bubbles typically would leak sternward and out from under the hull. New air would be injected forward to constantly refill the lubricating air pocket.

Scientists speculate that more effective drag-lowering systems using smaller "microbubbles" might be possible if someone could come up with a low-cost way to make the sub-millimeter bubbles. Winkler says that his company is working on a "super-microbubble generator" that would enable existing ship hull designs to be retrofitted with such technology. These systems would also require the installation of surface cavities in the hulls.

The big issue then becomes maintaining stable coverage of nearly the entire hull surface so that rough seas do not simply wash away the bubbles. Continuous, maximal coverage is the key to success; every millisecond that a section of hull contacts water directly contributes to drag. This means ships might have to be equipped with radar and laser sensors that detect oncoming waves, which could permit constant adjustment of air flow in time to compensate for rough seas.

Although the costs of this air-carpet technology have not been fully worked out, Winkler says that adding relatively simple air cavity systems into new ship construction would add 2 to 3 percent to building costs.