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.