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.