Archive for November, 2009

Sailboat surfaces, a tricky problem

Sunday, November 8th, 2009

There are two exotic ways to reduce drag on a sailboat hull. One can add riblets or make the boat sail over a cushion of air.

Dennis Conner and his America’s Cup team coated the hull of Stars and Stripes with riblets, which are tiny groves running parallel to the direction of the water flow past the hull. Riblets may have helped the United States re-capture the 1987 America’s Cup.

Some have speculated that dolphins’ spectacular swimming speed may be associated with their expulsion of ethylene oxide. This gas would act as a cushion between their skin and the water.

Dolphins almost certainly don’t sweat enough gas to make a difference, but the idea of using a gas to reduce drag has been around for a long time. One could put a “bubbler” near the bow of a sailboat. In principle, the boat would then sail over a thin film of air. This would almost certainly be illegal and impractical.

Recently, Jonathan Rothstein and others at the University of Massachusetts – Amherst have considered physics related to both riblets and the gas cushion. They measured the drag on a surface with grooves. There is a difference. The groves are filled with air. The air is encouraged to remain in the groves because the solid surface is made of a “super-hydrophobic” material. That means it hates water. Small scale experiments on this structure show a real reduction in surface drag. It is not clear how this drag reduction would scale up to the size of a sailboat hull.

Surface

It is also far from clear that a finely grooved super-hydrophobic sailboat hull would be practical or affordable, and many would consider it to be unfair competition. The riblets were declared illegal on all racing class sailboats shortly after the 1987 America’s Cup competition.