Physics of Sailing

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.

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.

It is a bit of a stretch to draw an analogy between a sailboat’s wake and special relativity. A picture of a wake and the space-time diagram of relatively can be drawn to appear similar. This is an amusing coincidence, not a deep insight.

The boat on the left produces a wake with a fixed angle. This angle does not depend on the speed of the sailboat. Fast or slow, the wake angle is the same. Outside this angle, there are no wake waves. If one only watched the water, the boat would not be noticed outside the wake.

The picture on the right shows that an event that happens at the point marked with the star cannot be observed until light from the event has time to get to an observer. Outside this “light cone” the event cannot be noticed. Just as the wake angle does not depend on the boat speed, the shape of the relativity diagram depends only on the speed of light. It does not depend on the speed of the object that produced the event, or the observer who detects it.

Of course there are differences. Light travels a lot faster than a wake.

In the final analysis, a capsize is a result of unbalanced torques. For small sailboats, the torque provided by the sailor’s position at the edge of the boat can provide the torque needed for stability. When a sailboat tips too far, the sailor’s torque is diminished as he is brought closer to the center of the boat. It takes a lot a practice to capsize as elegantly as this MC sailor.

When the wind is astern, the required restoring torque is small. The sailor can rest in comfort.

At this point, excessive heel means the sailor is contributing essentially no restoring torque.

This sailor’s final position clinging to the bottom might help a little. But it is too late.