Wax On, Wax Off?

May 24th, 2011

This may be silly because it questions some basic ideas from fluid mechanics.

The “no-slip” condition of fluid mechanics tells us that waxing a sailboat hull is a waste of time. Could it be that this condition is wrong? Perhaps so. In recent years, there have been some experiments suggesting that hydrophobic surfaces (like wax) manage to produce less drag.


The picture suggests that there might be something different than ordinary water near the hydrophobic (water repellent) surface. One possibility is that tiny bubbles of air or water vapor inhabit the nooks and crannies of the surface. This is consistent with the observation that the reduced drag appears to be smaller on atomically smooth surfaces. There are more radical theories that say water could adapt a quasi-layered structure near the surface.

Wax is not the most hydrophobic material. The reported experiments use compounds related to silane, which is methane with the carbon atom replaced by a silicon atom. In principle, a sailor should coat his boat’s hull with dimethyldichlorosilane to achieve maximum speed. This would probably be expensive.

Two of many references are: Physical Review E, Volume 80, article 060601 published in 2009, and Physics of Fluids, Volume 16, page 4635 published in 2004.

Faster than the wind

February 16th, 2011


All sailors know that it is impossible to sail faster than the wind when sailing downwind. This picture shows a violation of this well-known rule. How does it work? Power from the wheels, which brakes the car, is used to power the fan. The fan pushes the car ahead. If the wind is from behind, the push from the fan can be greater than the breaking force from the wheels.  Apparently the first person to think of this was a student, not a professor, not a famous sailor, not an engineer.

This car would not work on a sailboat. The drag from the water is too great. However, it should be possible to construct an iceboat which works on the same principle.

The Wind Is Dying

December 23rd, 2010

Wind speed has significantly decreased in the 29 years from 1979 to 2008. In extreme cases, the wind decrease was a significant 15%. More specifically, the wind decreased at 73% of measuring stations which were 10 meters above the surface (about mast height for many smaller sailboats). The measurements were mostly from Europe, but also from the United States, China and Russia.

This wind decrease is attributed to an increased average roughness of the earth’s surface. A large part of the roughness increase is attributed to additional vegetation (more trees). Reforestation, abandoned farms and increased carbon dioxide levels all lead to more tall trees. New tall buildings also contribute to surface roughness.

The significance of this for sailors is not clear, since sailors generally try to steer clear of tall trees and large buildings. The wind decrease may have no effect on wind energy, since a wind decrease at 50 to 100 meters (where wind turbines operate) has not been seen.

This is a summary of results reported by Robert Vautard and others at the Climate Science and Environment Laboratory in France and the European Centre for Medium-Range Forecasts in the UK. [Nature Geoscience 3, 756, 2010, various web articles and Physics Today, December 2010, page 25.]


August 6th, 2010

Normally, when sailing to windward one picks a sailing direction which maximizes the boat’s progress in the direction of the wind. However, in variable winds it is often better to bear away from the wind to increase speed at the expense of direction. The pictures give an exaggerated and oversimplified example.


In this first picture, the green boat on starboard is slightly ahead of the red boat on port. The green boat passes just in front of the red boat. Then there is a 45degee wind shift to the left.  The green boat tacks. In the final position at the top of the figure, the green boat lead has increased because it was sailing toward the wind shift.


In the second picture, the green boat anticipates the wind shift and sails more to the left. Although this forces the green boat to pass behind the red boat, the green boat’s lead has been increased by this course change.

Man Overboard

July 7th, 2010


According to Rudy Maxa (Savvy Traveler) sailboat racers in Sidney Australia (long ago) intentionally jettisoned crew members half way through the race. Because it makes physical sense, this strange story may even be true.

Weight slows a sailboat. Roughly, a 4% increase in total weight (boat plus crew) decreases the downwind speed by about 1%. On the other hand, crew weight is needed to balance the boat when sailing upwind. Sailors are faced with a weight dilemma because heavy crew is needed only half the time.

There is a way around this weight problem for races where the first half is upwind and the second half is downwind. One simply dumps the crew into the water at the windward mark. The crew takes on additional responsibility; swim to shore or get rescued.

Being thrown overboard after only half the race obviously requires dedicated crews who really love sailing. As “Blondie” (another sailing expert) said:

Man overboard, sinking in a sea of love.

Man overboard; he jumped, didn’t need a shove.

It is clear that a lighter boat will be faster. An estimate that a 4% weight increase leads to a 1% speed decrease is based on some simplifying physical assumptions. They are listerd here.

1) Water’s drag force on the hull opposes the driving force of the wind. The water’s drag is equal in magnitude to the wind’s force.

2) The drag is proportional to the submerged cross sectional area of the hull, which is nearly proportional to the total weight.

3) The drag force is also proportional to the square of the boat speed.

4) The total drag force would stay the same if the speed decreased by 2% at the same time that the weight increased by 4%. This follows because a 2% decrease in boat speed corresponds to a 4% decrease in the square of the boat speed.

5) For downwind sailing, slower boat speed means an increased apparent wind speed (with reference to the boat). Combining the decreased boat speed with the increased apparent wind speed means the boat must slow by only about half as much (1%) when its weight is increased by 4%.

Sail shape and mathematics

February 15th, 2010

If a sail is fastened at its foot and luff, the cut of the sail alone does not determined its shape. For this geometry, an “ideal” sail shape is generally not obtainable.

However, if one could fasten all points of the sail edge to a rigid structure, one could essentially determine the entire sail shape. The only requirement is that the sail must have positive curvature (be cup-shaped, not flat and not saddle-shaped) at each interior point.

Thanks to Warren Smith for pointing out this oversight.  The mathematics of this is based on work by Cauchy, Pogorelov and Alexandrov.

Paul Elvstrom and hiking

February 15th, 2010

I was incorrect in claiming that Paul Elvstrom invented hiking. However, he did enormously improve the technique and the hiking equipment. Thanks to James Harper for pointing out my error.


January 6th, 2010

The is an advertisement. I recommend reading the book “Physics of Sailing.” Since I wrote it, this recommendation may be prejudiced.


Left-handed sailors?

December 10th, 2009


A recent collection of signatures from members of the Saratoga Lake Sailing Club revealed an unusually large number of “lefties.” Is the a coincidence, a sign of a good sailor, or a characteristic of people living in obscure places?

Sailboat surfaces, a tricky problem

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.


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.