Archive for the ‘Technical’ Category

Man Overboard

Wednesday, 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%.

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.


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.

Wakes and relativity

Thursday, October 22nd, 2009

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.

Capsize made easy

Wednesday, October 21st, 2009

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.

Air and Water

Thursday, August 6th, 2009

Air and water are the sailor’s fluids. (Alcohol doesn’t count.) Just two numbers determine the intrinsic properties of these fluids.

PROPERTY #1: The density is a measure of a mass of a given volume of the fluid. Water has a density of 1000kilograms per cubic meter. Water is about 800 times as dense as air.

Density is more precisely defined by;


PROPERTY #2: The kinematic viscity is a measure of fluid friction. Viscosity slows fluid motion. The larger the kinematic viscosity, the more quickly the motion stops. Air’s kinematic viscosity is about 15 times as large as that of water. Fill a hollow globe half way up with water and rotate it so the air and water inside are spinning. Stop the globe and the air’s friction will stop the air much faster than the water’s friction will stop the water. The magnitude of air’s friction is smaller, but its density is so small that it takes very little force to stop the spinning.