Archive for July, 2010

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%.