Archive for the ‘Uncategorized’ Category

Wax On, Wax Off?

Tuesday, 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.

Surface

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.

The Wind Is Dying

Thursday, 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.]

Book

Wednesday, January 6th, 2010

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

Boookcrop

Left-handed sailors?

Thursday, December 10th, 2009

NewTwoHands

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?

Wind shadow mystery

Tuesday, September 8th, 2009

Just as one can duck behind a building to avoid the wind, sails reduce the wind speed in the downwind direction. If one sailboat is to windward of another sailboat, the downwind boat must sail in a reduced wind. The boat on the right in the figure will be sailing in reduced wind.

Shadow2

Theory predicts that the wind speed reduction should decrease as the two-thirds power of the distance downwind. The theoretical ratio of the wind speed in the shadow to the true wind speed is shown below.

Shadow

This theory predicts a wind shadow with a long tail. It also predicts the same reduction ratio for light winds and strong winds.

Most sailors would not believe this theory. Traditional sailing lore says the wind shadow is more important in light winds, and the wind shadow becomes insignificant after about half a dozen boat lengths.

Perhaps the difference between theory and reality lies in the turbulent nature of the wind. A swirling wind would more effectively eliminate the wind shadow. If the wind’s relative turbulence increases with the wind speed, this would explain why the shadows are more important in light winds.

My other boat is a Sunfish

Tuesday, June 30th, 2009

BIG boat

Just a beginning

Thursday, June 25th, 2009

Soon topics of sailing physics will appear here. This site will augment the book “Physics of Sailing” which will be published soon.