Musings about science, technology, philosophy, religion, education.
Friday, November 12, 2010
Faster Than The Wind
I was wrong. A year ago, my future son-in-law asked me if a sailboat could travel faster than the wind. I said no. If the sailboat were travelling faster than the wind, then, to a person on the boat, the wind would seem to be coming from the opposite direction. This applies to downwind sailing, not to tacking against the wind. My mistake was going with my intuition, not my analysis.
I have studied many websites explaining how a sailboat can go (at an upwind angle) faster than the wind. Most explanations are flawed. Many quote Bernoulli’s principle, which isn’t an explanation. In the same way, physicists have stopped using Bernoulli’s principle to explain why planes can fly. (Otherwise, how could they fly upside down?)
Some websites contain text or comments from readers linking the claim that a wind-propelled craft, on land or water, can travel faster than the wind to a claim of perpetual motion. This, of course, is faulty thinking. Such a vehicle is getting its energy from the wind, and the wind is losing energy to the craft. No wind, no motion. Nothing magic about that.
But, for those still worried about what pushes a boat faster than the wind, here’s a simple explanation. First, what pushes a tacking sailboat forward against the wind? The answer is, the water. The wind pushes the sail, which is attached to the boat. The sail pushes the boat, which includes the keel. The keel pushes the water back (at a slight angle) and the water pushes the boat forward [Newton's 3rd, action and reaction.] Remove the keel (e.g.. raise the centerboard on a tiny sailboat) and the wind shoves the boat back, no matter which direction the boat is facing.
The faster a tacking sailboat travels, the greater the force of the wind on the sail, the greater the force of the keel on the water, the greater the forward force of the water on the boat. The limiting factor is not the speed of the wind but the friction of the water on the hull.
Going downwind faster than the wind (DWFTTW) is a little more anti-intuitive. A great blog entry with videos that demonstrates that an object can move faster than the object (and wind is millions of “objects”, air molecules) that propels it is http://dwfttw.blogspot.com/.