Monday, March 03, 2008

The Physics of Sailing

Thanks to my Dad, I've been sailing almost yearly since I was a kid, but my understanding of how boats and sails work has always been somewhat, um ... operational. That is, I know how to keep the boat from tipping over in certain situations by doing certain things, and I know the general strategies for going in various directions (tacking upwind, etc.)

But, there is probably no sport where exploitation of the laws of physics is so central or so apparent, so I always wondered about the details of how it all works. The problem is just engineering-y enough that it doesn't really crop up in either physics undergrad or grad courses (at least the ones I took). I think this ignorance is somewhat general in the physics community since "How Boats Work" was the topic of at least one informal but heated "cookies" discussion in grad school (which is actually an excellent way to learn about stuff like this, I've found).

Anyway, Physics Today rides to the rescue with a fun and detailed review article by Byron Anderson entitled The Physics of Sailing. Part of this discussion, inevitably involves the physics of wings and lift generation and the various popular yet incorrect theories of lift. To wit:
"Like airplane wings, sails exploit Bernoulli's principle. An airplane wing is designed to cause the air moving over its top to move faster than the air moving along its undersurface. That results in lower pressure above the wing than below it. The pressure difference generates the lift provided by the wing. There is much discussion of whether the pressure difference arises entirely from the Bernoulli effect or partly from the wing's impact and redirection of the air. ... A NASA website has an excellent discussion of the various contributions to lift by an airplane wing. It disputes the conventional simple version of wing theory and emphasizes that lift is produced by the turning of the fluid flow."

But there is a lot more to sailing than sails, and the article goes into some depth on the physics of keel shape, turbulence, wave resistance, hull speed. Plus there are lots of fun little facts -- like the fact that maximum speed comes with the wind perpendicular to the direction of motion. Or that iceboats can reach speeds three times faster than the windspeed and hydrofoils can get up to twice windspeed. Awesome! Anyway, I thought it was a fun read.


LeahC said...

Hey! Physics of Sailing! My dad was a high school physics teacher for 25 years and recently retired and lives on his 40' sailboat. So two things you might be interested in, 1) he has a blog (who doesn't) which he has had for the past few years. My stepmom and him saild out of Chicago about 3 years ago and have made it as far south as St. Maarten

2) He wrote a book which I bet would address many questions you have : The Why Book of Sailing.

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Tim said...

Hey Leah - that's a cool blog. I am very impressed they have kept blogging while living on a boat for several years. And thanks for the tip on the book - sounds really interesting...