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post #1 of Old 10-12-2000 Thread Starter
Mark Matthews
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Principles of Sailing

I know how to sail, but I don't know many of the actual rules or principles. Can you help?


Mark Matthews responds:

There is a lot of mystery over how sailboats work since the medium they use, the wind, isn't directly observable. Basically a sail is nothing more than a vertical airplane wing, or foil, that can be adjusted. Sailboats are actually pulled through the water on most points of sail by a force known as lift. A sailboat can't sail directly into the wind. If you point a sail directly into the wind, it flaps around-much like a flag on a flag pole will. If you head the boat away from the direction of the wind by about 45 degrees, however, and sheet or pull the sails all the way in, the boat starts moving. What is happening is that the air molecules are divided up by the sail, which is foil shaped. Air flows a greater distance on the outside of the sail than it does on the inside of the sail. The air on the outside of the sail accelerates to meet up with the air on the inside of the sail. This creates a difference in air pressure which causes lift and pulls the boat forward. Interestingly, similar forces are at work beneath the water as well. The keel or centerboard is like a sail in the water, and while it may also provide ballast for the boat, it converts the forces at work into forward momentum. Without the underwater resistance, the boat would just go sideways.

So how does a sailboat travel to a point directly up wind? Basically, it takes a lot of tacks as it zigs and zags its way upwind. A general guideline for sail trim is that when pointed as close to the wind as possible, the sails are trimmed in all the way. When going dead downwind, or with the wind behind, the sails are let all the way out. On this point of sail, the boat is no longer being pulled by the wind, but is actually being pushed. When the wind is perpendicular to the boat, the sails are half-way in, half-way out.

I'd refer you to some other articles on our site which will be able to provide you with more information. Look, for example, in our Learning to Sail Collection—specifically I'd recommend taking a look at Steve Colgate's articles.

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