How the wind causes pressure on sails and why that’s important
You’ve probably heard the America’s Cup experts talking about the pressure being better over on the starboard lay line. In the language of sailors seeking to dazzle television viewers with their expertise, that means there’s more

Wind blowing over a sail causes differences in air pressure that provide forward drive for a sailboat.

wind over on the right. How does air pressure equate to wind and what does it mean to ordinary boaters? Well, it should mean something to sailboat owners because, simply put, wind creates areas of high and low pressure on sails, and that’s what makes them work when you are sailing across or obliquely into the wind.When I was a kid, I had no difficulty understanding that an airplane wing, which is simply a horizontal sail, tended to rise if you tilted it slightly upward in front. The air hitting the bottom of the wing simply forced it up like a wedge. But the further I progressed with science lessons, the more ridiculous my theory appeared. What was happening, my mentors explained with great patience, was that air molecules passing over the rounded top of the wing had a longer course to follow than those sliding along the straight bottom, so they had to move faster to meet up with the bottom molecules at the aft edge of the wing. If you speed up a stream of molecules, the pressure drops; therefore, with less pressure on top and greater pressure below, the wing was forced upward.Unfortunately, no sooner had I learned that theory by rote than they changed it again. These days, the theory of how a sail gains lift to windward is much more complicated.No matter. For the purposes of celestial navigation, most of us assume that the sun revolves around the Earth. We know it’s not true, but it brings the right results. The same applies to yacht sails: one side of a sail sucks (negative pressure), the other side shoves (positive pressure). It’s caused by air pressure that depends on the angle of the wind. That’s all you really need to know.See also Sails and “Lift.”

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