The Physics of Sailing

5 minutes

Hello! Welcome to this 5 minute explanation of how sailboats work. (By the way, this is from the Learn to Sail course here on the site.) Knowing how sails work will be your most valuable skill. If you understand sailing theory, the basics of actually sailing will feel surprisingly intuitive. And as you progress as a sailor, these concepts will help you sail faster and safer in any conditions.

a. Windward and Leeward

It’s time you learn your first sailing terms — windward and leeward (pronounced loo’ard).

The term windward simply means “towards the wind” and leeward means “away from the wind.” For example, the windward side of a boat is the side the wind hits first. The opposite side is “to leeward.”

These terms show up frequently, because the wind nearly always blows across a sailboat, rather than from directly ahead or behind.

The wind in this photo is blowing across the boat from right to left, so we’re seeing the windward side of the sail. Note the roundness of that red stripe.

b. Sail Shape

The science of sailing starts with the rounded, triangular shape of the sail. The sail’s frontmost edge connects with the mast and its bottom edge runs horizontally along the boom. The sail itself is cut with some roundness, which lets the sail billow just a bit.

c. Push and Lift

With a rounded sail, the resulting curvature allows for two types of power — push from the wind hitting the windward side of the sail and lift from the wind rushing along the leeward side.

Push from windward is intuitive. The wind hits the sail and pushes it in the same direction.

Lift is a little trickier to explain. Similar to airplane wings, sails create pressure differences in the air that pull the boat forward.

Here’s an example: with the boom (and thus, the sail, since they’re connected) at about 45º to the oncoming wind, the wind splits at the mast to the sail’s windward and leeward sides. The wind pushes against the windward side, which fills the sail and creates a wing-like curve. The leeward wind sticks to this curve (thanks to physics) and speeds up to keep up with its windward sibling. This results in a drop in pressure which creates a suction that pulls the boat to leeward*.

*For a more detailed explanation, look up Bernoulli’s Principle.

The wind splitting at the mast and curving along the sail’s leeward side.

Lift can’t happen without this curved sail shape, so a bit of wind should always be hitting the windward side of the sail to keep it full and round. If there’s not enough wind to fill the sail from the windward side, the sails won’t work. They’ll flap in the breeze like a flag.

d. The Keel

In our example, the combined power of push from windward and lift from leeward is not directed straight ahead, but at an angle. The keel, or vertical fin on the underside of the boat, resists the resulting sideways motion and guides the boat in a straight line. In short, the power from the sails plus the lateral resistance of the keel equals blissful forward motion.

Congratulations — that’s the basic science of sailing! It takes some sailors years to figure this out. Feel free to take a break. Then get ready for the good stuff.

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