The physics of windsurfing

What do an airplane and a sail have in common? What do billiards and sailing have in common? Today, we will answer both of these questions.

Here a ball, there a ball

Let us remind you that, in the physical world, any action has an equal and opposite reaction. Newton’s third law guarantees that if you put pressure on something, it presses back.
When one ball hits another ball on a billiard table, it transfers part of its energy to the second ball, but at the same time, it also repels it and changes its own direction. That energy is transferred perpendicularly to the direction of motion of the ball.

The wind is air particles in motion, which we can imagine as millions of microscopic balls. When they hit the windsurfing sail, these air particles push it like balls on a pool table push each other.

The most productive way to push anything is at a 45-degree angle, as in the picture below, where the direction of "pushing" is indicated by the red arrow, and the yellow arrow indicates the wind movement when it collides with the sail.

Valerya Milovanova / Windy.app

So why does a board move forward when the wind blowing at a straight angle from the side of the board should seemingly move it sideways? This is why a board needs a keel — an underwater «fin.»

Water particles also resist when being pushed somewhere. In this case, they are pushed by the wide side of the keel, which gets its energy from the sail above the water. The water pushes the keel and the whole board in response, and the board does not drift in the direction of the wind.

Instead, the board moves forward because the water resistance is lower at the front of the board than in other directions. Because the keel has a sharp side at the front, the area of collision with the water is small.

The blue arrow indicates the wide side of the keel, where the area of water resistance is strong—the red arrow shows the sharp side, with minimal drag.

Not just Newton

You may have heard that a sail works like an airplane wing. Here lies another physical principle: the faster the particlesof water or air move, the lower the pressure in the flow, according to Bernoulli’s law.

This provides the lifting force for the wing. When hitting the wing, the airflow is split into two parts—above and below the wing. The shape of the wing, usually more convex at the top and flat at the bottom, makes the flow of air above move faster than the flow below.

Air consists of particles, so it moves in layers. You can see that the lines of motion of the layers above the wing become longer than those below it.

This is because of the law of conservation of mass: the volume of gas passing through an area must remain constant. But the wing forces the air above to travel further in the same amount of time than the air below the wing. In order for the law to hold, the air above is accelerated.

Because of the difference in velocity, the air at the top has a lower pressure than the air at the bottom—Bernoulli’s law kicks in. It turns out that the air from below pushes the wing upwards, where there is simply less air. That’s why airplanes fly.

What does this have to do with a sail? The profile of a sail from above is similar to the profile of an airplane wing from the side: the convex shape also makes the air separate, causing lift force "from below"—where the wind blows.

How to find the angle for a sail

Newton’s and Bernoulli’s laws complement each other to provide lift for wings and sails. But how does a windsurfer find the best angle for the sail, to go not only fast, but faster than the wind?

Any instructor or aerodynamic modeler will tell you that it is best to keep the sail at a 45-degree angle to the wind while it is blowing sideways, as in the picture in the first part of this article. The other direction, such as the wind directly behind the board, the «crosswind» desired by sailors, is not ideal as it can cause the sail to wobble.

But at higher speeds, athletes hold the sail at a sharper angle, which helps them more. Why is that?

The fact is that there are several winds. The true wind is the one that blows at you while you are standing still, and the artificial or headwind blows from the front of the board because the air is resisting your forward motion.

The faster you move, the more noticeable the headwind effect becomes. This is the same wind you feel in the palm of your hand when you stick your arm out of the window of a moving car.

The direction of the true and headwinds overlap, and together, they form the apparent wind, the direction of which is different from both winds. At high speeds, when the headwind is strong, the windsurfer adjusts the sail to the apparent wind.

Robert Stump / Unsplash

It turns out that first, the windsurfer needs to understand where the true wind is blowing from: this can be seen by instruments or the surface of the water, vegetation on the shore, and flags on the beach.

After catching 45 degrees to the true wind and speeding up, a windsurfer should turn in different directions and assess how the direction of the apparent wind changes. The true wind can change, too, so it is important to be flexible and experiment with different angles and courses.

Text: Jason Bright, a journalist, and a traveler

Cover photo: Ludomił Sawicki / Unsplash