Hello!! today is the turn to talk a little bit about physics, and to explain how a windsurf sail works. While it is not necessary to be an engineer or physicist to practice this sport, it never hurts to know the reasons behind the things to understand their behavior.

The parts of the sail and their function

Parts of the Windsurf Sail

To start we will go to the basics, defining the parts of the sail and describing their function.


The luff is the area of ​​the sail from where the wind enters. Controlling its curvature with downhaul we can act on the sail profile and leech twist.


These elements are designed to keep the proper profile on the sail. One of the great dilemmas of design has always been to be able to establish a profile aerodynamically suitable, but at the same time flexible to allow the course changes and the maneuvers. The inclusion of battens and the more stiffer materials have allowed the evolution of the sails in a substantial way.


It is the area that remains above the imaginary line joining the clew with head. As we will see later, this is one of the most important areas of the sail. Its correct functioning allows the profile of the sail to adapt to the wind flow.


It is the area under the imaginary line that joins the clew with the tack. In speed oriented sails, the foot is designed to close the space between the sail and the board to maximize power. Wave or freestyle sails are designed with low bending on the foot to maximize maneuverability.

Adjustment points

  • Head – Of the three adjustment points of the sail, this is the one we leave fixed when rigging. Although some sails come with this point adjustable (vario-top), their function is to allow the use of several mast lengths and not to actively participate in the sail setting.
  • Tack – It is the set point with which we will control, on one hand the luff curvature, and on the other the leech twist. Both are very important things as we will see later.
  • Clew – It is the set point with which we will control the profile of the sail and its operation as an “airplane wing”.

Laminar flow and airplane wing effect

Without going deep into the study of fluid dynamics, certainly one of the most complex areas of mechanics. I need to define the concepts of laminar flow and turbulent flow.

Turbulent flow

The path defined by a particular fluid particle (in this case air) is called flow line. We will say that the flow is turbulent when it is irregular, chaotic and unpredictable. The particles move in a disorderly fashion and form small eddies. It is represented by disordered flow lines.

An example that I hope helps to clarify the concept, is the operation of the old galleon sails. The sail was oriented perpendicular to the wind and the drive was maximum but inefficient. They also have the big problem that it couldn’t be oriented towards the desired course, but only sail with the wind.

It is easy enough to visualize the wind bouncing off the surface of the sail in a disorderly way and escaping through the sides. Nowadays the sails, as long as they are well adjusted and well positioned with respect to the wind, don’t work like this. As we will see next.

Laminar flow

SustentaciónWe say that is a laminar flow when it is perfectly ordered, stratified and smooth, so that the fluid moves in parallel sheets without intermingling. It is represented by parallel flow lines that adapt to the surface of the solid element that are in its path, but without losing order. The best example to represent this is the wing of an airplane.

Aerodynamic profile

The design of an object to take full advantage of the forces generated by the variation of speed and pressure when this object is placed inside an air flow or another fluid, it’s called aerodynamic profile. The upper curvature of an aircraft wing complies with this principle. Because of this, the flow of air entering through the top is accelerated and the one incoming underneath brakes.

The Bernoulli Principle indicates that the internal pressure of the fluid decreases as the speed increases. Therefore, the pressure under the wing is greater than the pressure on top of the wing. This generates a lift force according to Newton’s 3rd Law. This in aeronautics is called just “lift”.

Empuje - ESIf instead of thinking about the wing of an airplane we think of a windsurfing sail, we see that the aerodynamic profile is similar and its functioning as well. The main difference is that the airplane uses engines to generate the air flow that reaches the wing while in windsurfing we depend on the wind, both in direction and intensity. In our sport, the wind passes through the aerodynamic profile of the sail with a greater speed on the leeward side than on the windward side. This generates a pushing force that makes the board to move.

You may be thinking that the sail shouldn’t be positioned that way. It’s true, now we go to details…..

Angle of attack

Angle of attackIt is time to define the concept of Angle of Attack, which is very important in aeronautics and also in windsurfing. In the previous section we saw how the Bernoulli Principle generates a lift force on the wing of the plane.

If we now turn the wing a little bit in the way indicated in the image, we help the aerodynamic profile of the wing by increasing the difference of speeds between the top and the bottom.

This is true up to a certain point. As the angle of attack increases, the flow is finding it more difficult to go trough the aerodynamic profile and begins a transition from laminar to turbulent.

When this happens, the differential pressure on both sides of the wing decreases and there is a risk of stall. This is because the lift force is not enough to keep the plane in the air. In the same way (although considerably less dangerous!!!), when the angle of attack on our windsurfing sail is very large, the drive force transmitted to the board decreases and we lose speed.

It’s studied that:

Angulo 1Up to an angle of 8 ° almost no turbulent flow is generated on the leeward side of the sail. The drive is greater than if the angle of attack is 0 °.



Angulo 2At an angle of about 12 degrees, the transition to turbulent flow starts on the leeward side but away from the luff, more toward the clew. Even more drive is generated.


Angulo 3With an angle around 16º to 18º, the transition to turbulent flow approaches the luff, the drive is maximum, but we get close to the so-called critical angle of attack, from which the drive begins to go down.



Angulo 4From 18º to 20º, the drive is already falling and we will begin to feel strongly the heel component of the pushing force, a clear symptom that a catapult is coming :-)


Apparent wind

So, if the angle between my sail and the wind to maximize drive is between 16º and 18º, why do those who make slalom or formula goes with the sail almost parallel to the board?
The answer to this, is that the angle must be measured with respect to the apparent wind and not with respect to the true wind. This means that while being quiet or at low speeds, the ideal angle of attack is the one mentioned before. But as we increase speed this angle varies according to the direction of the apparent wind.

Let’s look at it graphically with the image we used to illustrate the concept of laminar flow.

Now we know the concept of Angle of Attack so we adjust it to show us the ideal position to start moving with the board..

VelaESDrive & Heel

As we can see, the force generated isn’t in the course direction. It forms an angle equal to the attack angle with respect to it.
Therefore we will decompose this force into two vectors. One in the sense of course and we will call it Drive. And another in a perpendicular direction that we will call it Heel. For the purposes of this article we will forget about the Heel since enough matter for a specific analysis that we’ll do in another time.

Viento Aparente2ESAs we have our sail perfectly positioned, we starts to move and to get more and more speed, then we realize that the direction of the wind changes.

This happens because our own movement generates wind, the apparent wind. That apparent wind is the one that actually our sail recieves. It’s formed by the composition of the true wind vector and the board speed vector. This is one of the most important consepts to understand windsurfing.

Graphically it would be as shown in the image on the left. The red “apparent wind” vector is larger than the “real wind” vector. This explains why speed records in windsurfing are superior to the existing wind at the time of measurement. Also the heel grows and we must position correctly to counteract it (besides using a suitable fin).

The leech

To finish, we will see a very important thing for the correct functioning of the sail, the leech.

What is the leech for? Well, it has two main functions that designers use to a greater or lesser extent. Depending on the overall performance of the sail.

  • Leech twist helps to manage the gusts or the strong winds, making the sail more controllable.
  • The correct design of the leech prevents the sail from stalling at the top due to the wind gradient.

Baluma ESTo explain this we return to the apparent wind scheme of the previous image, but now we add another variable, the wind gradient, which increases with height. That’s to say, the wind our sail receives at boom height doesn’t have the same intensity that the wind received the head. Therefore, the apparent wind also changes. As much in intensity as in direction, as shown with the green arrow in the image.

To compensate this effect, the angle of attack of our sail at the top should be different from the angle of attack at the bottom. That could be achieved by twisting the leech.


  • The sail is an aerodynamic profile designed to maximize the drive generated by the wind passing through it.
  • The downhaul adjusts the luff curvature and the leech twist. Both elements are key in the design and efficiency of the sail. The manufacturer’s instructions must be followed as each part is related to the other. Sails with a flatter profile (freestyle for example), have a larger leech twist than those with more curved profile (slalom for example).
  • The outhaul adjusts the profile of the sail. For strong winds in general a flatter profile is required and for gentle winds more curved profile is needed.
  • Leech must twist according to design. Although it is possible to play with the downhaul, in general the modern sails are designed for a certain point of tension.

Very well, now you can participate in the talks about how to rig the sails and why this or why that. You can mention your friend Bernoulli (Dutch – 1700/1782) or your friend Newton (English – 1642/1727). Or even the Wright brothers if you want and you will surely have more than one laugh. :-)

I hope you enjoy it, see you soon and good winds!!

How a windsurf sail works
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