The Science of Sailing

Sailing is a sport that requires a lot of experience in dealing with boats and the more time you have spent offshore, the more confident you will feel. It is no big deal to feel a little bewildered when you first set foot on a boat – most people hear a complicated introduction into the theory of sailing, get intimidated and forget that sailing is mostly for having fun. Don’t make that mistake. Focus on the essential theory, bring your gear and then try to start “learning by doing” in small steps. This is also the reason why we have tried to keep this “science of sailing” section as concise as possible.

What allows a sailing boat to translate wind into a forward drive? Obviously, the sails catch part of the wind, send the energy down to the hull and push the vessel forward. However, if you want a more fundamental understanding of how boats manage to move on the water, you will have to take a closer look. There are essentially three forces that act on a sailing boat in the act of moving forward:

1.) The driving force: This is caused by the wind flowing across the sail, mostly the main sail and the spinnaker. To make good use of this force, it is important to keep the sail at a small angle to the wind – this is why constant trimming is crucial for optimizing the driving force. The angle requires simple adjustments.

2.) Sideways force: As the force that the wind presses into the sail does spread not only into the forward-direction (driving force), but partly also off the forward-direction, sideways forces are generated. The faster the boat is sailing, the smaller the sideways force. It becomes more apparent if the sail is pulled in too far. If you are sailing on upwind courses, you will always have to consider the effects of the sideways force – the drift off your steered course due to the sideways force is called leeway.

3.) Heeling force: The force that the wind presses into the sail causes this third sub-force, acting upon the hull to heel the boat. Quite mean, this one. Resistance against the heeling force is primarily provided by the keel and by the weight of the load. Practically speaking, this means that the people on the boat will often have to lean out in order to balance the heeling force during challenging upwind maneuvers.

It is important to know the three forces in order to be in a position to balance them out. As you will see – and quickly learn to feel faster than rationally understand – you will have to find the ideal trim for your sails in order to maximize the driving force with minimized sideways and heeling force. To do that, let the sail out so that it flaps in the wind. Then pull it back in until it just – and only just – stops to flap and generate the perfect drive. If you are a beginner, do this over and over again. You will notice how quickly you will get a feeling for the right trim and soon you will do it without thinking.

Let’s now concentrate for a second on the wind as such. There are ways in the nautical sense that describe relative terms of wind. If you stand still – in a boat or standing on solid ground – and you feel wind, it is called “true wind”. Naturally, as soon as we move, we will feel airflow. The sum of the true wind and the wind experienced through motion is called “apparent wind”. But how does wind – or rather airflow around a sail – push a boat forward?

Some physics to cover the basics

Sailors distinguish between several different modes in which a sailing vessel can move with respect to the wind. The most straight-forward mode is “downwind”, in which the boat moves in parallel to the wind direction. The wind blows directly into the sails and – if faster than the boat – gets caught. The force is then transmitted into the hull and the boat is pushed forward. This prohibits the boat from sailing faster than the wind.

Sailing in the opposite direction is not possible: if the wind hits the sails directly from the front, the boat will stop catching it, slow down and eventually stand still. Boats can sail at approximately 40 degrees to the wind at best; if you want to sail upwind, you will have to tack, that is, go in zigzag lines. Many sailors find this challenging and exciting.

If you sail close to the wind – and it is likely that you will do this often – you will use the shape of the sails themselves to generate airflow and thus the energy that pushes the boat forwards. Under these circumstances, the wind doesn’t even need to be faster than the boat – the sail acts essentially like the wing of an airplane. When the wind flows around a sail, it splits at the edge and partly streams along a concave surface, partly a convex one. The air that goes along the convex part of the sail chose the longer way and therefore, has to move faster. This causes its pressure to decrease, which “sucks” the sail to the leeward direction.

Translating wind into velocity

The forces occurring around the mast are divided into the driving force forward and the sideways force sideward. Speaking in physics-talk, these two forces can be expressed as vectors that sum up to a third force that we can actually see and experience on a boat: the total force, which moves the boat. The movement occurs mostly forward, but in theory, the force should cause the boat to drift strongly sideways. This is minimized through the use of the keel, and, to a lesser degree, of well-balanced buoyancy tanks.

Now you should understand how the three forces we discussed above come into play – driving, sideways and heeling force. Every boat has a center of effort, a spot at which all forces act. This spot is most likely somewhere in the mast, but shouldn’t bother us anymore. Note that lots of very large boats can sail faster than the wind; they use relative wind speeds (relative to the moving boat, that is) that occur when sailing at an angle to the wind. In that case, the friction between the hull and the water limits the speed of the boat.

That much about the theory of sailing. However, the more important part is the practice – once you have routine in your moves, twists and turns, you will notice that you won’t think a lot about vectors, forces and angles anymore.

Further Reading

Back to "sailing"

Wikipedia on the physics of sailing