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Theory of Sail Shape

It should be pretty obvious how a yacht sails downwind. It is pushed along by the wind in the same way as tall ships have pushed across the oceans by the trade winds for centuries. How then is it that a yacht can sail with the wind on the beam or forward of the beam? To understand that we need a small understanding of physics. As you may well remember from school physics lessons 'nature abhors a vacuum'. We will examine how this applies to boats by first using the example from slightly faster and higher vehicles.

The wing of an airplane is shaped in a way that minimises drag and maximises lift. The wing creates lift because it is curved on the upper side but not so curved on the underside. This curve means that as air travels from the front of the wing to the back of the wing it has further to travel over the top of the wing than on the underside of the wing. Because the wind travelling over the wing needs to travel further is must travel more quickly. This is not because the air molecules cannot get left behind (they can), it is because a vacuum is created on the top rear end of the wing and the air must speed up in order to fill it. If the pilot would like to change the amount of lift that is generated this can be done by adjusting the flaps. The sail, or sails, on a sailing yacht produce lift in a similar manner (albeit that the lift pulls the boat horizontally rather than vertically) and have similar controls to adjust the shape of the sail.

So we know that the air is moving more quickly on the leeward side of the sail (i.e. the outside) than on the windward side (i.e. the inside). How does this help the yacht move forwards? To understand this we need to know that air that is moving more quickly exerts less pressure (a law known as Bernouilli's Principle). This means that there is less pressure on the outside of the sail. As a result lift is created perpendicular to the chord which creates a lot of sideways movement, a lot of heel and only a little forward motion.

Up until now we have been considering the mainsail in isolation. The good news for sailors who would like to move forwards rather than sideways is that the jib and the keel achieve this. Let's look at each of these in turn.

The jib is a vital component of a fast upwind setup because as air approaches the boat it slows and bends (an effect known as "upwash"). The jib can be set at a greater angle (e.g. 10 degrees) than the mainsail (e.g. 0 degrees) because it is operating in bent air. Because the jib can be set at a greater angle it generates more forwards lift and less heeling force. The jib further helps by changing the angle that the air hits the mainsail at by creating a "slot" between the windward side of the jib and the leewards side of the mainsail. This slot helps a small amount of air all the way across the leeward side of the mainsail to the leech. As the air approaches the slot it is slowed but then it accelerates as it passes through the slot. When viewed from above the curve of the jib and the mainsail should appear as one nice continuous curved foil.

The sum of the forces produced is still predominantly a heeling force and has only a small forward component. This is where the keel comes in (and things get really clever). The keel is able to generate lift perpendicular to the angle that the water hits it because the water hits it very slightly from leeward because of the leeway that the yacht is creating. Because the water does not hit the keel straight on it sees an asymmetrical keel profile. This asymmetry causes water to flow more slowly on the leeward side of the keel and more quickly on the windward side which in turn creates pressure, causing the yacht to be sucked forwards and to windward. Amazing!

It is only the keel then that helps us sail into the wind and even then this only works when we are passing through the water at sufficient speed to generate lift. This is why it is vital that we always concentrate on generating speed before we try to point.

The question then must be, how do we set our sails to generate the maximum power? There is one short answer to this and two long answers.

The short answer is that we can alter the angle of attack of the sails and the boat. If we trim our sails in then power will increase (until they stall) and if we let them out again power will reduce. The opposite effect is created by the angle of the boat. As it heads up power will reduce but when it bears away again the power will increase.

The two long answers are discussed here: