Sailing Theory – Understand How a Sail Works
A sail can be likened to a wing in the way it works. When a wing moves forward, some air passes below the wing and some above. Due to a phenomenon known as the Coanda effect, air will tend to follow an adjacent surface that curves away from the flow as long as the curvature of the surface is not too great. When moving air changes direction, a force is generated.
A wing has a relatively flat bottom face, and a more rounded top surface. Since the wing has different shapes along its top and bottom faces, the air has to travel different distances, and thus at different speeds, across these faces. The faster moving air across the top face causes a region of low pressure, creating the lift that the wing needs.
A sail works in a similar fashion. As wind enters the front of the sail, it is split, with some passing along the windward side of the sail, and some to the leeward side. The wind passing to the leeward side is forced to travel a longer distance, and therefore has to travel faster, creating a low pressure region.
Similarly to the lift created in a wing, the low pressure created by changing the direction of the wind causes a force to be exerted on the sail. It is this force which is used to move the boat.
However, to utilize the force of the wind most efficiently, the sail has to harness the wind’s power efficiently. And to accomplish this wind has to deviate in direction over a sail’s surface as smoothly as possible. To generate the lift required, wind passing over both sides of the sail has to follow the curved profile of the sail surface. This is achieved with the correct amount of curve in the sail, and having the correct angle of the sail to the breeze.
To get the most amount of force moving the boat forwards, you need to deflect as much wind as possible around the sail.
Upwind sailing can be a real challenge, and is an aspect of sailing that takes a lot of practice and patience to develop. Some may initially find it a little difficult to grasp the concept of upwind sailing, but with the aid of vectors the process can be explained a little easier.
When wind enters the sail, it is forced to curve around the belly of the sail. This curve in the sail can be represented by a force acting at 90° to the sail. This force is made up of 2 components – 1 acting sideways on the boat, and another pushing the boat forwards. By using a fin, we minimize the amount of sideways slippage, and maximize forward motion.
The more you pull the sail in, the smaller angle will become, which will result in a smaller force pushing you forwards. The smaller the forward force pushing the boat, the slower the boat goes. Conversely, the more you let the sail out (basically up until the point before it starts to flap in the breeze), the greater the force forwards, and the faster the boat can potentially go.
The fin acts in a similar fashion to the tyres on a car. They both minimize sideways movement and allow easy forward movement. For example, if you push a car on an angle, it will resist moving diagonally, and instead will only move in a forward direction.
For this reason the fin should be all the way down to minimize sideways slippage.
Reaching is a comfortable and enjoyable angle of sailing for many sailors. In the right conditions and with a good setup you can get a dinghy up on the plane and moving across the water quickly.
Reaching is basically an extension of upwind sailing. The wind is coming from roughly 90° to the boat, the sail is eased out to create a nice flow of air over both sides of the sail, meaning that the forwards force is increased, and hence the boat can potentially go faster.
Due to the fact that the sideways force is now smaller relative to the forwards force, sideways slippage will be reduced. Some sailors opt to raise the fin about 1/2 way to create less drag through the water, and hence go faster.
Sailing downwind or running is basically when you are sailing in the same direction as the wind is blowing. The wind is coming from behind the boat, the sail is eased almost all the way out, meaning that the forwards force is maximized. One problem with downwind sailing is that, since you are traveling with the wind, the wind across the deck, and hence the wind that is being caught by the sail, is less. Another issue is that since all the forces are (almost) in alignment, the boat can tend to become unbalanced, and it can roll over on top of you.
Due to the fact that the sideways force is now minimal relative to the forwards force, sideways slippage will be reduced even more. Some sailors raise the fin as high as possible without interfering with the boom to create less drag through the water, and hence go faster.