Marine Technologies Yacht Design

The sail as an airfoil: Sailing upwind.

Have you ever wondered how a sail boat can go against the wind, when it is the wind that propulses it? This seemingly incredible behaviour is created by what I call the olive bone effect(fig 1). When you press an olive bone between your thumb and forefinger, the bone will fly forward, even if the pressure you exert is mostly perpendicular to the direction of motion. The same applies to a sailing boat. it is caught between the following two forces.

1. The force on the sail, oriented at around 90° from the wind direction (this is not really true, as we’ll see later, but it is a useful asumption for the purpose of this tutorial)

2. The force on the keel, oriented at around 90° from the direction of the boat.

In fig 1, the drag on the keel is created by the forward movement of the boat. it will therefore be zero when the boat is stopped. The sail force then has a forward component that accelerates the boat until the drag becomes big enough to stop the acceleration. We then sail at a constant speed. In other words, an aerodynamic force is exerted in a sideways direction by the wind as shown in fig 1. The keel prevents the boat from moving sideways by creating a lateral resistance force. These two forces combine to create the resultant force which moves the boat in a forward direction. The interaction of forces is what propels the boat up wind.

The sail as an airfoil: Sailing upwind.

Fig 2 is a diagram of the way the sail generates the force. The aerodynamic force arises from the fact that as the airflow curves around the sail, it covers a longer distance on the downwind side then on the upwind side. This forces the air on the downwind side to go faster, thus reducing it’s pressure, according to Bernoulli’s equation. We then have a pressure differential which causes the sail, and therefore the boat, to be "sucked" to the right, in the diagram. The same principle applies to all foils, be they sails, keels, rudders, airplane wings,etc... Fig 3 shows the force developped by a keel or a wing in a fluid flow.

The diagram shows a boat with a single sail, a catboat, for simplification. But the same applies for boats with a jib. The jib and mainsail combination is actually more efficient, area for area than a single sail. This is because the jib "prebends" the wind for the mainsail, as it were. In other words, it increases the pressure on the downwind side of the mainsail, thus allowing to be sheeted in more without stalling, developping more power. The mainsail has a similar effect on the jib, in that it reduces the pressure on the upwind side of the jib.

Another way to look at this phenomenon is that the jib is used to increase the airflow across the downwind side of the mainsail, helping to create even less pressure. This produces more lift, increasing the force developped. The use of the Venturi principal in this way on sailboats is called the slot effect.

Fig 3 shows a keel in a water flow. The principle is exactly the same, and is even more similar to what applies to an aircraft wing, the foil being rigid.