Through observation, a sailor knows that a sailboat pointing directly into the wind, known as being in irons, will not move forward and its sails will be sagging. Turning to the opposite direction, a sailboat running directly downwind can never travel faster than the wind speed. By using a triangularly shaped sail, how can a sailboat move into wind or even travel faster than the apparent wind speed? Sailing enthusiasts have many theories, but many of these ideas fail to explain what is truly happening on a sailboat. We can look to Daniel Bernoulli and Isaac Newton for a little help.
Bernoulli's Effect - Bernoulli discovered that a fluid passing through a constriction experiences a change in it behavior, it accelerates. Some wing airfoils reflect Bernoulli's observations. The classic example is a wing airfoil that has a curved upper surface and a flat lower surface. As the wing airfoil passes through the air, that part of the airstream passing over the top of the wing accelerates, while the speed of the air passing underneath it remains constant. This differential in air speed creates a region of high pressure under the wing called lift. Many compare the shape of a sail to a wing airfoil and claim this is the secret of a sail's unique abilities.
Laminar Airflow - Sail performance depends on striking a proper balance of angle of attack to the wind direction. This is where our friend Bernoulli can help in this discussion and we can introduce Isaac Newton's contribution to sailing. Sailing depends on the sail's ability to change the direction of wind striking it. It is this change in direction that produces a force that can be used to move the sailboat through the water. As an airfoil passes through the air, the air traveling over the top of the shape accelerates and remains in close contact with airfoil's surface until the angle of attack becomes too sharp. In an airplane, this is a stall. Laminar airflow describes this uninterrupted flow of air next to the surface of the airfoil. Air will follow the curved shape of an airfoil due to Coanda effect. Why is this important? As long as airflow is laminar or in contact with the airfoil surface, it will continue to be turned in the same direction of the airfoil's shape. This ensures the change in wind direction needed to drive the boat forward.
Newton's First and Second Laws - Isaac Newton identified two ways that sails move a vessel through the water. He described force in the formula: F=ma. Force is equal to mass times acceleration. Air is quite heavy at sea level (~10 ton per square meter) and following Newton's formula, you can see that accelerating an air mass over the curved surface of a sail will increase the force acting on a sailboat. Isaac also tells us that for every action, there is an equal and opposite reaction. Going back to the discussion of angle of attack, wind striking a sail turned across it is deflected and forced in a new direction. This is known as Newtonian lift and even a flat surface can produce it. If you have ever watched an airplane fly upside down, this is how it is accomplished.
A Keel is Needed - If force vectors developed through accelerating air traveling across a sail and those caused by deflection are greater than the oncoming wind, the sailboat will move forward into the wind, if it is prevented from moving sideways in the water. Sailboats use their centerline and keel to produce a counterbalance to the heeling force of the sails that keeps the boat moving forward into the wind. Without a keel, wind striking the sail would only move the sailboat downwind. As with the sail, there will be an optimum angle of attack for the keel as well.
Sails drive boat performance through use of physics and aerodynamics. Sailing force is provided by accelerating wind over a sail, changing its direction and use of a keel. The beauty of sailing is that there is always more to learn about this avocation and ways to become a better sailor.
