Wind, Sails, and Lift
A sailboat is able to move as a result of the interaction between its sails and the wind. Exactly how this interaction occurs, however, is slightly more complicated than one might expect. First, one must understand the relevant parts that allow the boat to harvest the energy of the wind. The mast stands about as high as the length of the sailboat and holds the top point of both the mainsail and jib. The mainsail is the main generator of movement, while the jib adds some power but greatly improves the sailboat’s handling.
It is easy to imagine a sailboat sailing with the wind: the wind pushes against the sails and propels the boat forward. Since the wind is moving faster than the boat the sails cause the wind to decelerate. The sails push against the wind while the wind pushes against the sails, causing the boat to move forward. While this type of movement is fairly intuitive, it is a very limited utilization of a sailboat. While a boat sailing with the wind can never sail as fast as the wind, a boat sailing with the wind at an angle can match or even surpass the speed of the wind. The key to this is a phenomenon known as “lift” that results from differences in pressure.
You may have heard the term lift used when describing how an airplane works. For the purpose of understanding how a sailboat works it is useful to think of a sail as an airplane wing tilted on its side. Imagine a boat sailing into the wind at a 45° angle relative to the direction of the wind. As the wind moves past the boat, it passes the curved sail on either side. When the wind moves across the outside of the curved sail the air accelerates. In accordance with Bernoulli’s Principle, this higher velocity corresponds to a lower pressure exerted on the sail. This creates a difference between the lower pressure air on one side of the sail and the higher pressure air on the other. As the air tries to move from high pressure to low pressure it exerts a force on the sail; this force is known as lift. The diagram below illustrates this interaction in detail from an aerial perspective.
Lift Force on a Sail
This is the basis for how a sailboat harnesses the kinetic energy of wind to propel itself; the pressure differential causes the sail to experience lift and be pushed in a certain direction. It is important to note that this direction is typically perpendicular or at least at an angle to the direction of the wind. The boat ends up experiencing more of a “side force” from the wind than a forward force. Exactly how this side force is used to move the boat forward is discussed in the “Keel and Motion Vectors” section. What is important to understand is that the lift force is relatively constant when the boat sails perpendicular to the wind. This is why a boat sailing with the wind cannot sail as fast as a boat sailing with the wind coming from the side. When a sailboat is pushed by the wind it cannot reach the actual speed of the wind because at that point it would feel no “push” from the wind itself. In this case, an increase in speed corresponds to a decrease in the force of the wind on the sails. This is not true for a boat traveling perpendicular to the wind and explains why some boats can even sail nearly twice the speed of the wind that is present.