Sailing Basics

    Sailing is a simple application of basic physics. We're going to look at the two main concepts, buoyancy and lift.

    Buoyancy, by definition, is the upward force exerted by a liquid on any immersed object. If the force of the liquid on the object is greater than that of the object on the liquid then the object will float. In other words float is dependent upon the density of the liquid and the volume of the object submerged.

    Because no two objects can occupy the same space at the same time all objects displace fluid when placed in a liquid. Archimedes principle states that the magnitude of the buoyant force always equals the weight of the fluid displaced by the object.

    Buoyancy can be applied to a sailboat. The boat is considered the object. The combined mass of everything on the boat is less than the force of the fluid acting against it, allowing the boat to float.

    Now that we know how the boat stays afloat, let's look at how the sails actually work. Lift is produced when air is moving over one side of a surface faster than over the other. The faster moving air decreases pressure while pressure on the other side remains the same, creating a net force, and moving the object. Bernoulli's equation, when applied to an ideal fluid, specifies that, in laminar flow, the sum of pressure (P), kinetic energy (1/2 rv2) and gravitational potential energy (rgy) has the same value at all points along a streamline. Figure 1 better demonstrates the Bernoulli effect by applying it to the wing of an airplane.

Bernoulli's Equation P + (1/2)rv2 + rgy = constant

Figure 1. The Bernoulli effect on an airplane wing.

    As the figure 2 shows, the sail of a boat acts just like a vertical airplane wing (imagine the above diagram is a top view of a sail). The air moving around the outside of the sail is at a lower pressure than the air filling the sail, providing push for the boat. This same principle applies to the keel of a sailboat. The keel is attached beneath the boat and is usually heavy so as to counterbalance the "weight" the wind adds to the sails. The keel acts in the water just as the sails act in air; water rushing faster over one side creates a decrease in pressure producing movement, but in the opposite direction of the sail. A keel's main purpose is to keep the boat on course, therefore a boat won't just move in the direction the wind is blowing but in the direction desired.

Figure 2. Wind direction vs. Boat direction