Physics of Motion: Fluid Dynamics of the Keel and Sail
There are two main parts to the physics of motion of sailboats: the keel and the sail.
Keel: The keel has two main functions
1. It acts as lateral resistance in the water against leeway movement.
2. Keeps the boat upright by countering the sideways force of the sail. The resistance of the keel against the sail's force is proportional to the square of the keel surface and the boat's speed; it also resists at a specific angle, which depends on the keel area to thickness ratio.
Sail: Force from the wind on the sails depends on the angle the wind hits and the trim of the sails. The basic fundamentals of sailing involve fluid dynamics and the flow of air across the sail (or water across the keel). When a sail is curved, there is more surface area on the outer edge that air must pass over. Because the same air stream is passing across the front and back of the sail, both parts must enter and exit the sail's surface at the same time, so the air across the larger surface speeds up. An increase in speed across a surface decreases the pressure, essentially 'pulling' the sail backwards with the pressure difference and making the sail fill out. This is the main driving force behind the movement of sailboats.