How Does a Sail Work?

Sailing With or Against the Wind

Sailing with Wind Astern versus Tacking
Sailing with wind astern--downwind--seems easy to understand: the wind fills the sails and pushes the boat forward, the same way that a heavy wind from behind pushes a person forward when they are walking. Sailing upwind is a bit trickier to understand, though. Sailboats accomplish this by sailing slightly off the wind at an angle of about 40 degrees to the direction of the wind, in a maneuver called "tacking."

Stick a hand out of the window of a moving vehicle with the palm tilted slightly upward, and it tries to fly. It does so because the air is being deflected down and a portion of the kinetic energy of the wind is transferred to the hand as a result of the wind's change of direction. This kinetic energy is then dissipated by lift: the hand moves upward. Likewise, the sail of a sailboat changes deflects the wind as the wind moves along the plane of the sail, and the kinetic energy of the wind is transferred to the sail as a result of the wind's change of direction, to be dissipated by the motion of the boat; the only difference between the hand's response to the wind and the sailboat's is the orientation of the events: the hand is moving in a vertical plane, while the sailboat is moving in a horizontal plane.

The Function of a Sailboat Keel
If the only difference between the "flying" hand and the sailboat is one of orientation; it would be easy to ask, "Why won't the sailboat move sideways?" The answer lies in two elements of the sailboat's design: the shape of the sailboat's hull and the keel, a relatively large, flat surface that protrudes into the water underneath the hull, parallel to the longitudinal axis of the boat.(see illustration). As the kinetic energy transferred to the sail tries to push the sail sideways, the keel largely prevents this, since the water pushes back against the keel and the weaker wind force, preventing the kinetic energy from being fully dissipated in motion to the side. Since the sailboat's hull offers less resistance to movement by moving forward rather than moving backward, the kinetic energy derived from the wind being deflected by the sail is dissipated in the sailboat's forward movement. This also explains why, under certain conditions, a sailboat tacking can move faster than the wind powering it.