Sailing boats vary according to the manner in which the boats are rigged, but the basic principles of sailing are the same for all sailboats.

Sailing downwind, or "running" before the wind, is the simplest point of sail to understand. When running downwind, the sailboat moves in the same direction as the wind is blowing. The sails on the boat are set at an approximate angle of 90° to the lengthwise centreline of the boat. The sails simply derive their power from the push of the wind (the green vector in the illustration).

When sailing off the wind, or across the wind, the wind blows over the boat from the side, or "abeam". This point of sail is called a "beam reach". The sails are set at approximately 45° from the centreline of the boat for a beam reach.

Now, in this point of sail, something "weird" happens: The wind exerts a pulling, rather than a pushing force on the sails! The sails act more like the wings of an aeroplane than a "wind bucket", as on a run. The shape of the sail and the wind direction cause the sail to act as an airfoil.

On a reach, the wind tries to flow past the mast, and the angle of the sail allows it to do so. But the sail is set at an angle, causing the wind to flow at a higher speed along the forward surface of the sail, and slower along the curved surface at the back. This difference in airflows creates an area of low pressure ahead of the sail, pulling the sail in that direction. So, the actual force exerted by the wind on the boat is at approximately right angles to the sail (the blue vector in the illustration).

If the hull of the sailboat had been perfectly flat, the pulling force of the wind would tend to drive the boat at an oblique angle. But every sailing boat is equipped with at least one fixed keel or a retractable centreboard (or daggerboard), which acts as a longitudinal plane, to prevent the boat from drifting sideways through the water. The bow plane exerts an effective force at an angle to the opposite side of the boat's centre axis (the red vector in the illustration).

The speed and actual course the boat will sail (the green vector in the illustration, representing forward motion), will be the resultant vector of the combined force of the wind and the resisting force of the keel.

When sailing an upwind course, the sails also have an airfoil action. The wind blows past the mast and the sail, which will be set close to the centreline of the boat. The mainsheet will be hauled in tight, which is where the term "close-hauled" comes from.

The air passing in front of the sail will flow faster than the air passing over the back of the sail. This causes a low-pressure area in front of the sail, a little foreward from abeam. And the bow plane will exert an opposing force to the opposite side of the boat's centreline.

Note that the resultant motion of the boat (the green vector in the illustration) is much smaller than when the sailboat is sailing at a reach. But the motion will still be forward, as long as the sailboat does not try to sail inside the no-sail-zone, where the wind blows almost straight down the centreline of the boat.