How a Sail Works

Sails convert the energy in the wind into forward movement of the boat.  They do this in the same way as wings of an aeroplane provide uplift.  The wind causes the sail to take a wing shape.  

 The wind passes around the sail and because the distance is greater on the leeward side of the sail, the wind must travel faster.  This means the pressure on the leeward side is lower than the pressure on the windward side.  

 This pressure difference results in a force acting perpendicular to the boom in the downwind direction.  This force can be resolved as a force on the boat pushing it forward and a force on the boat pushing it sideways.  The sideways force results in drift but is (for the most part) avoided by using a centreboard / daggerboard or Keel

 A sail has a particular angle to the wind at which it is most efficient.  This is usually 45 degrees, which means that boats can travel as close as 45 degrees to the wind.

 It is across wind that boats travel fastest, anywhere between 60 and 120 degrees from the direction of the wind. When moving in a downwind direction the sail no longer acts like a wing, but more like a parachute, catching the wind and moving the boat along with the wind.

What does a Keel do?

The keel or centerboard keeps the boat from being pushed sideways by the wind. The resistance from the hull and the keel translate the hydrodynamic lifting force to forward motion.

If you did not have a keel or centerboard the boat would simply be blown across the water like a balloon blows across the water. However, with a well-shaped hydrodynamic keel, the boat will sail mainly forward while sliding slightly to leeward (away from the direction the wind is blowing).

The force works on the keel like a thumb pressing on an orange seed. If you put an orange seed on a hard surface, like a table and place your thumb on the right point and push down, it will 'squirt' forward. The keel, a similar shape to the orange seed, does the same thing when a sideways force is placed on it and against the pressure of the water.

HOW TO SAIL -An overview

General Definitions

Leeward/Windward : The windward side of anything is the side from which the wind is coming from.  The leeward side is the side in shelter.  Also used as directions, e.g."another boat to windward".

Steerage : The ability to control the direction of your boat.  You need to have some forward movement for the rudder to have any effect on the direction of the boat.  Therefore, adequate steerage is really adequate speed for steering.

Points of Sail : The direction of the boat with respect to the wind.  The point of sail determines the sail and centerboard positions.  See a better explanation .

Tack : As in "starboard tack" and "port tack".  When you're sailing across the wind, the wind will be coming over one side of the boat.  If it comes over the starboard side, it's a starboard tack.  When running (with the wind coming over the stern) your tack is the side that the sail isn't on.  Very important for right-of-way rules.  See also a note on the word "tack"below.

A Note on the Word "Tack" : There are at least four meanings for the word "tack".

1. Tack: The side of the boat which the wind is coming over.  A starboard tack or a port tack.
2. Tack: A corner of a sail. The lower corner of the luff, the leading edge.
3. Tack: To turn the boat into the wind so that you change from one tack to another (e.g. change from starboard tack to port tack).
4. Tack: To get upwind you sail as close as you can to the wind (you're on a beat) and then keep changing direction and sailing close to the wind until you get to where you want to go.  You make a zig zag course.  This is known as tacking - you tack upwind.

Spinnaker : A large parachute type sail at the bow.  Used only when on a run.

Genoa : A large overlapping foresail which takes the place of the jib. Can be furled without taking it down to reduce sail.

Beat : A point of sail .  Also called "Close-Hauled".  When a boat is going as close to the wind as it can it is on a 'beat' or sailing to windward.

Run : A point of sail .  Heading downwind.  Boats travel faster on a run than on any other point of sail.

Sideways Drift : When a boat is crossing the wind at any angle it will experience a force transverse to the direction of travel.  This force tends to push the boat sideways. This is sideways drift.

Abeam : Across the boat.