Sailing is the skillful art of controlling the motion of a sailing ship or smaller boat, across a body of water. Sailing vessels are propelled by the force of the wind on sails. Today, for most people, sailing is recreation, an activity pursued for the joy of being on the water and pursuing the mastery of the skills needed to maneuver a sailboat in varying sea and wind conditions. Recreational sailing can be further divided into Racing, Cruising and "Daysailing."

Introduction

A boat moves as wind pushes on its sails. This is obvious when the boat is sailing downwind. The keel of a boat keeps it from strafing to the sides. This allows a boat to sail downwind but at an angle. The wind that blows on a sail is deflected off the sail, and that wind pushes against the sail too. The result is that the sail is not just pushed straight downwind, no matter which way it faces; but rather, it is pushed in the direction that it bows. This lets a boat pointed upwind, though at an angle and not straight upwind, to move forward as long as the sail is able to bow forward. If the sail is pointed just as the boat, then the boat will only be pushed sideways, yet if the sail is pointed into the wind, then it will not catch the wind at all. So, the sail must be pointed somewhere between.

• A boat moves forward with a resultant force proportional to the dot product of the forward vector of the boat and the force vector of the sail on the boat. The force of the sail on the boat is parallel to the average normal vector of the sail and is proportional to the dot product of the relative wind vector and the average normal vector of the sail. This proportionality is most obvious because depending on the angle of incidence of the wind on the sail, the effective area of the sail varies. So, a positive value for a resultant force on the boat requires that the angle of the sail be bound between the forward direction of the boat and the upwind source.

When sailing downwind, a boat catches up to the wind, so the wind cannot push as hard on the sails. When sailing at an angle to the wind, however, although the boat may move fast, it is not catching up to the wind, so the wind continues to push the boat faster. Of course, eventually the boat cannot move faster because of the water, but not before speeding along much faster than if it had been sailing directly downwind.

• The force on a sail is actually proportional to the relative velocity of the wind to the sail. Many factors complicate these generalities, however, such as the drag of the wind on the sails, the drag of moving through the water, the loss of wind force due to the boat heeling to a side so that the sails are pushed down and not just across the water.

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