Sailing Upwind

THE SAIL AS AN AIRFOIL - SAILING UPWIND

As a boat sails into the oncoming wind, the sail splits and bends the airstream. As long as the boat does not turn too close to the direction of the wind (the eye of the wind) the sail can bend the flow of air towards the stern of the boat. The wind's energy is utilized when the flow of air is bent this way resulting in forward movementof theboat.

As the windstream separates at the leading edge of the sail, (the luff), the flow of air passes along both the windward and leeward sides. While the flow of air tries to go straight, it is forced to follow the contour of the sail. The wind blowing across the windward side produces a pushing force referred to as drive. The wind flowing across the leeward, (back side) tries to pull away from the surface of the sail creating a lower air pressure area along the back of the sail. This pulling effect is called lift. The lifting force accounts for as much as 70% of the sailboat's power when sailing to windward.

The jib in the illustration is used to increase the flow of air across the back side of the mainsail, helping to create even less pressure. This produces more lift, increasing the pulling force to windward. The use of the venturi princi- pal in this way on sailboats is called the slot effect.

A strong aerodynamic force is exerted in a sideways direction by the wind as shown in the illustration. The keel, situated under the boat, prevents the boat from moving sideways by creating a lateral resistance force. These two forces combine to create the resultant force which moves the boat in a forward direction. The interaction of forces is what propels the boat up wind.

The sails should be trimmed so as to create the maximum lifting force. (See Trim) If the sail is out too far, the flow of air will not be bent as much as possible. If the sail is in too far, the flow of air will break away from the back side of the sail. Either case will result in less than maximum lift and less than optimum performance for the boat.