## The Angle of Attack

The angle of attack refers to the angle of the sail relative to the direction of the wind. In fact, there are two angles to talk about. One is the angle of the overall sail relative to the wind, and the other is the angle of the luff of the sail relative to the wind (Figure 15.7). The latter is self evident; the luff should be in line with the direction of the wind if the sail is trimmed properly. The former is defined as the angle of the chord of the sail relative to the wind. As either of these angles change, the effect the wind has on the sail changes, thereby changing the performance of the boat.

ANGLE OF ATTACK

Angle of attack of the luff of the sail.

Figure 15.7

There are two angles of attack to keep in mind when thinking about the way sails work.

Angle of attack of the luff of the sail.

SAIL FORCE—The sum of lift's drag

SAIL FORCE—The sum of lift's drag

Apparent Wind Direction if '

Figure 15.8

If a boat is accelerating, the lift forces that are moving the boat forward must be greater than the drag forces that are holding it back.

Figure 15.9

A simple representation of what happens to the wind when it is presented with a flat surface.