by Bill Gladstone

Understanding and controlling sail power is essential to optimizing upwind performance.

The Three Sources of Sail Power

1. Angle of Attack
2. Sail Depth (Draft)
3. Sail Twist

Total sail power is the sum of power from the three sources. Sail trimmers first try to get the boat to full power (neither underpowered or overpowered), and then adjust the mix of power from angle of attack, depth and twist to match the sailing conditions and optimize performance. When overpowered or underpowered, we work to decrease or increase power.

Sailmakers design and build sails that are both fast and can be adjusted to perform well in a range of conditions. As sail trimmers, our goals parallel those of the sailmaker: First, achieve the designed shape and second, fine tune the shape to the conditions. We’ll look more at fine tuning sail shape after we define each of the three sources of sail power, as well as some other characteristics of sail shape.

1. Angle of Attack

The first source of power is angle of attack. At zero angle of attack, the sail is luffing. Trim in to increase angle of attack and thus, to increase power. Ease the sails out, and power is reduced. Angle of attack is also controlled by the driver. Bear off to increase power, and head up to reduce power.

Power increases as angle of attack increases, up to the point of a stall. When angle of attack is too great, flow stalls and power drops quickly.

Trimming in, as shown (A-B), increases angle of attack and power. The driver can also increase angle of attack and power by bearing off (C-D). Ease sails or head up to reduce power


Go here to find out about the other two power sources from the North Sails website...