Why Sails Twist

This article originally appeared on windwing.com and is now available via the Internet Archive

Since much material is often subsequently lost to public view, we are placing a copy here for future use.

Why Sails Need Twist

There are three reasons for twist in a sail. In descending order of magnitude and importance they are: I - Increase the wind range II - Compensate for the wind 'gradient' III - Delay circulation-induced tip stall. Following are brief technical discussions of these items relative to sail twist.

Reason I - Increased Wind Range The aerodynamic force generated by a sail is proportional to the square of the wind velocity. The force is also directly proportional to the force coefficient (determined by shape and sheeting angle), the air's density and the sail's square area. The following formula defines these relationships: File:Plotforceequation.gif Allowing the head of the sail to 'twist off' in response to increased aerodynamic loading at higher wind velocities 'smooths' this non-linear response. The 'sheeting angle' and resulting force in the upper sections of the sail are thereby reduced allowing sailors to maintain control when sailing 'overpowered' or in gusty conditions. The following plot demonstrates this effect.

The total aerodynamic force may be split into two components - a lift component which is perpendicular to the flow and a drag component which is in the same direction of the flow. These components are both proportional to the 'sheeting' angle. As the sheeting angle increases, lift increases to a maximum which is reached at the critical 'stall' angle. Above the stall angle, a rapid and significant loss of lift results. The following plot illustrates the effect of 'sheeting' angle on the lift coefficient for a camberless sail. (A 'cambered' sail will have a non-zero positive coefficient of lift at zero sheeting angle due to its 'pre-inflated' mechanically induced shape.)

The relationship between the lift and drag components is a measure of sail efficiency. At zero sheeting angle, the sail still has a drag component due to the frontal area (shape) and 'wetted' surface (overall area.) As the sheeting angle and lift increase, the drag also increases. This additional drag is known as 'induced' drag. The following plot illustrates a typical L / D relationship:

Reason II - Wind Gradient or 'Shear' The wind increases logarithmically with altitude above the water. A compensatory 'twist' in the sail maintains an optimum spanwise angle of attack which improves efficiency and performance. Following are plots of the wind as a function of height above the water.

Windspeed vs. Height plots compliments of W. L. Kleb

Reason III - Circulation-Induced Tip Stall The discontinuity at the head (sail and then nothing) causes 3-dimensional circulation flow at the tip. A component of this flow is in a direction which serves to increase the angle of attack. At high (near critical) angles of attack, the circulation flow is increased resulting in tip stall. In aircraft, this phenomena is reduced by 'washout' (twist), a more stall resistant foil section, a winglet, or a combination of all three. In a sail, twist serves to decrease the angle of attack at the head thereby reducing tip stall.and improving efficiency and performance.