Twist

Any surface, no matter how smooth, presents an area of friction to the air passing by it, and water is no exception. When wind blows across a body of water the surface friction slows the speed of the air molecules closest to the surface leaving those particles higher off the water travelling at a higher velocity. Some scientists believe that over a height of 60 feet there can be as much as a 50-percent gain in wind speed (Figure 6.3). With this increase in speed comes a change in apparent wind angle, i.e., the angle of the wind relative to the orientation of the boat as it moves through the water (Figure 6.4). We will learn more about apparent wind speed and its effect on apparent wind angles in the chapter about sail trim, but for now this is a brief description of the difference between the two.

Headstay

Masthead Looking Down on Headsail

Headstay

Masthead Looking Down on Headsail

Apparent wind is pulled aft toward the top of the mast as true wind increases.

True Wind Speed at 10 knots

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