Similar to steady flight, we've got to keep the air flowing over the sails smoothly. Anytime the angle of the wind across the lifting side of the sail becomes too great, the sail stalls. Wind molecules detach from the lifting side of the sail and turbulent flow is produced and thus reducing the lift.
The result is not as dramatic as in an airplane but you will get a noticeable loss in speed if your sail is producing turbulent flow of air when set at the wrong angle.
On this animation, click and drag on the slider bar. Watch the airflow as you increase the sail angle to the wind. At the beginning it is set to its optimum, but as you pull in the sail, watch the turbulent flow begin to spawn.
Figure 1.3-3 Turbulent Airflow Behind the Sail
To ensure smooth (laminar) flow of air over the sail and thus the greatest forward producing force on the sail we've got to make sure that we're positioning the sail at its optimum setting based on the wind direction relative to the boat.
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