Wind Gradient - Sailing

Sailing

In sailing, wind gradient affects sailboats by presenting a different wind speed to the sail at different heights along the mast. The direction also varies with height, but sailors refer to this as "wind shear."

The mast head instruments indication of apparent wind speed and direction is different from what the sailor sees and feels near the surface. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind gradient can be factored into the selection of twist in the sail design, but this can be difficult to predict since the wind gradient may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.

According to one source, the wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface. This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters and with a statement by the Australian Government Bureau of Meterology according to which differences can be as little as 5% in unstable air.

In kitesurfing, the wind gradient is even more important, because the power kite is flown on 20-30m lines, and the kitesurfer can use the kite to jump off the water, bringing the kite to even greater heights above the sea surface.