PostHeaderIcon Wind Shear

refers to a change in wind direction with altitude. On a micro-scale, the angle of the boat relative to the wind (TWA) is different at deck level than at the masthead. When shear is present, boatspeed and heel angle will be lower on the tack in the direction of the shear (skinny tack), and higher on the tack away from the shear (fat tack). This will require you to set up your sails with different profiles on one tack versus the other. There could be up to 15 degrees difference in TWA from masthead to deck level along a 60 foot mast.

The presence of shear is more likely when there is a big temperature differential between the air and water—especially when it’s warm air over cold water. The warm, humid air flows in the gradient wind direction. The air in the boundary layer immediately above the water is colder and heavier, and thus more resistant to flow. It is this resistance that causes the difference in the wind’s direction–and often its velocity. As the water surface heats up throughout the day, the boundary air layer heats up and mixes with the gradient air mass, reducing shear. See also: wind gradient.

Shear diminishes the accuracy of visual wind clues on the water, and reduces the value of the information available from electronic instruments as their sensors are typically at the masthead. See also: left sheer and right sheer.

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