Note link just below if you don't get an answer. My home page magazine articles, lectures. Anders Ansar, Kungsholms Str. Modified September 2000. You are visitor number 1853 since May 1997.
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Let us say the direction of the wind is shifting plus minus 15 degrees from its mean direction. Assume both boats are sailing on starboard tack, in a 15 degrees lift. Now the wind shifts 10 degrees anti clockwise. This means that starboard tack still is favored with a 5 degrees lift. And they start to sail in a 5 degrees header.