Relative wind

Relative wind is the wind measured relative to the ship, the apparent wind expressed by its direction relative to the ship’s head, in points or degrees on the bow or quarter rather than as a true compass direction. A masthead anemometer reads relative wind speed and direction; the navigator combines that with the ship’s course and speed on a wind triangle to recover the true wind for the weather log. The apparent wind is the resultant of true wind and the ship’s motion, and the relative wind angle is its bearing off the bow. That angle is what matters for sail trim, for the safe sector for helicopter operations, and for working out the true wind reported in synoptic observations.