Apparent Wind

The wind velocity experienced on a moving vessel, resulting from the actual wind combined with the vessel’s motion.

It is a vector quantity with both speed and direction. A masthead instrument measures apparent wind relative to the moving vessel; true-wind information is calculated by combining that measurement with vessel motion and, depending on the system, heading, speed through water or speed over ground.

Sails respond directly to apparent wind, so trim and apparent wind angle change as the vessel accelerates, slows or alters course. Apparent wind commonly moves forward and increases with speed when sailing across or towards the wind, but its behaviour depends on the full vector relationship rather than a universal rule.