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1. boat velocity - 2. reciprocal of boat velocity- 3. true wind velocity- 4. Apparent Wind velocity

Apparent wind is the wind felt by an observer who is moving in the presence of wind.

The stationary observer sees the wind in the true direction from which it is coming. When the observer is moving in the presence of wind, the wind perceived by the observer is different from that seen by the observer when he is stationary. The moving observer sees the wind relative to his motion and this is in classical mechanics, the relative velocity of the wind with respect to the observer. This is in sailing terminology called the apparent wind, which is the only wind that the sailor sees.

The Apparent Wind is the vector sum of the True Wind Velocity minus the Boat Velocity'

For a stationary observer, both apparent wind and true wind are the same.

Apparent Wind velocity has a speed and direction in knots and degrees . True Wind velocity is the speed in knots and direction of the actual wind. The boat velocity also has speed and direction of motion components.

The Apparent Wind on-board is often quoted as a speed measured by a masthead transducer that shows wind speed in knots and wind direction in degrees relative to the bow of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat velocity are input.

In sailing, and especially in sailboat racing, apparent wind is a vitally important factor, when determining the Points of sail a sailboat can successfully travel in. Some boats are able to sail faster than the true wind. These include fast multi-hulls and some extreme monohulls. Land-sailors also usually fall into this category, since they tend to have far less drag than their water-borne peers.

In fixed-wing aircraft, apparent wind is what is experienced on-board and it determines the necessary speeds for takeoff and landing. Aircraft carriers generally steam directly upwind at maximum speed, in order to increase apparent wind and reduce the necessary takeoff velocity. Land-based airport traffic generally take off and land facing upwind for the same reason.