Apparent wind

Apparent wind is the wind experienced by a moving object.

Apparent wind in sailing

In sailing, the apparent wind is the actual flow of air acting upon a sail, or the wind as it appears to the sailor. It differs from the true or prevailing wind seen by a stationary observer in speed and direction.In nautical terminology, these properties of the "apparent wind" are expressed in knots and degrees.

Definition of apparent wind

The "Apparent wind" is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer.

"Apparent wind" velocity is the vector sum of the "true wind" and the headwind an object would experience in still air. The headwind velocity in still air is negative the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the "Velocity of the wind minus the Velocity of the object".

Calculating velocity and angle

$A\; =\; sqrt\{((W*cos(a)-V)^2\; +\; (W*sin(a))^2)\}$

Where:
* $V$ = velocity (boat speed over ground)
* $W$ = true wind velocity
* $a$ = true pointing angle in degrees (0 = upwind, 180 = downwind)
* $A$ = apparent wind velocity

The angle of apparent wind ($b$) can be deducted from the measured velocity of the boat and wind using the inverse cosine in degrees ($AcosD$)

$b\; =\; AcosD(((W*cos(a)-V)/sqrt\{((W*cos(a)-V)^2\; +\; (W*sin(a))^2)\})$

Instruments

The apparent wind on-board is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane that measures wind speed in knots and wind direction in degrees relative to the heading of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat velocity are input.

Implications on sailing speeds

In sailboat racing, and especially in speed sailing, apparent wind is a vitally important factor, when determining the points of sail a sail-boat can effectively travel in. A vessel travelling at increasing speed relative to the "prevailing wind" will encounter the wind driving the sail at a decreasing angle and increasing velocity. Eventually, the increased drag and diminished degree of efficiency of a sail at extremely low angles will cause a loss of accelerating force. This constitutes the main limitation to the speed of wind-driven vessels and vehicles.

Windsurfers and certain types of boats are able to sail faster than the true wind. These include fast multihulls and some planing monohulls. Ice-sailors and land-sailors also usually fall into this category, because of their relatively low amount of drag or friction.

Other areas of relevance

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

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