Apparent wind
[citation needed] In sailing, apparent wind is the speed and direction of wind indicated by a wind instrument (anemometer) on a moving craft (on water, land or ice) in undisturbed air.
[1] In nautical terminology, apparent wind is measured in knots and degrees.
Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following:[2][3][4] In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the ground or geographical wind.
[citation needed] The apparent wind on board (a boat) 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 speed and direction are input.
[citation needed] In sailboat racing, and especially in speed sailing, apparent wind is a vital factor, when determining the points of sail a sailboat can effectively travel in.
A vessel traveling 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.
[citation needed] 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.
[citation needed] The AC72 foiling catamarans used in the America's Cup are an example of this phenomenon, as the boats sail through the water at up to double the environmental wind speed.
The effect of this is to radically change the apparent wind direction when sailing "downwind".
In these boats the forward speed is so great that the apparent wind is always forward—at an angle that varies between 2 and 4 degrees to the wing sail.
[5] 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, as well as most mid-sized and large birds generally take off and land facing upwind for the same reason.
) can be calculated from the measured velocity of the boat and wind using the inverse cosine in degrees (
Note: Due to quadrant ambiguity, this equation for
is only valid when the apparent winds are coming from the starboard direction (0° < β < 180°).
For port apparent winds (180° < β < 360° or 0° > β > -180°), the true pointing angle (α) has the opposite sign:
