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Apparent wind is the wind experienced by a moving object.
Definition of apparent wind
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 inverse of 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.
Apparent wind in sailing
In sailing, the apparent wind is the actual flow of air acting upon a sail. It is the wind as it appears to the sailor on a moving vessel. It differs in speed and direction from the true wind that is experienced by a stationary observer. In nautical terminology, these properties of the apparent wind are normally expressed in knots and degrees. On boats, apparent wind is measured (see "Instruments" below) or "felt on face / skin" if on a dinghy or looking at any telltales or wind indicators on board. True wind needs to be calculated or stop the boat.
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:
- Leeway (or drift on power vessels) - it is very seldom that a boat is pointing in the direction it is going, and on a sailboat the angle of leeway is the difference between the heading of the boat and its actual track through the water. This must be corrected for when converting apparent wind angle to true wind direction. The same effect is found when the boat is altering course.
- Mast twist - the rigging loads often put a significant amount of torsion on the mast, especially if the rig has runners, so it is twisted along its length
- Mast rotation - many racing multihulls have a mast that can be rotated, so the anemometer reading needs to be corrected by the angle of rotation of the mast
- Heel angle - this is a simple trigonometric correction
- Upwash from the sails - the airflow around the top of the mast is distorted by the presence of the sails. This effect varies with the sails set at the time, the wind speed and the point of sail, but is noticed by the true wind angle changing from port to starboard tack, and the true wind speed changing from when beating to running
- Boat motions - as the masthead is so distant from the centre of motion of the boat, inertial effect on both the wind vane and the anemometer cups can be significant when the boat is moving, especially when pitching and rolling
- Wind shear - there can be a significant change in both wind speed and direction between the water's surface and the top of the mast, especially in conditions of unstable, light airs. The wind instruments are just measuring conditions at the masthead, and these are not necessarily the same at all heights
Whilst in non-tidal waters it is valid to define the true wind as the wind when the vessel is stationary, but where there is a water flow (whether from tides or sailing on a river) this also has an effect on the wind experienced, and this is independent of the motion of the boat through the water. This is commonly factored out by defining the true wind as the wind experienced when the boat is drifting with the water (but moving with respect to the sea bed), and then defining the wind when the boat is stationary with respect to the sea bed as the ground or geographical wind.
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 velocity and direction are input.
Implications on sailing speeds
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.
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.
In the foiling AC72 America's cup catamarans, the boats sail through the water at up to double the environmental wind strength. 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. This means that AC72's are effectively tacking downwind, although at a greater angle than the normal 45-degree upwind angle, usually between 50 and 70 degrees.
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, as well as most mid-sized and large birds generally take off and land facing upwind for the same reason.
Calculating apparent velocity and angle
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- = velocity (boat speed over ground, always => 0)
- = true wind velocity (always => 0)
- = true pointing angle in degrees (0 = upwind, 180 = downwind)
- = apparent wind velocity (always => 0)
The above formula is derived from the Law of cosines and using .
The angle of apparent wind () can be calculated from the measured velocity of the boat and wind using the inverse cosine in degrees ()
If the velocity of the boat and the velocity and the angle of the apparent wind are known, for instance from a measurement, the true wind velocity and direction can be calculated with:
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:
- Thornton, Tim. The Offshore Yacht. Adlard Coles.
- Marchaj, C.A. The AeroHydrodynamics of Sailing. Adlard Coles.
- "Sailing Instruments Calibration". Ockam Instruments. Retrieved 10 June 2015.
- TVNZ Live America's cup Broadcast. Interview with Tom Schnackenburg. 22/9/2013