Apparent wind is the
wind experienced by a moving object.
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.
The ''
velocity of the apparent wind'' is the
vector sum of the ''velocity of the headwind'' (which is the velocity a moving object would experience in still air) plus the ''velocity of the true wind''. The headwind is the
additive inverse
In mathematics, the additive inverse of a number is the number that, when added to , yields zero. This number is also known as the opposite (number), sign change, and negation. For a real number, it reverses its sign: the additive inverse (opp ...
of the object's velocity; therefore, the ''velocity of the apparent wind'' can also be defined as a vector sum of the ''velocity of the true wind'' minus the ''velocity of the object''.
Apparent wind in sailing
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. It is composed of the ''combined'' speeds and directions of the craft and wind observed by a ''stationary'' wind instrument—the ''true wind''. A true wind coming from the bow increases the apparent wind induced by the speed of the craft, coming from the stern it decreases apparent wind, and coming from the side the apparent wind angle and speed change according to the combined speed and direction of each the craft and the true wind. Apparent wind is important to sailors in order to set sail angle with respect to the wind and to anticipate how much power the wind will generate on a
point of sail
A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface.
The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. ...
. Apparent wind differs in speed and direction from the true wind that is experienced by a stationary observer and composed of the true wind speed (TWS) and true wind direction (TWD) or the TWS and true wind angle (TWA) relative to the boat if it were stationary.
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:
*
Leeway (or drift on power vessels) - it is very seldom that a craft is pointing in the direction it is going, and on a sailboat the angle of leeway is the difference between the heading of the craft 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 craft 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
multihull
A multihull is a boat or ship with more than one hull, whereas a vessel with a single hull is a monohull. The most common multihulls are catamarans (with two hulls), and trimarans (with three hulls). There are other types, with four or more h ...
s 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 craft, inertial effect on both the wind vane and the anemometer cups can be significant when the craft 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
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''.
Instruments
The apparent wind on board (a boat) is often quoted as a speed measured by a
masthead
Masthead may refer to:
* Nameplate (publishing), the banner name on the front page of a newspaper or periodical (UK "masthead")
* Masthead (American publishing), details of the owners, publisher, departments, officers, contributors and address d ...
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.
Implications on sailing speeds
In
sailboat racing
The sport of sailing involves a variety of competitive sailing formats that are sanctioned through various sailing federations and yacht clubs. Racing disciplines include matches within a fleet of sailing craft, between a pair thereof or among t ...
, and especially in
speed sailing, apparent wind is a vital factor, when determining the
points of sail
A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface.
The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. ...
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
multihull
A multihull is a boat or ship with more than one hull, whereas a vessel with a single hull is a monohull. The most common multihulls are catamarans (with two hulls), and trimarans (with three hulls). There are other types, with four or more h ...
s and some
planing monohulls.
Ice-sailors and
land-sailors also usually fall into this category, because of their relatively low amount of
drag
Drag or The Drag may refer to:
Places
* Drag, Norway, a village in Tysfjord municipality, Nordland, Norway
* ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania
* Drag (Austin, Texas), the portion of Guadalupe Street adj ...
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.
[TVNZ Live America's cup Broadcast. Interview with Tom Schnackenburg. 22/9/2013]
Other areas of relevance
In
fixed-wing aircraft
A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinc ...
, apparent wind is what is experienced on board, and it determines the necessary speeds for take-off and landing.
Aircraft carrier
An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...
s 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
Where:
*
= velocity (boat speed over water, 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
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determining how large or small a physical quantity is as compared ...
, the true wind velocity and direction can be calculated with:
and
''Note:'' Due to quadrant ambiguity, this equation for
is only valid when the apparent winds are coming from the
starboard
Port and starboard are nautical terms for watercraft and aircraft, referring respectively to the left and right sides of the vessel, when aboard and facing the bow (front).
Vessels with bilateral symmetry have left and right halves which are ...
direction (0° < ''β'' < 180°). For
port apparent winds (180° < ''β'' < 360° or 0° > ''β'' > -180°), the true pointing angle (''α'') has the opposite sign:
References
External links
* http://www.csgnetwork.com/twscorcalc.html
* https://www.tecepe.com.br/nav/inav_c11.htm.
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