Furuno WS-200 Operator's Manual page 20

There are two components to any wind measurement: speed and direction. By con-
vention, the wind direction is an angle representing the direction from which the
wind is blowing. Sometimes this angle is referenced to true or magnetic north, and
sometimes it is referenced to the bow of the vessel. Both true and apparent wind
use these same references.
Consider the case of a vessel proceeding at a speed of 15 knots in calm air. An
observer on board would experience a wind of 15 knots from dead ahead. This ap-
parent wind would be due solely to the motion of the boat. If a true wind of 15 knots
was blowing from the stern, an observer would experience dead calm—no appar-
ent wind. That is because the boat is moving at the same speed and in the same
direction as the surrounding air.
15 knots
true wind
ent wind speed and direction, the speed and course over ground of the vessel, the
compass heading, and the local magnetic variation. Note that heading and course
are not the same thing: heading is the direction the bow of the vessel is pointing,
while course is the direction the vessel is traveling. Heading and course may differ
due to the effects of wind and current.
Because true wind is calculated using the data from several sensors, its accuracy
depends on the accuracy of all the raw data used in the calculation. For instance,
if the electronic compass is located near iron or a similar magnetic disturbance, the
heading will be incorrect, and the true wind calculation will therefore be in error,
perhaps by quite a bit. In another example, the speed and course over ground pro-
vided by the GPS receiver are averaged over time. If the boat is performing ma-
neuvers, changing speed and/or direction, then it will take a few seconds for the
SOG and COG values to "catch up". The reported true wind values will therefore
also be incorrect until the vessel reaches a steady-state condition, traveling in a
straight line at a constant speed.
Air Temperature Sensor
The Weather Station instrument includes a built-in negative-temperature-coeffi-
cient thermistor that measures the ambient air temperature. This NTC thermistor is
located in a thermally isolated region of the Weather Station housing that is open
to the outside air.
15 knots
COG
21.2 knots
apparent wind
Now, consider the more complicated situ-
ation of a vessel proceeding at 15 knots
with a true wind of 15 knots blowing from
the side (see the figure at right). To an ob-
server on board, the apparent wind would
be 21.2 knots blowing from an angle 45º
off the bow.
In order to calculate the true wind speed
and direction when on board a moving
vessel, it is necessary to know the appar-
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