Airmar PB200 Owner's Manual & Installation Instructions - page 22
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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
apparent 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
apparent wind. That is because the boat is moving at the same speed and in the
same direction as the surrounding air.
Now, consider the more complicated situation of a vessel proceeding at 15 knots
with a true wind of 15 knots blowing from the side (see Figure 12). To an observer
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 apparent 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. The WeatherStation Instrument can provide true wind speed and
direction only if all of the data is available. The speed and course over ground
must be provided by a GPS receiver––either built-in or networked. The heading
may be provided by either the built-in electronic compass or by an external
networked compass.
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 provided by the GPS receiver are averaged over time. If the boat is
performing maneuvers, 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.
Figure 12. Apparent wind
15 knots
15 knots
true wind
COG
21.2 knots
apparent wind
Copyright © 2007 Airmar Technology Corp.