For reasons previously discussed in this chapter and for reporting
purposes, Quartermasters must be able to compute the direction and
velocity of the true wind. The following discussion contains instructions
for observing the wind speed and direction and computing true wind
data (speed, direction, gusts, and shifts).
The movement of the ship affects the wind speed observed by both the
ships anemometers and hand-held anemometer. Relative wind is
measured from the direction and speed from which the wind appears to
be blowing. Relative wind seldom coincides with true wind because the
direction and speed of the relative wind are affected by the ships
movement. For example, if your ship is heading north at 10 knots and
true wind is blowing from the south at 10 knots, there appears to be no
wind at all. In another example, your ship is heading north and the
wind appears to be blowing in on the port bow, but the true wind is
actually coming from the port quarter. In our discussion of the different
types of wind, refer to the following explanations:
1. True wind (TW) is the velocity and direction from which the true
wind is blowing.
2. Relative wind (RW) is the velocity and relative direction from which
the wind is blowing in relation to ships heading (SH).
3. Apparent wind (AW) is the velocity and true direction from which
the relative wind is blowing. For example, if your ship is heading
090° and the relative wind is blowing in on your starboard bow (045°) at
15 knots, the apparent wind is from 135°T at 15 knots. The formula for
apparent wind is: AW=RW+SH.
Wind speed (including gusts and squalls) is observed, computed, and
reported in nautical miles per hour (knots) to the nearest whole knot.
Since the true wind must be computed, the chance of committing an
error is increased. The wind data reported is used as criteria for wind,
storm, and high seas warnings. Care must be taken whenever computing
true wind. Wind data can be observed using the following methods
listed in order of preference:
1. Installed anemometer
2. Hand-held anemometer
3. Visual estimation