WIND MEASUREMENT
Consists of determining the direction from which the wind is blowing and the speed of the wind. Wind direction is measured by a wind vane, and wind speed by an anemometer.
A wind vane is a device pivoted on a vertical shaft with more surface area on one side of the pivot than on the other, so that the wind exerts more force on one side, causing the smaller end to point into the wind.
A anemometer consists of cups mounted on short horizontal arms attached to a longer vertical shaft which rotates as the wind blows against the cup.
TRUE AND APPARENT WIND
You can use a Maneuvering Board to determine both your speed and direction of the true wind by means of the speed triangle.
Apparent wind is the force and the relative direction from which the wind blows, as measured aboard a moving vessel. It can also be expressed as a true direction.
In this triangle, the vector (er) represents the course and speed of the ship, the vector (ew) the direction and speed of the relative or apparent wind, and the vector (ew) is the direction and speed of the true wind. The vector (er) is plotted first, the vector (rw) is then plotted from (r) in the direction the apparent wind is blowing, the length of (rw) represent the speed of the apparent wind. The third vector (ew) represents the direction and speed of the true wind. True wind is the force and true direction from which the wind blows, as measured at a fixed point on the earth.
Here is a example, your ship is underway, on course 030°, speed 15 knots, and the true direction of the apparent wind is 062° at 20 knots.
Draw the speed triangle as using a scale of 2: l.
The vector (er) represents your course and speed. From (r) plot the relative speed vector (rw) in the direction of 242° (the apparent wind direction, 062° plus 180°), and to a length representing 20 knots, this is labeled (w) join (e) and (w) this vector, (ew) represents the true wind direction, from 109.5°, and its speed, 10.8 knots.
