Relative versus True Winds
The wind that you measure on board a moving ship takes into account the true wind as well as the speed and direction the ship is moving. You therefore need to correct the relative wind you measured to determine the true wind speed and direction.
AG2 Ferguson: "If you're assigned to a smaller ship, you may need to use a wind wheel, or a wind computer, to compute true wind. A wind wheel consists of an oval base-plate and a clear plastic compass rose, fastened at a central pivot point. The wind wheel allows you to compute true wind versus the relative wind affected by the ship's motion. Directions for using the wind wheel are printed on the reverse side of the wind plate. When calculating true wind based off relative wind, you need to know the current relative wind and the ship's speed. For example, we have a ship speed and direction of 020 and 15 knots. So, we rotate the plastic inner wheel to 020, and we indicate on the white portion of the wheel the speed of the ship, which is 15 knots. So, we slide the center to 15 knots, ensuring that 020 is indicated on the due north portion of the white. Then we take our relative winds, which would be 090 at 20 knots. We come over to 090 and follow the line over to the indicator of 20 knots. Make an "X" or a dot, whichever you prefer, and then you take this grease pencil dot and center it at 000 on the wheel, and zero the wheel out. Wherever that "X" lies is your true wind. So, we have true wind at 153 at 25 knots. Because it's wind and we can't record it as 153, we would round that so that it ends in a zero, making it 150 at 25 knots."
Your anemometer indicates winds of 30 kts. The ship is currently moving at 20 kts into the wind. What is the true wind speed? Choose the best answer.
The correct answer is d).
The ship's speed into the winds will add 20 kts to the actual wind speed. So if the anemometer registers 30 kts, you need to subtract 20 kts to get the actual speed. Note that the conversion to true wind is only this easy when the ship is sailing directly into the wind. Some shipboard anemometers can automatically adjust the wind measurement to take the ship's motion into account.