Wind – Apparent & True
When your boat is moving, the wind indicator (or your wetted thumb) shows ‘apparent’ wind. This means that the direction and strength of the wind includes your movement, i.e. if you are going downwind, the wind feels less, but if you are going into the wind, it feels stronger. True wind is the actual force of the wind if you were stationary. Try to give your True wind when talking to other boats.
Wind vectors: See diagram below.
Step by step: how to calculate true wind (diagram below):
- Draw a line representing your True course and speed, for example: course 000° T, speed 5 knots. Choose a scale to use, say 5 centimetres for 5 knots (one cm per knot).
- Place an arrow at the head of the line where your bow is pointing.
- Draw the apparent wind speed and direction, i.e. where the wind is blowing to, with the head of the arrow pointing at the stern of your course and speed line. In our example, that is approximately 040° off the ship’s bow at 10 knots, therefore the line would be ten centimetres long. (The drawing below is not to scale.)
- The True wind is the resultant vector drawn from the tail of the apparent wind line to the head of your vessel’s course and speed line.
- Measure the True wind line using the scale you have been using for the rest of the vector (in this example, measure how many centimetres the True wind line is, that will be your True wind speed in knots).
- Measure the angle of the True wind. You now have the True wind speed and direction.
Rules to follow
- Speed is always drawn to the same scale for each line of the vector.
- Direction is always drawn in the same type of bearing, e.g. True, Magnetic or Compass.
- True is best, then you can calculate the True wind (direction the vessel is going to and direction the wind is coming from).
- The head of the arrow must be placed at the head of the line, indicating direction.
- Apparent wind blows to your stern (on your course and speed line).
- True wind blows to your bow (on your course and speed arrow).