Understanding the difference and importance of the concept of Real Wind and Apparent Wind is fundamental to sailing. For sailing, we use both mechanical and electronic instruments to make measurements of these parameters.
If we measure the wind speed and direction is mounted on land, then the readings obtained are those of the true wind speed and direction. If we take this same instrument and mount it on a boat moving through the water, then the reading will be very different from the one taken on land. These readings show the apparent wind speed and direction in relation to the boat. The term “relative to” is used because the apparent wind is applicable only to the particular boat we are looking at and is not the same for other boats sailing in the same area.
If the true wind is blowing at 20 knots from the south and the boat is travelling at 15 knots in a southerly direction, then the wind speed measured on the instrument will be the sum of the two speeds (35 knots) and the wind direction will be from the south. Similarly, if the boat is travelling at 15 knots north, the measured wind speed will be the difference between the two speeds (5 knots) coming from the south. It’s a matter of vectors.
This is a simple and easy to understand scheme:
Fig.1 The “Windex” indicates the apparent wind direction.
The situation gets a little more complicated if the boat is travelling at 15 knots east. What wind speed and direction will the instrument show? To arrive at this answer, we will have to resort to something called a vector. A vector is a line with an arrowhead. The length of the line represents the speed and the direction of the arrowhead shows the direction in which the wind or boat is moving.
There are some websites that help you to calculate the value of the real wind, depending on the apparent one.
The importance of windex
Fig. 2 A sailboat’s wind instrument (display and masthead transducer) . You can calculate the wind through the apparent wind direction and speed along with the actual boat speed.
Image Wind equipment: https://www.raymarine.com