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True Wind & Apparent Wind
If an instrument for measuring the speed and direction of the wind is mounted on shore then the readings
obtained are those of the speed and direction of the true wind. If we take
this same instrument, and mount it on a boat that is moving through the water,
then the readings will be quite different from those taken on shore. These
readings show the speed and direction of the apparent wind relative to the
boat. I have used the term "relative to" 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.
Let's look at it in greater detail. 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, i.e.
35 knots and the direction of the wind will be from the south.
Similarly, if the boat is travelling at 15 knots in a northerly direction, then the wind speed measured will be the
difference of the two speeds, i.e. 5 knots coming from the south. Most people will have no difficulty understanding
this. The situation gets a bit more complicated if the boat travels at 15 knots in an easterly direction. What wind
speed and direction will the instrument show? To arrive at this answer, we will
have to resort to using something called a vector. A vector is a line with an
arrowhead. It can be used to represent the speed and direction of anything you
like, whether it is the wind, a boat or a windsurfing board. 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.
In the diagram above, the vector TW represents the speed and direction of the true wind, BV represents the speed and direction of the boat and AW represents the speed and direction of the apparent wind. TW has a length representing 20 knots coming from the south. BV has a length representing 15 knots going to the east. AW is the result of something called vector addition and represents the apparent wind relative to the boat. Its length represents 25 knots. The angle between AW and BV, shown as ^aw, is 53 degrees. The apparent wind therefore has a speed of 25 knots coming from a direction 53 degrees south of east.
It is the apparent wind that acts on the sail, not the true wind, and that is
why it is so important to understand how it behaves. In the rest of this
article we will look at how the apparent wind changes in speed and direction as
the speed and direction of the board changes relative to the true wind.
Copyright 2000 Denis Wee. All rights reserved.