Sailing on a Beam Reach

To understand the effect of apparent wind on a windsurfing board let us look at the case of a board that is sailing on a beam reach as shown in the diagrams below. They show in sequence how the apparent wind changes in speed and direction with different board speeds while the true wind TW remains constant at 20 knots at an angle of 90 degrees to the board.

In the first diagram the board is travelling at a speed of 15 knots. The interaction of board velocity BV and true wind produces an apparent wind, represented in speed and direction by the vector AW. Its speed is 25 knots and it is at an angle ^aw = 53 degrees.

Fig. 3

As the board speeds up, the apparent wind increases in speed and moves further forward. This is shown in the next diagram. Here the board has reached a speed of 20 knots. The apparent wind has now increased to 28 knots and is now at an angle ^aw = 45 deg.

Fig. 4

In the third diagram we assume that the speed of the board has gone up even more to reach a speed of 25 knots. The apparent wind speed has now reached 32 knots and its angle ^aw has gone down to 39 deg.

Fig. 5

What this shows is that as the board speed increases, the apparent wind speed also increases. With a higher apparent wind speed, more power is generated in the sail and this drives the board to a higher speed which in turn creates a higher apparent wind speed. Does this therefore mean that the board speed and apparent wind can go on increasing indefinitely? The answer is no. There are three factors which limit the speed of the board. The faster the board travels, the higher will be the drag of the board in the water and the higher the drag caused by the sailor's own wind resistance. More importantly, as the board speed increases, the angle of the apparent wind ^aw decreases. The sail has to be sheeted further and further in so that it maintains a correct trim in relation to the the angle of the apparent wind. This results in a smaller and smaller forward component of driving force on the sail. The speed of the board will reach a limit when the driving force on the sail will just be sufficient to overcome the sailor's wind resistance and the drag of the board in the water. In order to go any faster, we will have to increase the angle ^aw by going into a broad reach. In the next section we will see what happens when we do that.

Next: Broadreach

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Copyright 2000 Denis Wee. All rights reserved.