Sailing on a Broad Reach

If we start bearing away from the beam reach to go onto a broad reach what happens to the apparent wind is shown in the next three diagrams. The true wind is still at 20 knots. In the first diagram, the board has turned away from the wind so that the angle ^tw is now 110 degrees. The apparent wind has dropped from 32 knots to 26 knots but its angle ^aw has increased to 46 deg. In spite of the drop in wind speed the increased angle of the apparent wind allows the sail to be sheeted out slightly, thereby increasing the forward component of driving force on the sail which results in a higher board speed.

Fig. 6

We shall assume the board speed has now gone up to 30 knots. The apparent wind speed will also be 30 knots and its angle ^aw has decreased to 39 degrees as shown in the second diagram. Once again, because of the smaller apparent wind angle, the sail has to be sheeted in, reducing the driving force on the sail and the board cannot go any faster.

Fig. 7

Figure 8 shows what happens if we keep bearing away until the true wind angle ^tw reaches 140 degrees. The apparent wind speed will drop to 19 knots and its angle ^aw will increase to 41 degrees. This much-reduced apparent wind speed leads to a loss in sail power and the board will start to slow down. This causes a further reduction in the apparent wind speed and a further increase in the angle ^aw. At a board speed of 20 knots, the apparent wind speed will be 14 knots and the angle ^aw will increase to 70 degrees. If we continue to bear away from the wind, the apparent wind speed will drop very rapidly and the board will eventually stop planing.

Fig. 8

At this point in time, I should point out to readers that the board speeds I have assumed are only for the purpose of illustrating what happens when the board is sailed at various angles to the true wind. In practice, such board speeds may or may not be actually attained.

To summarise what I have said earlier we see that, as we bear away from a beam reach, the speed of the apparent wind will start to decrease, however its angle ^aw will increase thus allowing the sail to deliver more power to drive the board forward which in turn increases the speed of the board. As we continue bearing away downwind, there comes a point when there is no further advantage to be gained by a larger apparent wind angle and the lower apparent wind speed causes the board to slow down. When sailing on a very broad reach, the board will slow down so much that it cannot continue planing. To keep it on the plane the board will have to be turned back upwind. Exactly the same thing happens if the speed of the true wind decreases.

In practical situations the wind is seldom constant. There will be gusts and lulls. To keep a board on a plane and at the same time to sail as broad a course as possible, the sailor will have to luff up on the lulls and bear down on the gusts. Competitive sailors will pump vigorously going downwind even in strong winds, not so much to gain speed, but to stay on the plane while sailing as broad a course as possible. This is where a big sail will have a clear advantage over a smaller one. A sailor carrying a larger sail will be able to keep planing on a broader course.

Apart from changes in the true wind speed, there could be other reasons for changes in the speed of the board. The most common cause is the need to negotiate the waves. When sailing downwind, the waves are usually travelling in the same direction as the board. The board will slow down when it is climbing the back of a wave and speed up when it is surfing down the face of a wave. The angle ^aw of the apparent wind will therefore be alternately increasing and decreasing. The sail will therefore have to be correspondingly sheeted out and sheeted in to maintain the optimum angle of trim.

Next: Sailing Upwind

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Copyright 2000 Denis Wee. All rights reserved.