Sailing Upwind

It is well known that in marginal planing conditions it is possible to plane on a beam reach but it will not be possible to plane going upwind. This fact is particularly relevant to sailors who are keen participants in Formula Windsurfing races where the minimum wind speed at the starting line before a race can be started is 7 knots. Most modern boards equipped with large sails can plane in about 6 to 7 knots of true wind. Let us look at what happens to the apparent wind under these conditions.

We will start by looking at a board on a beam reach planing at a speed of 11 knots in a true wind speed of 7 knots. The apparent wind will be 13 knots at an angle ^aw of 32 degrees. If the board is now turned upwind so that it is now sailing at an angle of 60 degrees to the true wind the apparent wind will immediately go up to 16 knots but the angle ^aw will be reduced to 23 degrees. In spite of the increase in the apparent wind speed, the smaller angle will not provide sufficient forward driving force on the sail to keep the board planing. It therefore stops planing and its speed will drop back to about 6 - 7 knots. To keep planing, the board will have to be turned downwind again towards the beam reach position. As the true wind speed increases, it will be possible to plane at angles progressively closer and closer to the true wind. The higher the true wind speed, the higher the board will point.

Fig. 9

The implication of this in racing is that in marginal planing conditions (6 - 9 knots) a board that is planing upwind will have a velocity made good (VMG) to the windward mark that is less than the VMG of a similar board that is slogging upwind at a speed of 6 - 7 knots. In such conditions the sailor has to make a decision whether to continue planing or to start slogging. The situation becomes more deceptive if the wind at the start of the upwind leg is 10 knots or more but gradually drops back to 8 knots or less. The sailor may then not be aware that although he is still planing in the lower wind speed, he is not pointing as high and is losing ground in the windward direction.

A Word About Pumping
The action of pumping the sail introduces an additional element into the wind vector diagram. Its effect is to move the apparent wind backwards and increase the angle of attack. This is beneficial when sailing upwind because it enables you to point higher. If you are trying to fetch a windward mark that seems to be just out of reach, you will notice that pumping the sail all the way to the mark will enable you to round it quite easily.

When going downwind on a very broad reach, the sailor has to ensure that pumping is done carefully and correctly if it is not to be counterproductive. The apparent wind is already aft of the beam position and the pumping action will move it even further back. It is therefore very easy to exceed the optimum angle of attack and get into the oversheeted position. This is particularly noticeable when sailing downwind on a longboard in very light winds. In such conditions the sailor should try to avoid a fanning action which results in sheeting the sail in and instead pump the sail by pulling it without changing the angle of the plane of the sail while at the same time making sure that there is always maximum resistance on the sail. If the resistance decreases during the pumping stroke, it will indicate that the sail has become oversheeted. It helps if, prior to the pumping stroke, the sail is sheeted out beyond the zero angle of attack position and as it is pulled in it will increase the angle of attack and go through a little beyond the maximum. This will help to ensure that the pumping stroke is optimised.

Next: Forces Acting on the Sail

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Copyright 2000 Denis Wee. All rights reserved.