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Sail Trim and Stability

Action of the Wind on a Sail

The wind consists of moving quantities of air, the weight of which should not be underestimated, which hit the sails and rebound to go off in another direction, much like a tennis ball when it hits a racket. An elastic shock takes place molecules of air are deviated with a certain force and in turn an opposing force of the same strength affects the sail.

Considering the situations shown in the drawing: (Referring to Fig.22)

a) The sail is not drawing, it spills; a few light whirls slow down the wind.

b) The sail has been hauled in a bit to stop spilling. There is no more turbulence, air currents are smoothly deviated. A force F acts on the sail and balances the force F’ which makes the air current diverge.

c) The sail is hauled in a bit more, the deflection of the air currents is increased; the necessary action and consequently the reaction on the sail, are stronger.

d) If we keep on hauling in, the force will continue to increase but air turbulence will appear.

e) By hauling in still more, there will be a sudden change in the flow of air. Large whirls will be formed; air will be slowed down and diverted. The force on the sail will diminish. A change has been made from laminar flow to turbulent flow, and the sail is ‘stalled’.

The effect of this transition is very important because it slows down the speed of the boat considerably. Easing of the sheets is not enough: one must almost go to the point of spilling to find a steady flow again.

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