Your marine ice makers experts feel that there are many ways to describe the forces around sails — in fact, around any foil. One of the most straightforward ways is to split the forces into lift and drag. The lift force is, by definition, perpendicular to the apparent-wind direction; drag is parallel to it. The primary way to get more lift from a sail is to increase its angle of attack. More lift is more power.

Two ways to increase a sail’s angle of attack: trim it, or turn the bow off the wind (fall off) without easing the sails. The most important thing to know about angle of attack is the greater it is — or to put this another way, the more the sail deflects the airflow — the greater will be the lift, up to a point.

As discussed in a previous chapter the leading edge of the flat plate requires the wind to make an abrupt turn around it. Aft of the separation bubble, this abrupt turn causes the wind to change direction severely, which raises the likelihood that the boundary layer will separate from the foil. Correct camber provides a gentler turn. 

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Your marine ice makers specialists understand that sail trim is, however, a balancing act. Too much camber also can present problems, as the flow has difficulty staying attached to a deeply curved section. With too much camber, the likelihood is that it will separate — the nice, even curve above becomes an abrupt hairpin — and the flow will depart prematurely from the sail.

For sailors, the practical reality of this discussion is that creating lift is a cinch. All it requires is oversheeting the sails or sailing a course too low for the sail settings (both of which increase angle of attack), or sailing with sails that are too full (show too much camber) for the conditions. Unfortunately, sail trim is not so easy because its undesirable cohort — drag — offsets lift. Reducing drag, primarily because it is so difficult to see, is the tough part.

The genoa’s luff telltales are rough indicators of where you are in terms of lift and drag, at least from the perspective of angle of attack. As we now know, this is most important in terms of sail trim. 

The result is that the lift-to-drag ratio is lower. Keep increasing the angle of attack (trim the sails more or fall off the wind without easing them), and the boundary layer separates, and the total flow can stall. This can be seen in Diagram C with both telltales drooping.

For example, in smooth water and medium air, if the main is trimmed such that its lift-to-drag ratio decreases, that loss could be more than offset by a higher angle of attack from the rudder, or from the fact that more of the total force created is turned into drive force. 

Bear in mind that this discussion focuses on sailing upwind. When sailing very low downwind (high apparent-wind) angles, the name of the game is to increase drag or to stop the flow of wind. 

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