HOW IT WORKS OPTIMISING SAIL TRIM

It is difficult to describe ‘correct’ sail shape, but the three most important things to consider are: sail draft (the fullness of the sail), draft position, and twist (controlled by the kicker /vang and leech tension).

SAIL DRAFT 

Sail draft (chord depth) is an imaginary line from luff to leech called the chord. Chord depth can then be expressed as the ratio percentage between the maximum draft (d) and chord length (c). Draft stripes or seams in the sail can be used to estimate the depth. It is quite difficult to measure, so cruising sailors will use their eyes and experience to estimate draft.

DRAFT POSITION

The distance from the luff to where you find the maximum draft in the sail is called the draft position. Draft forward gives a lower lift/drag ratio, and you can’t point as high as with the draft aft. But it is a more forgiving shape, making it easier to steer and giving a wider ‘groove’. The ‘groove’ is a narrow course range determined by a combination of your sail trim, boat speed and pointing ability. Once ‘in the groove’ your boat comes alive and travels at maximum efficiency. Draft forward is therefore more suitable in rougher conditions or for a less experienced helmsman.

Draft aft gives a better lift/drag ratio than draft forward, allowing you to point higher. The sail will, though, stall more easily if the boat is not steered correctly. Draft aft sail shape is therefore best in easier conditions (medium winds and flat seas).