Excerpt from The Complete Sailor by David Seidman
A sail’s airfoil shape is described by its cross section. The imaginary reference line from the luff to the leech is the chord, and a perpendicular from the chord to the sail is the draft, or depth. The location of maximum draft along the chord is the draft position, expressed as a percentage of the chord’s length aft of the luff. The ratio of the maximum draft to the chord is the sail’s camber, an indication of fullness.
Since there is no universal all-purpose sail shape, sailmakers put in what they believe to be (sailmaking is not yet a foolproof science) the proper camber and draft position for close-hauled sailing at a theoretical “average” wind strength of 10 to 12 mph. Typically the camber of a mainsail might be 1:10, and the draft position about 40% of the chord aft of the luff. In all other winds the sail’s shape will need to be altered.
It might help to think of a sail’s shape in terms of an airplane’s wing. A relatively slow biplane’s wing is well rounded on top with considerable draft located almost midway back. A moderate-speed prop fighter has less draft and it is farther forward. And the wind of a fast jet is almost flat, with a small amount of draft located very far forward.
To better see what shape your sails are taking, use black tape to make temporary draft stripes on the sail, or have your sailmakers put on permanent ones.
To read more about sails, sail performance and pick up additional sailing advice, get your copy of The Complete Sailor from Practical Sailor.
