Sails today are very different from their ancestors in the period before the polyester revolution. Shape has always been the most important factor in sail efficiency, and time was when you chose your most suitable canvas for the conditions, pulled it up and sheeted it in. Some cruising sails are still made like this. They work well enough, but the cloth from which they are cut often means that their performance potential is nowhere near that of a modern sail whose geometry can be modified to suit the wind and sea. Such equipment has worked its way into cruisers following the lead set by racing yachts, whose hi-tech vanguard have now moved on to cloths of such sophistication and stability that the shape cut into their sails is barely compromised until they literally burst.
The maximum camber of a sail should be somewhat forward of the middle of its cross-section. In practice this varies to a degree with what sort of sail it is and how hard the wind is blowing. The power of an aerofoil depends upon its depth of curvature, so a baggy sail will drive you along in light airs far more effectively than a flat one. As the breeze hardens, the power of the fullcut sail will become too great for the boat. It must then be flattened or reefed, if either is possible; or changed for a different sail if not.
This requirement is underlined by the fact that as the wind increases, a sail naturally becomes fuller and the point of maximum camber is blown aft towards the leech. Both these results are the opposite of what is desirable, and something must be done to mitigate them.
Article Source: http://EzineArticles.com/1574546