On 24 Dec 2005 21:47:05 -0800, "
wrote:

So, how does one evaluate the shape of a sail?

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The objective of sail shape is to maintain a proper foil section,
appropriate to the wind strength. The key measurements are depth of
the draft, and position of the draft along the chord length. If you
have horizontal stripes on your sails you can quantify these metrics
by taking pictures upwards from mid-way along the foot of the sail,
printing the picture, and measuring graphically. The optimal depth
and position of maximum draft varies according to wind speed and can
be adjusted to a certain extent with sail controls such as outhaul
tension, halyard tension, mast bend, etc. In general, the position of
maximum draft should be just forward of the chord center, and the
optimal amount of draft decreases as wind strength increases (thus
reducing side force).

Older sails tend to stretch out of shape in such a way the draft tends
to move aft which is an inefficient shape. They also develop wrinkles
and uneven shape which are also slow.

North sails had some excellent books at one time on sail shape.

This is the definitive work on a scientific level but it is expensive
and VERY technical:

http://tinyurl.com/dg2q4

Here are a couple of others:

http://tinyurl.com/curwo

http://tinyurl.com/bg3xv