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Evaluating old sails
On 24 Dec 2005 21:47:05 -0800, "
wrote: So, how does one evaluate the shape of a sail? ======================================== 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 |
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