Wednesday, February 11, 2015

Sail design

Long ago, in 1983, I was building a two-masted 20' sharpie.  Naturally, it was a custom design and would require custom sails.  In order to save money, I would sew my own sails.  Sailcloth was ordered from the Sailrite company which is still in business.  At that time it was headed by Jim Grant (and may still be) who put out a quarterly newsletter.  As a customer, I received a newsletter which included an article on "broadseaming", a method of varying the width of seam overlaps in the fabric panels of a sail so as to control the shape, specifically the draft, of a sail.  Broadseaming was considered to be an art and somewhat of a mystery (at least in the published literature).  Just as plywood hull shapes can be described mathematically, I thought that sails made from stabilized fabric panels could be similarly described.  I wrote a letter to Sailrite outlining such an idea.  Later, upon receiving the July 1984 Sailrite newsletter, I discovered that Jim Grant had made my letter into an article titled, "The Mathematics of Sail Design".  Although CAD technology has progressed greatly since 1983, that article is included here because a recent internet discussion suggested that even now little has be made public.

Jim Grant overstated the difficulty of the mathematics required for these computations.  An inexpensive pocket scientific calculator provided necessary results for me, and the described method was used to design the sails for that 20' sharpie.  A photo of the sharpie is included in which you can see the shape of the sail draft by looking at the curved shadow that the straight sail boom makes on the sail surface.  The curvature appears to closely duplicate the cross-section shape of an airplane wing.
  I made one mistake in this sail design: the draft is too shallow.  Due to a lack of published data, I had to guess what the proper draft/chord ratio should be.  I guessed too shallow (it was about 7-8%); it should have been about 10-11%.  Although the boat handled okay, I have wondered how much more power the sails would have developed with a greater draft.