I asked Bill to post his comments - they're certainly valid for people endeavoring to compute a draft location "from scratch" especially in dacron sails, which are more responsive to battens than low-stretch materials.



I failed to emphasize that my mention of "40%" was quite specific to my sailmaker's computations for this particular sail. I hope nobody rushed out to the back yard and started aiming for that figure. :-D



Another thing about draft, leading edge and trailing edge:



As to the mast's impact on draft - - picture this: a properly rotated mast, especially a "Loose diamonds" rig but also a prebent rig - makes that thicker entry my I mentioned earlier. - - (you really visualize the underside of the "wing" as a straight line from the winwardmost point on the mast to the leach - - ((keeping the windward flow attached to the sail is more about smoothly letting the sail slip by - remember it's the higher pressure side of the sail. Laminar flow on the leeward side directly lowers the air pressure there, in accordance with Bernoulli's principle that faster fluids are under less pressure than slower fluids. So you rotate the mast for smooth transition on the leeward side.)) ) So the rotated mast, in that it thickens the entry *skews the baseline you're measuring the draft from, deepening the front more than the rear*. So it actually doesn't entirely counter the benefits of having your sail and battens draft being forward of center. My own opinion, then: it's simplistic to just "add in the mast's long dimension" to the batten's overall length when trying to compute draft position. You've got to draw it out, fill in the new baseline, measure the new draft, and a good cad/cam software package is probably the way to go.



All of which I'm assuming my sailmaker knows and allowed for when designing my sail - which is why I accepted his figure without worrying about recalculating anything.



If I were doing it from scratch, I'd definitely want to hear what Bill is saying here, and get all the advice from people who'd worked on the same or similar sails.



Aso, that bit with the sawhorses is priceless - some windless day I plan to fine tune my battens that way. (Yes, it's true, I confess it: if my passion were cars, I'd spend most of my spare time (when not driving them), tuning the engine in the driveway, tinkering with the carb(s), tweaking the tranny, cranking up the clutch etc. etc.)



Keep one hull dry!



Ed Norris