Hi Arne and all,
On reflection, this is a very long-winded way of asking a simple question which I ask from towards the end of line 5 to line 9 of the next paragraph!
I am following your suggestions to use barrel rounding to achieve camber, and reducing that rounding/camber as we go from Panel (P) 7 with 320mm of "round" (R), up to Panel 1 where R is only 50mm. I've only just absorbed the message in your instructions - yes, I'm pretty slow, and far too excitable at the same time - reminding sailmakers that each reduction in rounding between adjacent panels results in a shorter edge. From the 320mm of round in our P 7 and 6 to the 220mm of P 5 and 4, we lose about 27mm of length in the edge. My pea brain can't really envisage how this loss appears in the sewing of P6 and P5 together, as of course they still follow the batten length of 5900mm, but that extra has to go somewhere! I then started thinking "what if my templates' rounds don't match, or in other words what if the curves are different shapes"? Certainly I have tried to let the fibreglass batten spline lie in an even curve, but the 6m long table is not dead flat so the spline springs freeer on some bits of the table than others which means that I had to manipulate the spline a little here and there with weights to get it fair. Was I building in variations in my curves by doing that? This got me thinking how maybe I would have done this whole template-drawing-out-thing differently if I'd thought of this before....
.....and that is to very carefully make a template out of really good plywood or maybe a double thickness of painter's (lining) paper for the panel with biggest round (P7 and 6), making sure that the straight edge really is straight and that the curve is just as fair as possible. Having cut the cloth for 7 and 6 using that template, I would then have reduced the round on this good durable template by simply calculating that 220mm, the new round, is 68.75% of 320mm, the old round. I then make a mark every, say 300mm along the template showing the new curve at 68.75% of the width of the template at each 300mm station, safe in the knowledge that the form of my curve for R 220mm will match as close as possible the form of the curve for R 320mm, even if the 320 one isn't perfect.. BTW I took care to measure the widths at each station with the steel rule reasonably perpendicular to the curve, and not perpendicular to the straight edge of the template. (For the four lower panels the template for the parallelogram part is thick lining paper, with the rounding template made from horrible fluffy hard-board material, not nice crisp plywood. The fluffy stuff I cut with a small cheap Japanese pull saw which has very fine teeth and an incredibly thin blade and is ideal for following a thin line accurately. The three panels above are all one piece of lining paper which includes the rounds).
As I go up the sail, the differences in the edge length between panels will become less and less. I've tried to even out these differences as much as possible. But where DOES that extra rounding go, especially the 13mm or so from P6 to P5?!
Cheers,
Pol.