Arne wrote: ... break down and confess; your first JR sail [shelf foot] panels were built over a plywood mould to get them right…
Yes, I confess …. Well, no it wasn’t quite like that.
I confess I knew nothing about canvas, or sewing machines, and was afraid to pick up the hot knife. So, I made a mock-up of a jib panel using some bits of old plywood I had, just to convince myself what one of these critters would look like in reality. It’s a good thing I did, because I made a mistake with the camber and had to make a second one. (Also, little did I know that a shelf foot panel in soft cloth actually doesn’t look like the rigid model, but anyway it gave me the confidence to proceed). Then I looked at the basting tape, and wondered how to apply it when I only have one pair of hands. Suddenly a brainwave – I thought, seeing I have the plywood model, why not use it as a tailer’s dummy and lay the cloth over it. Suddenly I realised I wouldn’t even need to draw the shapes out on the cloth. Just drape the cloth over, and cut around the “chine” in situ with a hot knife, with total confidence that everything must fit. It made the whole job so easy, and quick, and gave so much confidence in the finished product, that I decided to do the mains the same way, and made a mains dummy as well.
There’s only a day’s work in lofting and making a plywood dummy, and it made sense for my Amiina Mk ll sail (which has all identical panels except for the top triangle).
I think, for a small sail with identical panels, I'd do it again. Otherwise, if a confidence-builder is needed, just make a first one to scale, out of cardboard.
David, I seriously considered broad seam for the mains. Maybe the only type of junk sail panel that seriously calls for shelf foot is the jibs of a SJR. I wouldn’t use any other cut but shelf foot for the jibs. But for anything else, I don’t know enough to have an opinion. I just didn’t have the confidence, after reading the text books, that I could do it right with broad seam, or just how much camber I would actually get. It’s a bit of a mystery to me. (Is “arcane” the right word? I don’t have time left to gain the necessary experience). I made the top triangle (low camber and unsplit) using, not exactly broad seam but some long “patched darts” which have a similar effect on shape, plus a bit of rounding – and it did work out OK, so perhaps a broad seam construction is not so difficult to design. Now that I have learned that a single curved seam does not produce a "chine" (as it would do if we used plywood or tin plate) but rather it gives the whole cloth panel a fair curve – I might look at broad seam again with a little bit better understanding. If cloth is laid parallel with the leech, as it probably ought to be, then there are going to be seams there anyway, so I guess there is further economy in just tapering the seams. The thing is … how much? A beginner can’t visualise the final result. A spreadsheet table of numbers just doesn't give me a visual picture.
So, unlike Arne, I regard shelf foot as an alternative method not unsuitable for beginners, because you can make a cardboard or plywood model first, which gives some understanding, something visual to start with, and thus confidence to proceed. Its faux-intuitive, if that’s a word – after making one out of cardboard, you can then convince (delude) yourself that you understand how a shelf foot sail takes up its shape, and then pick up the hot knife with confidence.
(That said, I have no doubt the easiest and quickest method by far is to be found in Arne's Files. nothing I have seen yet surpasses Arne's Files as a confidence-builder, to get one up and out of the arm chair. I suspect Annie's book and Arne's Files are up there with PJR as required reading for all students of the rig).
The shelf foot method gives you a bit more control over where the maximum camber will be, (compared with other methods?) if that matters much, but apart from that, the panel bellies out no different from any other cambered panel as far as I can see. I’ll probably use the method again because I am confident in it now, and I like the simple geometry (or should I say trigonometry) which is all that is required to get a “predictable” result - but not because I now think the finished product is a superior cut. And it has to be admitted, there is a bit more work in it, and a little more cloth wasted.
(The other reason, for me, is because my rig of choice is SJR, and I can’t see the point of SJR unless the jibs are as large and aerodynamically “correct” as you can possibly make them, with the maximum camber exactly where you want it, in which case shelf foot is a no-brainer. For the jibs. That's for another thread).
Apart from that I am inclined to agree with Arne that whatever works, in practice, is going to be better than good enough for most people, and looking for that elusive extra 2% of performance is probably a fool’s errand - for most people, that is. Hats off to the few who never cease to strive for perfection, nothing would progress without them. Apart from that, it probably just comes down to whatever has the most intuitive appeal. The most important thing is - don't just think about it - do it!
Durability is probably the only other thing that matters in the end – although even then, for most people who only mess about, that’s probably not such a big deal either. I would imagine shelf foot is as durable as any other cut, depending on the details that apply to all sails?
Except... I think it is difficult to work out what is the best thing to do on the ends of the lenses, of a shelf foot sail, to make that area of the sail with the batten ends, strong – there is a little bit too much going on there. Darren might have worked out the best solution there, where heavy duty is called for, if there is not too much cloth to punch through. We'll see. I used layers of tabling - and added gussets of short lengths of rather light webbing across the leech ends of the half-lenses, but did not sew the panels together. That's not going to last. I should have sewn the panels together or (better) used full double lenses, and I think then maybe a short strip of light webbing across the double lense might be enough on a small sail. - but then its a hard spot. A bolt rope or a full length of webbing right down the leech seems a bit agricultural to me, but I don't know, it seems to work on Arne's sails. I dunno.
The biggest problem in SJR is across the luff ends of the lenses, you want to keep a nice shape there. The shape won't matter so much with the luff of a contiguous sail, but its still difficult because, unlike at the leech end, the sail is trying to build up camber quite quickly there. It will be interesting to see how that detail works out in Darren's sail. I don’t think Paul has not yet quite reached perfection at the luff ends of the lenses, and perhaps that part of Pango’s sail could be improved.
Leech. Beautiful.
Luff. Would it help if the lense pairs spanned a a wide centre strip? 
So there is fertile ground for further discussion on the structural details of a shelf foot sail.
I make these comments from the position of being a learner who has perhaps just reached level 101.