Paul: "The note worthy thing about this sail is it has 30° shelves, the angled shelves reduce the "baggyness" of the sail and enable it to fill quicker when the breeze gets up. "
David: Best practice, IMHO. Less baggyness than a horizontal shelf, camber over more of the height of the panel than a 45˚ shelf, easier to calculate the width of shelf than 45˚ (1/2/√3 triangle).
I think Paul is on the right track by using an angle other than horizontal for the shelf foot, for the reason he has given, but as for the optimal angle, I am not altogether sure of the reasons given by David as to why 30 degree shelf should be all that different from 45 degrees, which has been used quite a lot in recent years since Slieve documented the angle shelf foot method in his SJR sails a few years ago. The 45 was probably a fairly arbitrary choice at the time and 30 may well be better, though I can't really see that there would be a lot of difference in practice, and would be interested to know more.
While a shelf foot sail panel made from plywood or tin plate would have camber over “more of the height of the panel” if the shelf angle is 30 degrees as compared with 45 degrees, this attribute seems to disappear almost completely when the panel is made from of soft cloth. In reality, the inflated soft cloth panel in cross section takes up a curve, spreading the camber smoothly from top to bottom, with it’s maximum at about the middle. [I'm not explaining this to David, he knows a lot more than I do - this is for anyone else, who may not know]
Please excuse the poor schematic diagram. And I’ve turned everything on its side.
The questions is relating to the curve the soft cloth takes up in reality, as we traverse from the top to the bottom of the panel: would the shape of the curve be discernibly different when comparing a 30 degree with a 45 degree shelf? I am inclined to think there would be very little difference. Indeed, a nicely made barrel-rounded sail which has no shelf at all, yields a cross section shape which is surprisingly similar to that of an angle shelf foot sail.
(As an aside, as we know, a shelf foot sail generally ends up with more camber in the middle of the panels than the actual designed (or "tin plate" camber) and Arne (in the previous post) probably asks the most relevant question).
I am trying to understand what is meant by “bagginess” as opposed to camber. The shape of the curve might be discernibly different if we were to make a more extreme comparison, say between a horizontal shelf foot sail and a 30 degree (or a 45 degree) shelf foot sail. Certainly the horizontal shelf foot sail, if given the same designed camber, would have a great deal more actual camber in the middle of the panel, to the point where, uninflated, it could look “saggy” or “baggy”.
I think David's final assertion – ease of calculation - I don't quite understand. There is a little bit of trigonometry which is unavoidable. The part I found most difficult was visualising the three dimensional shape, and how to loft it onto a flat surface, something I am not very good at. But all the arithmetic can be done with a calculator or a spread sheet. I can't see how that can be avoided by whatever angle (other than horizontal) is chosen. Anyway, perhaps that is a reasonable reason for choosing one shelf angle over a not very different other.
There is a trade-off between minimising “bagginess” and (up to a point) maximising camber. The experienced sail makers, such as David and Paul, are in a better position to make judgement calls in these matters than ordinary folk such as me. Perhaps it just comes down to a question of what feels right.
Summary: Paul has made a beautiful sail which performs to all hopes and expectations, and the use of 30 degrees for shelf, is an interesting feature. It’s good to see someone try something a little different, and 30 may well prove to be closer to an optimum than 45 – who knows? Any difference in that regard seems likely to me to be pretty slight. The noteworthy thing, in my view, is the use of angled shelf as opposed to horizontal shelf, rather than the 30 degrees itself, although 30 may prove to have been a very good choice. The other noteworthy thing, undeniably - Paul's beautiful workmanship – and happy client.
Addition: Distribution of camber between top and bottom of the panel – you won’t see much better than this, and there are no shelfs here at all (that’s only the shade caused by a quite lovely camber from rounding alone).
(Distribution of camber along the chord (ie fore and aft) is a different matter and perhaps of more relevance to the SJR than the contiguous sail. It does seem that the angled shelf foot method allows more camber to be crammed into the first 20% of the foil, and a blunter entry angle, than the barrel-rounding method. But this is another story, I'm not sure how much this would be relevant to conventional junk sails, and in any case, it's not really anything to do with choice of shelf angle).
Another thing - these all seem very small details. Here is a horizontal shelf foot sail (also made by Paul some years ago) with panels inflating beautifully, and I was very surprised given that the cloth was not particularly light, that the sail actually inflated quite quickly, as I recall we started out in only about 5 kn of breeze picking up later to about 10. Quite a powerful sail actually, I think we were all pretty impressed at the time. (For those who don't know, that's Pango when Paul made this sail for Zane).
A whopping amount of camber here, a combination of: the camber given to the shelf, and the angle of the shelf (zero degrees here). I guess that is what is meant by "baggyness", but it inflated all right and set quite well. The horizontal shelfs are accentuated here by the choice of cloth colour. Quite impressive. In very light conditions I expect those bags would droop a bit - quite a lot perhaps. I guess, with hindsight, a reduction in camber, or an increase in shelf angle (or both) might be an improvement in this case.
(That's Marcus who couldn't resist a climb up the battens).