Anonymous wrote:

I wonder,
has the actual, resulting camber in the lower panels of Pango’s sail been measured? I am asking because I wonder if a general rule of thumb has been established to find the ratio between the ‘camber’ of the half-lenses and the resulting camber in the panel.

On my sails, the max rounding of the barrel curve is roughly 55% of the resulting camber I aim for. However, this varies a bit with varying ratio between height (P) and width (B) of each panel. That’s why I found ‘the chain calculator’ to be useful.

Arne

 

Have not measured it but I have found on average, maximum camber on a shelf foot sail is about 2% more than the calculated camber. So if I want a 8% sail I use 6% and 8% for a 10% sail and so on. However it is not an absolute measure as the width of the panel also effects the outcome (wider panels tend to develop more camber) so one needs to use a little judgement gleaned from experience. I don't loose sleep over it as I'm not building sails for an America's Cup boat. Should I bother to measure, it's more to satisfy my curiosity than anything else. Also the narrower panels and lower aspect ratios tend to make the camber appear more dramatic than it's actually is.

Over all, I'm pretty satisfied with the sail on Zane's Pango, the boat is fast and weatherly. Despite Zane's lack of experience, he has done well.

Just to add a little more to the very clear explanations by Graeme and Ueli:

The 90˚ shelf is actually as easy to plot and cut as the barrel cut of Arne's methods, as the panel is a simple quadrilateral and only the shelf or lens has a convex curve to be plotted. The one down-side is that it does put a little too much extra cloth in, which has to be lifted and inflated in light airs.

Which is why Slieve came up with the 45˚ shelf, and a spreadsheet method of calculating the offsets (the panel has a concave curved edge, and the shelf or lens has a convex edge).

To get a good combination of ease of calculation with effectiveness, a 60˚ shelf or lens works well. A 60˚/30˚ triangle has sides in the ratio of 1:2:root 3. So the panel can have a concave curve with an offset of 1 unit, to match a shelf or lens with a convex curve with an offset of 2 units, at any point along them. Not quite accurate as regards depth of camber, but we're talking sailmaking here, not precision engineering. The middle of the panel will belly out from the theoretical 1.732 (root 3) units to 2 units - 2 units being equivalent to the aimed-for camber at any point. Also, it's easier to baste and sew a seam with these two curves than with a curve and a straight edge. This method of calculation also works with tucks and broad seam, which extend into the convex edge of a panel twice as far as the amount of that convexity.

hi mark

the 'shelf foot' is a piece of sailcloth added at the top and bottom of the panels to define the profile of the sail (to connect the profiled sail to a straight batten…)

sometimes the part added at the bottom of a panel and the one added to the top of the next panel downwards are made of one single piece of cloth. this is often called 'lens cut' because of the lens shape of those double profiled elements. but in fact it's still a shelf foot.

ueli

edit: graeme was faster.

TO Annie hi, can anyone explain to me by what is meant by 'shelf-foot' method when it comes to sailmaking.  Also in Sydney Australia are their any other junk rig fans who might just be able to show me a bit about sailing a junk-rigid boat. I'm currently finishing off an eight foot lug-rigged mini cruiser design called 'Ocean Explorer' by Michael Storrer & hope to be out sailing soon in her  but would love to start learning more a bout the ins & outs of Junk sailing. Regards Mark

Well..

I surely agree that Pango’s sail looks great, even spectacularly great with the contrasting colours. It may even be faster than sails with camber achieved in simpler ways, but it comes at a price: It requires more skills and workspace than, for instance my amateur methods. Not everyone has a Paul Thompson or Chris Scanes around who can do it for them. For professional sail makers, I am sure the shelf foot method will move up to be the standard, at least in the lower panels with a lot of camber. I see nothing wrong in that.

Still, most junkies are more or less forced to make their own sails, both for economic and logistic reasons.

Sooo, IMHO, the best construction method is the one you can afford to buy, or can manage yourself.

Arne