Jami Jokinen wrote:Yes, 

I get this - it is the same method that Arne suggests and I also used with my first sail. This is easy, because the max curve point is known.

But where is the max curve of the broadseamed edge?

(And did we now enter the stupid question category?)

Jami

I keep saying that making a junk sail is far from rocket science. No one has yet made an accurate algorithm which describes the resulting camber as a function of the round we cut along the edges of each panel.  If there were such an algorithm, then one could decide for some favourite foil shape (resulting camber) and the resulting amount and shape of the Round-curve along the panel-edges would be given. 
I know Slieve is keen to keep the curve of the camber  all the way to the luff. I, on the other hand don't bother so much, as the mast will ruin that curve on one tack anyway. 

It would take a long and exhausting match-race series to optimise this camber. My half-informed guess is that the amount of camber (6, 8, 10%) will be the most dominant factor.

Arne

Without a CAD program or large drawing board, you'll need to go back to the traditional sailmaking methods. Having established where the corners were, and then how much depth of curvature he wanted, and where the maximum depth would fall, the sailmaker would use a long flexible batten to draw a line through the points. Alternatively, he "threw" his roll of webbing through those points, arranged it in a fair curve by eye and pinned it down.

A good simplified way of drawing the shape for the upper and lower edges of a JR panel is to plot a point at the luff corner; another that is at the maximum depth of curvature and 35% of the chord back from the luff; and another point that is on the straight line between the corners and 70% of the chord back from the luff; and then, firstly draw a circular arc through those three points and secondly a straight tangent to this arc that passes through the corner at the leech.

Easier to draw than to describe – I hope you can make sense of this.

Hi Jami,

No, it's not a silly question. And I hope this is not a silly answer.

Faced with the same sort of problem some years back, and realising that it was a question that would have to be answered many times I tried to write a spreadsheet to do the necessary work for me, and although not necessarily perfect the result has given satisfactory answers. I still use it for all sails. It gives answers which are very similar to Arne's method for the amount of round required.

If you look in my files on the website section - Junk Information, - Public Domain files by-, you will find -

C and SJ Appendix 3 12-05-12a.pdf

Universal Round and Broadseam Calculator.xls

The results require the broadseam to be split over at least two seams, or semi-seams, and placed well forward in the panel.

There is more information on page 12 and page 19 of C and SJ P1-22 12-03-17c.pdf 

Have a look, and come back if you have any problems with it.

Meanwhile, may I Wish you all a Merry Christmas and and Happy New Year,

Cheers, Slieve.

Jami, I think that is exactly how to do it. I will come back after Christmas on thoughts about how much to shorten the edge. Slieve McGalliard has already produced some thoughts about it.

Arne