Anonymous wrote:

Nice work, Darren!

Just one point: a while ago, I worked out for Paul Fay roughly what effect adding camber had on the diagonal measurement of the panel when measured along the surface of the cloth - it seemed that the batten angle had to be increased somewhat, with increasing % of camber, if neutral or positive stagger was to be maintained. The attached PDF has some guidance on how much. Paul told me that this worked out perfectly when he made his sails.

David, I've saved your PDF, it's a nice analysis, although the angles required look really high to me at first glance.  I'll have to check this on my 1/2 scale practice panel to really convince myself.

 I think it is possible to figure this out graphically for each sail using a method similar to the one I just described for the shelf foot (Late at night thinking taking place, so nobody trust me).  If you take your airfoil and rotate it and rescale it (x-axis only) so that it is snapped to the lower and and upper corner of the panels, then you can use the "Total Length of Selection" tool to figure out the diagonal length of the cambered fabric on the panel.  Then as per PJR Fig. 2.11, if your batten is longer than the distance you just measured you'll have positive stagger.  For Leeways sail with 8% camber and 10^o battens the cambered diagonal is 5074mm against a 5000mm batten :-(    I think the Paul Fay-style standing luff parrels with the tapered mast will keep the foresail stagger OK, but I'll have to think about the mainsail because I canted it on the mast.

Oh, and another point: a 60˚/30˚ triangle has sides in the ratio 1:2:✓3 so that a shelf width of 2 units, sewn together with a panel concavity of 1 unit, gives a theoretical panel camber of 1.732 units. As the cloth isn't stiff like tinplate, but will be blown into a curve by the wind, the actual camber is something in the same neck of the woods as 2 units - this is sailmaking we're talking about, not precision engineering ;-)

So cut a shelf width equal to the camber you're aiming for, and cut the panel concave by half of that amount, and you won't be far wrong.

I make no guarantees that the below method works, just that so far I haven't been able to prove that it doesn't

I think Slieve has a very nice summary of the different ways to put camber into panels in Chapter 3 of the draft of his book, so I won't repeat it here.  Round and Round and Broadseam have seen a lot of use to make cambered panels, but not as many shelf-foot sails have been made (Paul Thompson and Sebastian Hentschell are two of the few I'm aware of that have used it for multiple sails).  The method is super-intuitive if you use a flat-shelf-foot where the airfoil shape lies horizontal and is stitched to the main sail panel at 90^o.  However, this leaves a lot of "extra" fabric in the sail.  Instead angling the foot to 30^o or 40^o towards the centre of the main panel, is probably a happy medium of getting as much camber across the height of the panel as you can without having a sail that is unduly baggy in very light airs.  Also, it's interesting to do something a bit different.....

The trigonometry isn't really difficult, and some members in the past have figured out ways to automate the process.  I was inspired by fellow JRA member Erik Menzel, who has done a really nice job of drawing his entire rig in 3D CAD and then having it "unwrap" it to flat 2D shapes that can be cut from cloth.  However, Erik's method requires you to draw the entire rig in 3D and the free version of Sketchup he used to do it is no longer available.

However, 2D CAD is pretty easy to learn and can be had for free or cheap (I use QCAD Pro, which is inexpensive, but I think the free version should work for this, there are also other free CAD programs that have similar commands).  So, I wondered if there was a simple way to use 2D CAD to make shelf foot panels.  I started with an excel spreadsheet to use trig to calculate all the coordinates to transpose the wanted airfoil into a 30^o shelf foot and the relief in the panel to which it would mate.  However, I had an "a ha" moment when I realized the calculations for all the different airfoil coordinate points where just yielding a simple ratio.  Thus, the attached PDF shows my attempt at a simple graphical way to build 30^o shelf-foot panels.  I've tested my results against paper models of Round and Broadseam panels using the same base airfoil, and I've also used the method to see if I can get the same result Erik does using 3D CAD.  Paper is pretty unforgiving, so the Round and Broadseam panels have minor kinks in them, but otherwise they are pretty similar to the shelf-foot, with identical max camber (within the limits of small models) and profiles from luff to leach at mid panel height.  The Round and Broadseam panels do have a more curved profile in cross section between the battens.  I was able to duplicate Erik's work to within a centimetre and since it wasn't clear whether his or my airfoil was superior or where, in either of or workflows, the discrepancy might have crept in, I shrugged on got on with the next stage of designing the sails.

The PDF below was really just my notes to start and although I've expanded it for here, it probably isn't the most efficient workflow, it's just what I used to get myself to the next stage of design.  Also, if this is considered a success much of the credit needs to go to Erik who talked me through the initial trigonometry and who shared his 3D designs with me that made it much simpler to check if things were working in 2D.  Of course, if there is a flaw then that belongs entirely to me.  Enjoy.