Jan,

Understood. I was going to camber the lower panels since they are straight panels and I hope to get a more consistent shape. I may need to aid camber to the top panels also. On further reading and photo study I am wondering about the need for an extended flat center section. If the point of maximum forward camber is at 35% then a short transition full panel width and then a longer run rearward to the leech. Say 35%, 15 % transition and 50 % rear taper to the leech? I’ll make a pattern tonight and take a look at it. Taking the sail off tomorrow.

Jan, 

On this polytarp sail I plan to add camber to only the 4 bottom panels. If you look closely at the photo of the flat sail the top two panels trailing edge looks irregular. It is actually straight but is beginning to flutter as the winds increase. Mildly annoying in light winds but above 15 it really flutters. I will take the opportunity to cut the leech with a curve on the top two panels. This one piece sail allows pretty quick modifications. It is taking longer to decide the length of the darts than to actually sew them in. It should only take a couple of days to sew the modifications in. The wnext sail will be cloth using one of the “Arne” patterns. I’ll need to figure out the best sail for the Peep Hen. It’s a fat little thing with a fixed mast position! 
Curtis

I made toppanel for my sail using this method. Works good !

link

You don't have to cut the sail. Just make a pleat.

All,

I have spent the last year sailing my flat JR poly tarp sail with adequate performance for my 

normal gunkholing but now is the time to add camber to the sail. I spent last week in 

the OBX 130, a camp and sail raid type event here in the North Carolina,USA sounds. 

Winds were 15 to 30 mph daily. At 10 - 15 I could make 60 to 65 degrees to windward. 

Reefing the sail was terrible for performance. At one point barely sailing a beam reach

 and making 1 knot towards goal while sailing at 3.8 knots with 18 nautical miles to 

go I had to withdraw and trailer to the next campsite. I have a few weeks before returning

 to other projects and would like to add camber to the flat sail by using darts, thus the 

interest in the position of maximum camber. 

So far, I have the maximum camber starting at 35% from the luff, a straight section 

with full panel width for 24% and a tapering section of the last 40% to the leech.

Curiously, the plans sketch I have recommends a 2 inch dart in the leading and trailing 

edges of panels but this produces a very large camber depth both by calculation and 

by using the chain calculator. By using a less drastic dart ( 1 inch) I was able to get 

the camber depth to a more reasonable value. See the photo of the chain calculator 

and the drawing. A photo is included from last week of my flat sail in 10 mph winds. 

I realize darts are not the most efficient way of adding camber but should work as I 

rework this sail. Sewing a new sail will have to wait until winter.

Thoughts or advice on positions of camber or depth of camber in a dart modified sail. 

Thank you, Curtis

Paul and Arne

SJR Amiina rig

I have no comparative results to add, but as my sail is a little different and it does perform well - (how well? I have no comparative information so don't know).

The foil shape on both jibs and mains is very blunt entry, very rounded at the luff, (not what I would call a sharp entry/gentle curve at the luff), and the max camber occurs at about 35% or thereabouts on both mains and jibs. I wouldn't say it points exceptionally high, but it does point quite well and I would say the windward performance is good, both tacks the same, tacks easily. Camber plenty - I haven't got around to measuring it with a ruler but it is probably around 10% jibs and 8% mains or maybe a little more, if I remember the numbers correctly when I built it.

(slight lee helm in light wind, can be fixed by shifting weight forward in the cockpit. Considerable weather helm when heeling, addressed by shifting weight to back of cockpit and raising the swinging centreboard a little. That's all dinghy stuff and more to do with the hull than the sail, I think).