Duckworks article, and to Arne Kverneland of the Yahoo Junk Rig forum for inspiration.
I've gradually realized that I've been deterred from some expeditions because of the limited reefing capabilities of traditional Oceanic sails. The Chinese junk rig is famous for being the quickest and easiest to reef, but it has also gained a poor reputation for windward ability in modern adaptations. Modern stiff sail fabrics can be blamed for much of this problem. The junk sail is traditionally cut as a flat panel and this worked well with old style stretchy fabric which would form an airfoil shape when the wind started blowing. When polyester fabric was used in a flat cut sail, no shape was produced and neither was much lift.
My heavy duty polytarp cambered junk sail is made up of individual barrel shaped panels that are sewn together at the batten locations. The battens are two pieces screwed together through the fabric and are intentionally very stiff. The bottom batten is also the boom and the yard at the top is a stiffer spar.
The leech along the bottom three panels has a 5" extension called a Gurney flap. There is some argument about the effectiveness of this device, but I will stay with it for a while and cut it off if it does more harm than good. So far it doesn't seem to hurt the otherwise excellent performance. I don't have any GPS tacking angle data yet due to a battery problem, but will get that the next time I'm out. It's winter here and sailing days are few and far between.