Arne and David,
Thanks for contributing, one of the hardest parts I've found about figuring out cambered sails is making the jump between PJR and the current best practice (even with the excellent written work of Arne and Slieve and some personal tutorial from David Tyler). One problem is that although everything is in the forum archives and old magazines, it is difficult to sort through what ideas have been tried and subsequently been discarded or modified. Hong Kong Parrels are an example. They seem to have been widely used, often despised, and then partly replaced. But, I haven't found a clear explanation of the current recommended use of Hong Kong Parrels.
This post started out just about Hong Kong Parrels, but morphed into everything I think is not covered in PJR and must be found spread elsewhere here on the JRA. It is mostly my notes for my own benefit, but perhaps someone else would find it useful. If anyone has the time to read through it and find my errors, that would be appreciated. I should explicitly state that I am not complaining about the fact that info is spread throughout the JRA, things are still much easier for someone now than it was when all of this was being figured out originally and I'm grateful to all that have shared their work.
On the list of things that seem different or not explicit from PJR are:
Hong Kong parrels: reading through the forum, it looks like these are a necessary evil to avoid creases in cambered sails. Arne's earlier sails had many, and progressively over time folks have managed to find way to eliminate all or many of them, mostly through modifications around the throat hauling parrel. The problem with Hong Kong parrels is that the provide yet another line for things to get hung-up on. Also, if adjusted too tight it looks like Hong Kong Parrels can bend or break battens. It looks like the correct adjustment for Hong Kong parrels is normally slack, and only going taut when sailing close-hauled. Arne has suggested that thin bungee could be added to them to keep them out of the way when reefed, but let them stretch easily when the sail is lifted.
Throat/Luff Hauling Parrel: most cambered sails seem to combine these for one less line. It is now usually run from the yard to the first few battens below. Once adjusted the fully hauled sail should lie pretty close to the right position with no other standing or running lines adjusted other than the halyard and Throat/Luff Hauling Parrel.
Yard hauling Parrel: The yard hauling parrel can be moved higher (30-40cm) than the halyard to help peak up the yard.
Paul Fay Yard Hauling Parrel: Getting the yard to peak properly and the rest of the sail to hang naturally below seems to be the key to getting a low stress sail. Paul Fay redesigned the yard hauling parrel so that the free end is run to, and pulls down from, the forward end of the yard. This helps, rather than fights the luff hauling parrel in peaking up the yard. Peaking up the yard can further be assisted by moving the halyard sling point slightly aft of the middle of the yard.
Yard and Battens: The yard and battens of a cambered sail have to be heavier than the recommendations in PJR, this is because in a flat sail the loads are spread evenly across the battens and yard. However, in a cambered sail the loads are spread to the luff and leach. This requires stronger battens and yard to resist bending and breakage. However, I haven't found a recommendation on how to scale-up PJR battens for cambered panel rig. The batten spec database can give some guidance.
http://www.junkrigassociation.org/member_proj_forum/1443163
Batten rise angle: The PJR suggestions for batten rise to create positive stagger don't work with a cambered sail. Instead 10 degrees seems to be the replacement that is broadly used now. This is roughly double what PJR recommends. Even this amount alone won't create the desirable positive batten stagger.
Batten Stagger: The unusual stagger behaviour of the cambered panel sail can be tamed by using shorter batten parrels. I think Arne uses half-length ones. Paul Fay has used his version of standing luff parrels (connects both ends to the luff end of one batten), that get progressively shorter up the mast and thus induce positive stagger. Apparently, the PJR version of standing luff parrels can also be used (fixed line from one battens luff to the luff of the batten above it). These would have the advantage of remaining slack until the sail is hoisted. Either of the standing luff parrels are of more use on a schooner where you are unlikely to want to change the balance of the sails. I haven't figured out how Slieve's combined downhaul/batten parrels would work in this framework. The best material for these fixed parrels is not bare line (which tends to grab the mast or other lines), but either webbing (Arne), or line inside a semi-rigid plastic sleeve (Paul Fay).
Boom batten: The lowermost batten on cambered panel sail should be shortened by 4-5%. This is because cambered sails don't follow the stagger rules for flat sails and the resulting negative batten stagger can cause fouling of the sheets. Arne cured this by cutting 5% off the length of the bottom-most batten.
Halyard: The halyard drift in PJR is generous and David, Arne and others have cheated it and been OK. Arne's rule for this is that the angle between the halyard and the mast should not exceed 30 degrees, rather than specifying a ratio to the sail as PJR does. The minimum drift recommended is 60 to 70cm, but more is better. When using less drift than PJR recommends, you should probably use a halyard with less than three parts(Arne uses three part, David uses two part) to lower the wringing load on the mast when the sail is let out for running. This has the advantage of making the sail easier to reef (there is less overhaul resistance and the sail drops easier), but in increases the compression load on the mast and means you will have to use a winch for raising larger sails.
Schooners/Ketches: With cambered panel sails you are likely to need to sheet the mainsail to windward in order to point high. This can be accomplished by a horsed sheet (traveller) or with double sheets (port and starboard).
High aspect ratio sails: split sheets (upper and lower) or double sheets (port and starboard) are strongly recommended for a high AR sail. The steep sheeting angles present in a high AR sail mean that a single sheet system is unlikely to be able to control sail twist.
Sail Camber: This is a bit of a horses for courses kind of a thing. Generally, it looks like folks going offshore have been happiest with something under 9% in the lowest panel, and then gradually decreasing to little or no camber by the time you get to the triangular top panels. Even a bermudan storm trysail has some camber so it may not be necessary to have perfectly flat top panels. There is of course a group who supports flat sails for offshore work that only have induced camber from sail-set. You can also find examples of folks who would like more camber if they regularly find themselves wanting to sail to windward in light airs. Hull shape also seems to play into this. Heavier hulls that require more drive should consider a bit more camber. The disadvantage of more camber offshore seems primarily to be increased slatting of the sails when the boat rolls in light winds, increased chafe due to extra fabric in the reefed sail fluttering and a sail bundle that doesn't reef as neatly.
How to sew camber: The primary method seems to be Arne's well instructed method just using rounding to add camber. However, if I've read Slieves work correctly, it looks like there is not much more work to use a Round and Broadseam method. If you are using a fabric with some stretch it looks like just rounding will do well at the expense of some wrinkles that likely have little effect other than appearance. If you just use rounding the fabric should be left a bit slack along the batten. If you are using stiffer sail fabric, such as regular dacron sail material, then Round and Broadseam should result in fewer wrinkles and the sail can be stretched taut along the batten. The point of max camber should be around 35 to 40% of the sails chord. If you use the Round and Broadseam method of sewing, you can make the entry angle of the sail steeper for better performance (although there aren't really any tests of how much this benefits).