Jan, let me dispel the first misunderstanding immediately.
“As I understand it, you have to hold all of them (in Paul's case, two, or even three lines) in your hand (and retrieve them) while reefing, while easing the halyard with your other hand. At the end, you have to cleat all those lines.” No. Nothing like that.
The SJR (like any other properly set up junk rig) can be reefed or dropped completely, in an instant, with one hand, and with complete control of the sail. Just the same as the system you have already. The parrel-downhauls can be tweaked up later, when you have a moment to spare. They are held in jam cleats or cam cleats or clutch cleats or some other convenient system.
The ability to reef or drop the sail instantly, with control at all times over the loose canvas, is the only advantage and the only reason for having a junk sail on a dinghy.
If you have achieved that, with your system of parrels, then you already have a successful system.
I think your parrel system is quite ingenious and I can see immediately that it has a further advantage (in common with the rigid parrels of Steve’s Serenity) that it restrains the yard and battens from moving in a fore-and-aft direction, thus eliminating the need for the usual running parrels (LHP, HKP etc). Furthermore, again in common with the rigid parrels of Serenity, it seems to work perfectly well with an unconventional sail planform.
You have got that little inflatable dinghy to actually sail (which is quite an achievement) and you have a rig which can be managed safely, reefed quickly – and dropped instantly if needed. In that case, I suspect you have acheived at least 90% of what could be possible.
The running downhaul parrels which Slieve designed achieve a similar result (on a sail which has been designed with a conventional planform). It’s a good system, but it won’t work so well on your sail planform and I don’t think it would work very well on Serenity either.
If you over-think, and start trying to mix and match features from different packages, you are going to make a lot of complications – you might gain an advantage somewhere, and lose an advantage somewhere else.
I am not opposed to experimentation, but the aerodynamic theories of Marchaj are a long way from the actual aerodynamics of a little inflatable dinghy with a home-made sail. Your number one priority is sail-handling and safety and you seem to have achieved that very well. Next, you could look at making a better sail and fine-tuning for a slightly better performance, but the difference between good enough – and as good as it can possibly be – will be a fraction of a knot of extra speed – perhaps a slight improvement to windward but likely the difference will barely be measurable.
Yes, the luff of the main should be approximately at the centreline of the mast, with the slot forward of the mast, for the best results (in theory, at least). If you would like to shift your sail forward a few cm and see if it leads to a measurable improvement, why don’t you just shift your semi-rigid parrels aft a little, to bring the mains luffs alongside the mast? (I see from one of the other posts that you have tried to keep these parrels away from the sail panels because you fear that they will tangle with the sail in some way, but I can’t see a problem, why not try it?) If you find, as a result, that the sail is now a little too far forward and maybe causing a little bit of lee helm, then you will have added a little more knowledge to what you have. (In all likelihood that won’t be a problem, but you won’t know unless you try).
Your parrel system lacks any sort of downhaul. Junk sails don’t generally seem to need downhauls (and you currently have no problem with reefing or dropping the sail, so there is probably no need for downhauls on your sail either). It is possible that with downhauls the luffs might straighten out a little and the sail might set a little better – again I doubt if you will be able to measure the improvement – but the simplest way to try it would be simply to add a single simple downhaul to one or two of your battens – and when you have a moment to spare, just put a little bit of tension on them and see what happens, see if the set of the sail improves a little. (Not when things are happening quickly, of course, but when you have a moment to spare). (That’s how the Slieve-type parrels work also, you don’t need to manage everything at once).
The sail you have made is an excellent prototype for trying out these little things, and then, when you have decided what matters, make a new sail with proper cut and good cambers (see below) with confidence that everything is going to work.
If you want to continue with your square top planform, then I would suggest that you stick with your “pistol” parrels, or some other rigid type such as Steve’s “egg formers” as used on Serenity. If you want to use the spanned running parrel downhauls which Slieve developed, then I suggest you would be best to follow the Amiina planform, (the shape and proportions of that sail) and have the mains luffs close to the mast, and keep to the Amiina design package.
Either way, your number one priority in a little dinghy is to be able to reef and hand the sail quickly and reliably, if the wind suddenly gets too strong and everything starts to happen too quickly. I think you already know that.
Yes, I think Paul G has done a lovely job of his SJR and I can't congratulate him enough on his informative and delightful video. What a wonderful suprise to see Poppy in it as well! I am looking forward to seeing Paul's SJR Schooner rig.
With regard to the term "camber" it is probably time to remind people that there is an ambiguity between the meaning of the term "camber" as applied to a conventionally cut sail, and the meaning as applied to a shelf-foot cut sail. Strictly speaking, camber should be measured after the sail has been made, in the way Arne has suggested.
There are ways of calculating how much curvature or "round" to build into the sail panel in order to get the desired camber result.
The question is slightly different when making shelf-foot sails, as a camber is cut directly into the shelf which, after a little bit of simple trignometry gives the camber which the sail would have, in theory, if it were made from something rigid such as plywood or tin plate. But this is not the camber which the soft fabric sail will take up in reality. A shelf foot sail with a "designed" or "tin plate" camber of, say 10%, will have a camber which is different in reality (greater). So do not confuse the actual camber of a sail, with the designed camber of a shelf-foot sail. They are not the same thing, and if you want to use the word "camber" in a way which compares "apples with apples" then you need to measure the camber which is actually formed in the inflated sail panel.