Jan: a few things to think about over Christmas.
Here is a scaled down Amiina Mk2 rig with the bottom panel missing. The sail works quite well with one reef in, so this should work.
It is not accurate, because this sort of thing is better done with a proper drawing.
This is enough to convince me that a SJR can fit on your mast, and bring the centre of area of the sail quite a long way forward, as shown. This is as far forward as you will get using a junk rig, on that mast position, and whether it is far enough forward I don't know. (Actually I made a mistake, it can go a little further forward - the vertical line drawn here at the mast should represent the mast centreline, so you can move it forward another one or two cm to coincide with the centreline of your current mast.)
If you want to go to the trouble of making a little SJR, then I think it could be made to work. It would improve the situation. My guess is, you will still have weather helm if the swinging boards are in the vertical down position. I do not think you will get lee helm from the arrangement and should not need to make adjustments to the jibs, nor should you do that.
If the boat still has too much weather helm, there will be nothing more you can do to the sail. In that case I would put slightly longer blades on the swinging boards (they are not actually dagger boards) and deploy them at an angle - I mean, swing them back a little, not vertical.
Like this (exaggerated). Adjust the angle until you get the right helm balance.
This means you can utilise all of the mechanical arrangements you currently have, with no major or structural alterations.
Other comments:
Maybe I'm wrong, but the SJR seems to be a more efficient sail and better balanced. I have already made a model of this sail (made of agrotextile, 1:5 scale), I can also send a photo.
Please do send a photo. I am not sure if the SJR is a more efficient sail, but it certainly does work well if the jib cambers are made accurately. At least, that is what I know of the Amiina Mk2 SJR. That sail is a package, of more careful thought and design than its simple shape would suggest, and I would not attempt to design my own SJR, nor depart from the proportions of that sail.
"Better" balanced? The high balance will improve the situation on your boat with its mast in that position, and I personally prefer high balance (for other reasons), but not everyone will agree that high balance is "better" - lower balance junk sails are more usual.
Be warned: 33% is near to the limit of possible balance. If you make a mistake and end up with balance of higher than, perhaps 35%, you will have a failure. Yes, there is at least one recorded example where this happened, due to not calculating mast balance correctly. On a SJR ignore the slot when calculating balance. The mast centreline should divide the lower battens in the ratio 1:3 and 2:3 Then you will have close enough to 33% mast balance and will be safe.
Less than 30% mast balance for SJR makes no sense to me. I think if the balance is to be less than 30% a contiguous sail might be better. Other designers (such as Paul Th and Arne) have been pushing the balance of contiguous sails up towards 30% and can give better advice than I can about that. To make a SJR that will most benefit your situation, as well as to make the most rational use of the split rig, you need 33% balance. No more, no less. The battens must be rigid and the sail set firmly in position (which it will be with with the parrel-downhauls) - I have not experienced it, but I would not want the sail or its centre of effort to move forward and the rig to become aerodynamically unstable.
I also started making cardboard templates on a 1:1 scale, and bought 6 aluminum tubes 12mm x 1mm x 2000mm each.
It sounds as though you are committed. Good luck.
I'm just wondering how to make batten pockets and batten parrels. It is possible that I will make test batten parrels from a ribbon/rope tied in a knot.
You can read Arne's notes in the technical documents archive on this website, for how to make batten pockets.
I would not use conventional batten parrels for the SJR. For that sail, I think it is better to use running parrel-downhauls as described by Slieve in his notes, in the technical documents archive.
I want to make the batten pockets in such a way that the jiblets can be easily cutted, and moved along the batten if it is necessary to reduce their length/surface. And to reduce the mast balance this way ?
I wouldn't use this project as a platform for experimenting with SJR jibs. Just keep it simple, use the Amiina Mk2 planform which works. Just set the jibs up with a sheeting angle of 12 degrees and a designed camber of about 10%, The mains panels should have a little less camber - say 8%. (In reality, the measured camber of an inflated shelf foot panel will be a bit higher than the designed camber).
I'm still hesitating whether to simply make the first version using the origamii method and make appropriate eyelets on the edges of the batten pockets, which would make it possible to easily replace the panels with various other variants - with a different camber or made using a different method (shelves/barrels).
The origami method may well be suitable for this little project. It looks simple and easy. I have not seen it done so I don't know. You could contact Paul McKay.
If you decide to make sewn-camber jibs (instead of origami) then they are best made using angled shelf foot method as described by Slieve, and for these little jibs I would recommend spend one day and make a "tailer's dummy" as a mould to build them on. I can explain later (or see below), this is just my own idea, not thought necessary by other people, but I found it to be the most practical way to make and then to replicate a relatively small shelf foot sail panel, with accuracy, from very light cloth. Also, once the mould is made, it makes it extremely easy to mark out, cut and assemble and baste together the components of a shelf foot panel, and will probably save time in the long run if you are making three identical panels.
(Use the lightest cloth you can - the little jibs on a SJR sail the size we are talking about here are just pocket handkerchiefs. I looked at some rip-stop spinnaker nylon the other day, 48 gram/sqm looked enough for your small sail. I have also experimented with Tyvek - fabulously light for its strength, also lovely and soft, and has potential if you want to experiment. I made a couple of panels with it as an experiment and was quite impressed, although I think better with lighter stitching than I was using - I couldn't seem to get the sewing machine tensioned right for it, it is so light - maybe the seams could be glued. It is certainly strong enough for a little junk sail. (junk sails generally are subject to considerably less stress than a conventional western lug sail). Tyvek may not withstand too much UV. I would regard Tyvek as experimental, not proven, but with real potential for a tiny junk sail.).
Make sure you mark the jibs top and bottom as you make them - if you don't, then it is easy to get a jib panel upside down when you assemble the sail. (Don't ask me how I know this).
The mains panels can be made with shelf foot or barrel method, whichever you find easiest.
If you use the Amiina Mk2 planform, don't split the top panel. The top panel can be made using barrel method if you want. It is usual to have less camber in the top panel, or probably even flat, if you you like. I think I gave mine about 5% - I don't recall exactly.
(The tailer's dummy is described in this article here. Other people will say it is a waste of time. For a tiny shelf foot sail panel which must be fairly accurate, and will be replicated. such as the jibs on a Amiina-type sail, I would say it will save you time and you will achieve 100% accuracy. If they are not reasonably accurate, and very light weight, these tiny little jibs on a boat your size would be somewhat useless I would expect.
Also: do not increase the sail area shown here, unless you plan to use a taller mast. The mast height is important on this low yard-angle rig, and needs to be slightly taller than the highest point on the yard, as shown in the above diagram. Anyway, the above diagram looks to me as though the sail area is plenty, it looks like an increase on what you currently have. You don't want to over-canvas that little boat and the Amiina Mk2 sail is quite surprisingly powerful. If anything, I would make the sail a bit smaller than shown here, but you will know what best suits your little boat.)
That's pretty much all I can tell you, some of it is personal opinion, based on a certain amount of dinghy sailing, and very limited amount of experience building a sail - and quite a lot of experience at making mistakes in the process! Good luck. And Merry Christmas.
Edit I have pushed the Amiina Mk2 planform at tedious length but for good reasons. This sail reefs reliably, there are no issues with batten stagger, vertical leech ensures sheetlet tangling does not occur, the long yard ensures it never gets wrong side of the mast or the lifts, it does not seem to be prone to "fan-ups", the sail can be dumped into the lazyjacks (or better still a McGalliard-style sail-catcher) without issues etc etc - the stresses on the spars, sheeting forces etc are all very low, nothing other than the running parrel downhauls are needed to keep things in place - it is in fact a model of design harmony. Its brutal simplicity also means replicated sail panels rather than lofting each panel individually, so it is a relatively simple sail to make, as far as the SJR can be simple. I look upon these things through the eyes of a beginner (which I am) and doubt if you could improve on the design. It is a package of interdependent variables, the changing of any one of which, or its proportions, could undo that carefully balanced combination. Like the Johanna planform, it now has a proven track record as a modern cambered sail type that can be successfully made by an amateur sailmaker. The sail has its requirements and does not fit all situations - in fact, generally speaking it is possibly not the ideal planform for a dinghy - but I think it would fit nicely in your situation, albeit very much a miniature and requiring a bit of extra time in the making.