Robert
Your request for feedback deserves a response and as none of the split rig aficionados have yet done so, here are a few comments in the hope of provoking some further discussion. I have recently made an experimental SJR for a trailer boat about the same size as yours, motivated by the same objectives as you have listed and am just a little further down the track than you.
Your second objective (to be able to rig the boat and be up and running in 20 minutes) is where I am stuck at the present time. My goal is 10 minutes. I have not yet achieved even 20 minutes, but still working on it. Evidently the junk rig sits more happily on a moored boat, where time can be spent setting it up initially, whereafter it can be left in its stowed position, ready at all times for a quick hoist. Whereas, a trailer boat of 5 or 6 metres is just at the awkward size where the complete bundle of mast, sail and battens is getting a bit big and unwieldy to strike and keep together as one package. The need for partial or complete disassembly and reassembly between launches, requires a bit of extra thought. Sure, the free standing mast goes up in a jiffy, but from there on things slow down a little. In the words of Phil Bolger, there are a lot of sticks and bits of string and while the JR has some undoubted advantages, for a trailer boat: getting away smartly at the launching ramp appears not to be one of them. I hope to find a way, and will be interested to know how you get on.
Slieve, the originator of the rig on Poppy, together with Edward on the Amiina, have made a short sequence of developments resulting at present in the very parameters you have adopted for camber, sheeting angle and batten rise so I presume you are familiar with Slieve’s writings, and up-to-date with his later developments. Slieve is the obvious go-to person for discussions about further development, although there are others emerging. Also James on River Rat and Dave D. who has converted a Wayfarer have both helped me with comments which relate especially to SJR on their small trailer boats.
Merely copying what others have done successfully is the safer way, which is what I am doing, but with your proposed sail plan you are, as you say, experimenting a little. I have been very interested in the changes made to Amiina’s rig between the MkI and MkII versions, as they are evidently based on experience and reported to have resulted in an improvement. I note that apart from increasing camber and sheeting angle of the jibs, they decreased the over-all sail area, changed from 5 lower panels to 4, but maintained the sail’s proportions, in particular the ratio between height and width of the jib panels (a ratio of 1050/940 = about 1.1) - whereas on Cirrus you are proposing 5 relatively narrow panels, with somewhat unusually proportioned jibs (height to width looks like about .75) and a very large top panel. I would be interested in your reasons for trying this.
I am not at all sure about your suggested batten-to-batten jib leech stiffener, perhaps you should wait and see if there is any need. You may have noticed from photographs that Amiina now carries a tiny sail batten on each of the jibs, which each have a light, standing sheet back to the mast. I am not sure if this slight extra complication has been added to improve performance, or whether it is to overcome a problem from the increased camber and sheeting angle, as it is a slight departure from the KISS principle advocated by Slieve. I copied the Amiina MkII sail design and so far the jibs seem to set quite well without them, but there is plenty more I have yet to learn.
Slieve has always advocated moderation in sail area for the SJR. Some of us have fallen for the temptation to increase sail area. I confess to it, and I suspect James might have done the same (we will see, I am hoping James will post a report when he gets back from his coastal voyage to the recent AGM.) I think Slieve is going to be proved right, in the end, on that point. I think you have gone for too much sail area. After making a free standing mast, SJR sail and a set of battens, I am astounded at how very much heavier the whole bundle is, compared with the rig it replaces. I am now sceptical of the oft quoted adage that you can pile on sail area and the weight aloft and the relatively high centre of area of a junk sail “does not matter because it comes down when you reef.” That might be OK up to a point with a shorter mast and high peaked yard, but not backed up by any calculations, this seems to me to be a rather imprecise statement and might be worth re-considering, or toning down, in a small boat which is easily capable of capsizing. You are an experienced dinghy sailer and I suppose you are looking for good light weather performance. Still, I wonder if it might be an idea to contact John Welsford, the designer of your boat (which you can easily do) and see what he thinks about your proposed small increase in height and possibly not so small increase in weight of the mast and spars, together with your substantially increased sail area and its substantially higher disposition.
I made the mistake initially of placing the yard too high and had insufficient halyard drift. It looks as though your proposed rig is a bit the same. On the subject of drift, I also initially over-looked the requirement for sufficient drift below the boom. In order to reef with the spanned running parrel/downhauls, you need a drift down there of at least half a panel width plus a little more. It looks as though you will be able to place the turning blocks below deck level, so perhaps not an issue in your case.
Your suggestion of making the slot as 6% of chord length yields, in your case, a slot width of about 20cm or 8” which seems unusually large. You might be advised to check with the others before doing that. I think (from memory) Slieve has spoken of using mast diameter as a guide, which might give something like 10 or 12 cm in your case - and I note the slot on Amiina is 15 cm. I don’t know how important the slot width is, but it has been found in the past to add a little confusion to the definition of "balance" when applied to a split rig - with unhappy consequences in at least one recorded case. You probably already know that it is necessary to include the area of the slot together with the "area of sail in front of the mast" when doing the calculation. This so-called “balance” (a misnomer in my opinion) is a sensitive area which has provoked discussion and was perhaps a little ambiguous in the draft notes which Slieve provided. Slieve has since clarified his method for placing the mast in relation to sail area, in a forum post here, dated 24 May 2017 and I presume you are aware of that, and not experimenting too much with that.
I don’t think anyone has advocated the classic “slot effect” theory as a reason for the split rig, and I don't know much about it it, though I can point out to you that accelerating the speed of air flow over a surface results in a reduction, not an increase in pressure. Slieve has given a clear explanation of his reasons for developing the rig and he is happy with the results while acknowledging that other junk rig developments in other directions are also proving to be successful. The SJR (and aerojunk) rigs do give another option regarding mast placement, and do appeal to those who like to experiment.
Best of luck with your conversion. I look forward to your results and I hope some of the of the others who have already built functioning small-boat SJR rigs will chime in.