First of all, I have to say that the “top 3 panels SJR” that I superimposed on your photo just doesn’t look right. As I explained, it was drawn purely to make the point of where the mast might be placed to accommodate a 33% balance sail, with its geometric centre coinciding with the geometric centre of the old sail plan. And I was trying to copy the outline you had sketched. Personally, I don’t like that ultra low aspect ratio plan form, its too much of an extreme, unless you have a special reason for wanting to get that much sail area on one relatively short mast. I take it that was not your intention, so I will leave it there, with the summary that a higher aspect ratio, more normal looking split junk sail – with the geometric centre at close to that spot, will dictate a mast position about where shown, or very slightly aft of it if it is higher aspect ratio.
So what do we learn from this? The highest possible sail balance (about 33%) will dictate a mast position very close to or right on the position of the current mainmast – as far as we can judge from putting rough sketches onto a photograph. (Or, directly each side of that position if you are thinking of a side-by-side, or "biplane"). That’s about as far back as you can go on that boat. 33% is probably too much balance for a contiguous sail, so if that mast position (or mast positions if side-by-side) suit you (taking account of internal layout, position of bulkheads and structure for supporting a free standing mast etc) then a split junk rig is probably dictated.
Let us go to the other extreme now and consider a 10% balance sail, and see if we can find the other end of the spectrum of mast positions. This would be a contiguous sail of some kind, for sure. The principles for mast placement (with respect to geometric centre of the original rig) may be slightly different for a contiguous sail so you should get advice from someone like Arne, but I can give you a very rough indication of the range of mast positions available to you for a single mast junk rig (or side-by-side configuration) with the following diagram.
On the left, a 33% balance, low yard angle sail, mast positioned approximately at the original main mast position. Split sail. It’s a copy of Amiina Mk 2 sail designed by Slieve.
On the right: a 10% balance high yard angle sail, mast positioned somewhere between the first two fore cabin windows. Contiguous sail. its one of Arne’s, a low aspect ratio (1.8) Johanna type.
I haven’t bothered looking at sail area or done any calculations. Its purely to give you an apples-with-apples comparison so you can see the implications of sail dynamic balance, with regard to approximate mast position on that boat, so you can judge if you think a junk rig could be installed on this catamaran. To help you to decide if this catamaran is going to be the one for you. Whatever rig you choose, mast position will likely be within the range shown in the above diagram. (The masts shown here are about 10 M above top of tabernacle, but would be lower of course if the rigs were biplanes. And you might note that the high yard angle sail has a little more area than the low yard angle sail for the same mast length).
You could crib the mast just a little bit further forward, with an even lower balance sail, if you wanted to talk to David about his Weaverbird soft wingsail, which is, I would say, leading edge in junk sails.
(Imagine a side-by-side junk wing rig, how cool that would be. A real biplane). I expect the mast(s) would be close to the forward cabin first window, in placement, in the case of a weaverbird sail, as it has only a very small percentage of the sail area in front of the mast (ultra low balance).
So there you have it. That’s about the range of choice you will have.
If you need to get the mast right at the front of the cabin, as you originally proposed, then I think you will need a mizzen mast to make it balance. (ie helm balance).
You are still a long way from designing an actual sail plan, and there are others more expert who can help you with that.
Before you can get down to something more precise, there are some things you should do if you can: (1) sail the boat a little bit with her existing rig, and make sure you are happy with the helm balance. If the boat seems to have lee helm, or excessive weather helm, the time of changing the rig will be the time to fine tune where the new sail geometric centre will be positioned. (2) Obtain, or make, a sail plan showing the current rig, with accurate measurements of the sail dimensions and the precise position of the current masts. These two steps will give you or the sail designer the ability to pinpoint exactly the geometric centre of the current rig, and provide a good starting point in the design of a new sail plan. A photograph is not really good enough.
(Sail area, aspect ratio, sail type etc will also make tiny changes to where the ideal mast position should be. The designer of the sail you choose can help you with that, when the time comes).
You ask whether a split junk rig would be suitable for a twin-mast side-by-side or "biplane" arrangement. Well I would rather Slieve answer that question, I can only make a couple of comments and give you an amateur’s opinion. When Slieve designed the SJR one of his goals was to get good camber and lift in the very front part of the sail, and try to get efficiency to windward comparable to a modern Bermudan sail. I would say he has got pretty close to it. Anyway, there is no point in it if there are two sails and the airflow of one interferes with the other - which is something to consider in regard to ketch or schooner rig. But, and you can check with Slieve, I can’t see how there would be any problem there with side-by-side sails. On a broad reach, maybe? Perhaps someone will chime in and advise.
Anyway, it has been done. The first junk I ever sailed on (only a few years ago) was just such a rig. I had the pleasure and privilege of an outing on Oryx, at the time owned by Pete Hill who had designed the rig, with some input from Slieve I believe. It is quite a different type of split sail from the SJR (its a type of "aerojunk" rig) but it was a junk split sail biplane catamaran, and it went like a rocket.
Note the very low rig, but ample sail area. These masts are housed in sunk tabernacles which Pete designed and built.
I suggest you contact Pete and ask him if there are likely to be any issues with a biplane SJR. My own unqualified opinion is: if all the other factors harmonise (internal accommodation, deck structure) I would say it’s a great idea. You get a low rig, with the sail area divided between two rigs – the sails can be optimum aspect ratio and each sail smaller and easier to handle than one larger sail, and you could cram on a little extra sail area too. No doubt there are pros and cons of biplane rig catamarans, but there are enough of them around you should be able to contact one or two people who have actually done it and can advise you.
Hope that helps you to make a decision.
One further comment:
Ivan wrote: "When I look at smaller ~20 foot mono hulls, their JR masts seem small and simple and light. Easier to handle, cheaper, install etc. "
Yes, but be careful of drawing conclusions there. You might have a pair of masts that are the same length as a 20' mononhuil mast, but they will need to be stouter. A 20' monohuil heels over in a gust - your boat won't, it will be stiff and will require stronger masts than the equivalent length mast on a light monohull. I doubt very much if twin masts would be a cheaper option, but I expect it would give you ease of handling in some respects, because dealing with much smaller sails, easier to hoist etc.
Edit: looking ahead to the next post (from Mark) suggesting try sailing the boat without the mizzen, in order to asses whether it is actually necessary. An excellent suggestion, further reason for getting familiar with the existing rig before making changes. If that boat goes well to windward without the mizzen, then you could consider placing the geometric centre (and hence the mast(s)) of the new rig further forward.