Small scale split junk rig (SJR) conversion

  • 16 Jan 2023 18:14
    Reply # 13059498 on 13038596

    Dear Graeme,

    Sincere thanks for your thoughtful and detailed reply!! I’ve been mulling things over, and will reply when I’ve explored your recommendations. 

    Thanks again for the advice!

  • 13 Jan 2023 22:50
    Reply # 13056752 on 13038596
    Anonymous member (Administrator)

    Welcome to the JRA, Ignatius.

    Sweet Pea: what a delightful little dory-style dinghy. They were commercially produced in New Zealand and many years ago I acquired one of these (fibreglass shell only) but life intervened and soon after I sold it. I fancied it would row well, and sail well too without the need for a large sail area.

    I don’t think your plans are daft. A skittish little boat like this means, to me, the ability to reef or hand the sail in a jiffy is a requirement. Junk rig is worth the complication to achieve this, in my opinion. You need to give thought to arranging things so you can transition easily between sail and oars, while out on the water. Lazy jacks combined with lifts are a requirement of the junk rig -  I believe they are the key to this.

    If you are convinced that the mast needs to remain in its current position then probably a split junk rig is your only option, and by eye-balling your photo, looking at the proportion of the existing rig, I would say that with the mast in its original position you would be close enough for a 33% balance SJR to work.

    However, you are not quite right in thinking that the SJR is “a good way to get a lot of sail area on to a short mast”. Unfortunately, it is not. In order to get sufficient halyard span for this low yard-angle sail to work properly, the height of the masthead must be (minimum) as high as the peak of the yard, and a little higher would be desirable. This is a limitation on the sail area possible with that mast.

    (To get the greatest amount of sail on that mast, you need a high-peaked sprit rig. But you won’t be able to reef that so easily).

    I have used Slieve’s well-tried Amiina Mkll SJR sail and scaled it (using scale factor about 0.56) to make it match your photograph, as best I can. This gives a sail of about 5 sqm as far as I can work out, without proper scale drawings. It gives you some idea of what it would look like, and demonstrates that a SJR on that mast will not achieve your goal of increasing the sail area of your current rig.

    [Edit: That rig should have CE in about the same position as the original rig, but it doesn't look right - looks too far forward - photographs are deceiving. Need proper measurements, you can't really use photographs for this.]


    If you want an increase in sail area, in that mast position, I believe you will need to lengthen the mast (often done) in order to maintain the proportions of that sail (recommended) - or get another, longer piece of tube. But I would not increase the sail area too much above the 6.5 sq m the designer gave it. An open dory is not suited to a tall rig or too much sail area - and the SJR is a fairly powerful sail.

    But you do have other options. You are not as restricted as you may think, with regard to mast length or mast position.

    It all depends on how you use the boat. Day sailing off the beach, or camper-cruising?

    (option 2) It is not out of the question to install the mast through the buoyancy chamber – with a little bit of creativity it could be done, and without sacrificing the buoyancy compartment. Then you could (would need to) use a conventional, lower balance junk sail – a Johanna rig, perhaps, or one of Paul’s fanned sails. Or the Hasler/Mcleod sail ( from Hum) which you have drawn – although that sail looks to me to be too large for that boat (others may disagree). Anyway, that mast position is quite feasible. 

    (option 3) Another alternative would be to install the mast at the fore end of the buoyancy chamber. In that case the CE will probably be too far forward, but two-masted rigs solve that problem. Two-masted rigs have their own advantages, and are quite popular for camper-cruisers these days. The aft-raking mizzen (slightly offset) on this Drascombe Lugger gives the idea of what the mizzen might look like, and might suit your boat.

    Here's a little two masted open boat which has a full working mizzen.

    The mast heights are kept very low, as befits this sort of boat - and it sails very well. The sails here are Paul Thompson's fanned type. The boomkin is easily removed and stowed, of course

    Or, have a look at some of John Welsford’s camper-cruisers. You would need to do some calculations.

    I think the simplest solution is  option 1 with increased mast length - or perhaps option 2 possibly with the original length of mast as shown above -  the Johanna rig gives more sail area for a given length of mast than the SJR. In any case, if you decide to lengthen the mast, be careful you don’t over-power that boat. Look at the sail plans on dories (usually sprit rigs with very short mast(s)) – the expert on Dory rigs was R. D. (Pete) Culler – look at some of the sail plans in his book “Skiffs and Schooners” to get an idea of sail area and best disposition of sail in a dory. By the way, I think your boat is somewhat similar to the “Swampscot Dory”, the most beautiful of all dories – I think there is one of these in Culler’s book.

    I hope that helps, and gives you some ideas to think about. Perhaps someone else will chime in here.

    Put up a scale drawing showing the original sail plan.

    Last modified: 14 Jan 2023 10:00 | Anonymous member (Administrator)
  • 11 Jan 2023 09:07
    Reply # 13052526 on 13038596

    Hi, I'm new to the Junk Rig Association (JRA) and am following this discussion closely.

    I have a small dinghy that I'm looking to convert to Split Junk Rig (SJR). It is an excellent wee yacht and it gets along well under sail. My only complaint is that the wishbone rig is underpowered (see picture below) - just under 6.5m2. Oh and I've been bitten by the junk rig bug, so am keen to try my hand at converting it.

    I'm interested in SJR for three reasons:

    1) in-built buoyancy means that it is hard to move the mast position forward.

    2) the mast is quite short (4.49m) relative to the boat length (4.1m LOA), and SJR seems to be a good way to get a lot of sail area on to a short mast.

    3) I don't have any good ideas for replacing the mast, and the current mast has a goose-neck fitting protruding out the front (for the wishbone boom) that would foul on a non-split rig.

    I'm working my way through Practical Junk Rig (PJR) and the various articles and newsletters on this site, but thought I'd post here out of excitement of having finally joined the JRA after several years of lurking on the site, and as my conversion slowly becomes closer to being a reality.

    In addition to the picture under sail, I've included a photo of the buoyancy, and one on profile with the image of Hum from PJR aligned to illustrate how short the mast is.

    I'll post again when I've done some calculations of CE etc and with some profile sketches etc. However, please let me know if you think my plans are daft!!

    6 files
    Last modified: 11 Jan 2023 20:50 | Anonymous member
  • 03 Jan 2023 20:04
    Reply # 13043207 on 13038596
    Anonymous member (Administrator)

    Hi Dave, I was speculating on whether it is possible to get too small, with respect to SJR. One would expect a limit as to how small you could scale those panels down to and I did wonder if you might be approaching that limit. I just meant to express uncertainty.

    That trimaran clip you found - a tiny SJR really BELTING along on a broad reach seems to suggest quite small can still work well - at least in a breeze!! I must follow up and look at those other clips. The trimaran is a Bernd Kholer design and the sail is presumably the owner's.

    looking at the other vid clips: Evidently its been around for a few years.

    First hoist: he made the classic mistake (mast too short)

    No acknowledgement of Slieve ("... [SJR] popularised by a guy, I can't remember who it is, somebody over in Europe I'm thinking...") I guess a concept has truly become "main stream" when its originator's name is forgotten!

    The way the panels are setting, and what can be seen of the sheeting all look a little odd to me, some things not quite right in this early trial.

    Anyway, it sure goes well on a broad reach. There are quite a few good youtube clips. We don't seem to see it close hauled.

    Last modified: 04 Jan 2023 22:56 | Anonymous member (Administrator)
  • 03 Jan 2023 10:41
    Reply # 13042541 on 13038596

    Hi, Graeme,

    No, i didn't read anything negative in what you posted.  If you were referring to changing the CLR of the dinghy hull through either centerboard position or helm position, that is completely true and a good point made for sailors who may never have sailed a dinghy, but gone straight into keelboats where crew weight and position changes are less effective, especially short-handed sailors. 

    To reinforce your point, my centerboard brake wasn't working so I was constantly having to push it fully down as it kept creeping up.

    This was apparent as when the board crept up and the CLR moved aft, i had to put in more rudder to counteract the lee-helm, from maybe 2-3 degrees when board was fully down, to more than 10 degrees when the centreboard had crept up and a bit of lee-helm developed.

    I will indeed report further if I manage to get enough time to sail it. You have given me an incentive to do so.

    On the matter of small SJR, there was definitely a smaller one out there.  A small canoe type trimaran, which is where I got the idea to build the main panel using Slieve's panel method, but using 0 degrees sheeting angle.  It made sense to me at the time to use the same system that i was familiar with from the jiblet panels.

    Little Trí

    He has a few other videos from the Florida 120 showing his split junk rig


    Last modified: 03 Jan 2023 11:45 | Anonymous member
  • 03 Jan 2023 09:24
    Reply # 13042489 on 13038596
    Anonymous member (Administrator)

    Thanks for the reports guys - two interesting takes on the concept of small SJR: Dave's Wayfarer and Roger W's Pathfinder.

    There's another one too: Roger S's laser Panic

    Perhaps the thread should have been entitled "Small scale SJRs" 

    Last modified: 03 Jan 2023 09:27 | Anonymous member (Administrator)
  • 30 Dec 2022 12:39
    Reply # 13038954 on 13038596

    The SJR on my John Welsford Pathfinder is 15m2. This is split across four panels. It has been used for thee years now and I am very happy with it. In designing the rig I did settle on the mast position first. As supporting an unstayed mast in a small boat was the primary design limitation. I then calculated the size and balance of the SJR to get the centre of effort of the sail in the right place for the hull. Over the last couple of years there have only been two further modifications made. I have doubled the depth of the sail bag. It needs to be at least twice the depth of the sail bundle diameter. I also reinforced the first Batton because in strong winds when I was reduced to one panel it was bending a bit more than was comfortable. I think I could improve the cut of the first panel and jiblets but it hasn’t warranted getting the sewing machine out yet.

  • 29 Dec 2022 23:56
    Message # 13038596
    Anonymous member (Administrator)

    Some more thoughts during the Christmas vacation.

    I have made a copy of David D’s post on the “O'day Daysailer ll Mod and Conversion” thread, and pasted it into this new thread, because I think it is worthy of a reply. Dave made some interesting points. Here it is:


    While I'm no expert on the SJR rig,. regarding the Aspect Ratio of the SJR, the fact that it consists of 2 separate surfaces, each with their own camber built in, taking each surface on its own, both jiblet and main actually have a very high aspect ratio individually.

    So I wonder if, what might look like a low aspect ratio sail  at first glance, (because we are thinking of jiblet and main as one sail, instead of two separate sails, each with a high aspect ratio), will actually work quite well.

    Another advantage of SJR, i think, is that it is better balanced, in that when the sail is in any position other than close-hauled, the fact that a good portion of the sail is on the upwind side of the mast, producing its own force more inboard than a Hasler/Arne type sail, means that, with less sail area producing force on the leeward side it probably suffers less from weather helm and produces less heel.

    Another thing to remember about a dinghy with no ballast for cruising , as opposed to one used mainly for racing, is that one isn't looking to extract maximum performance at all times, so a smaller sail than the racing size is probably more than adequate for cruising. Extracting the last 5-10% performance to win a race is not necessary when cruising.

    Cruising is about relaxing.

    A taller mast exerts more leverage  in your dinghy, with no ballast to counteract that leverage, so reduced sail size and reduced mast height as a consequence is a bonus.

    Graeme has a lot more practical experience of using the SJR than I ( this year, this year, I promise myself), but the little i HAVE sailed mine with 95-100sqft rather than the usual 140-145sqft usual on the Wayfarer, i didn't feel there was a lack of sail area, solo at any rate.

    If the wind dies, fire up the outboard, or break out the oars.

    Regarding building the SJR, I managed to build mine using Slieve's notes and drawings and quite a few questions on the forum.

    Never having built a sail or used a sewing machine before, the sail still turned out well, setting nicely on the mast

    I used paper and sticky tape to draw out a panel model and get my head around what Slieve actually meant in his notes, which once i begán to understand them, are actually very good.

    Try making a single panel from paper first. That will bring better understanding. You are building both camber AND sheeting into the shape of the upper and lower lense panels.

    The 45 degree shelf makes for a sail which looks better and hangs better when there is no wind than a 90 degree shelf foot which IIRC, was one of Slieve's criteria when he designed it. To look better and less "baggy" and need less wind to fill out.

    It also happens to perform very well.

    So I wouldn't rule out the SJR as a smaller sail, as for cruising you won't be needing a racing sail area and the SJR is a good performer anyway.

    But there are many happy customers for the Arne type sail also.  

    Down to personal preference, and if the mast position and sheeting angles work for you.

    Cheers, Dave D.

    Dave, I have read your description of your Wayfarer SJR conversion in JRA Magazine #79, along with the above post. The magazine article is an honest report and it contains some good information.

     So far, I think your SJR is the smallest made, so it will be very good, once you have had the chance to give it a good trial, if you could report back perhaps on this thread.

    [edit: I chopped a big chunk out of this post because it reads a bit negative, which I had not intended. My apologies.]

     CLR, CE etc issues can be over-ridden, to a some extent at least, by simply shifting one’s weight or adjusting the centreboard. That’s all just part of sailing a dinghy, and shifting your weight forward in light airs may be all you need to do.

    I will be keen to know how you get on. I would also be keen to hear from anyone else with a smallish SJR.

    Last modified: 09 Jan 2023 15:54 | Anonymous member (Administrator)
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