Jamie, you wrote, 'The part of the jib camber shape and the arc of a circle -idea is something I can’t understand, though. Could you shine more light on this? '

Em? I'll try. I can list the problems, but am still looking for the answer.

Overall from jib luff to main leech the rig is an airfoil. The desired shape for a foil, even only one canvas thick is fairly well known, with a rise angle, maximum camber point between 30 and 40 % chord and ending in a straight run to to the leech. When you split the foil, with jibs and main then there is twice the problem in deciding each foil shape separately and how they combine to make a combined foil. Experience from Bermudan rigs is a great help here, but, and this is the problem, but the angled shelf foot jib panels in the SJR cannot be tensioned down the leech so will sag away, and effectively change the shape of the resultant airfoil.

Initially I built the jibs using a typical Bermudan jib shape, and as expected the leech fell away in the middle so that the shape of the leech when viewed from the rear was a fairly smooth curve and not be what was wanted (which was Graeme's tin-plate shape). Still I had to start somewhere. Many experiments were tried with mini sheetlets pulling the middle bit of the leech either aft or towards the battens with mixer results. The Mk2 Amiina rig has batten pockets included to see if the shape could be forced closer to the shape we thought would be best. A lot has been learned, and the jibs were modified before the original draft notes on the rig were written, and a correction was included which helped. As ever, simplicity is a key issue, and I have been wondering if a good/ adequate shape would result from a camber drawn as a simple arc of a circle. Easy for the builder, it would be only after making a model or two, as I did for the first rig that it might be able to see how it would look. At a guess it might require a slight (1 deg) increase in sheeting angle.

Having retired from sailing I'm not rushing to start more experiments, so I'm afraid this is only theory.

As for the Galion 22, until more is known I would suggest sticking to the early section with modifications as it is known to work.

Does this help, Jami? (Oops! I've just realised that in the past I may have spelt you name with an 'e' on the end. Sorry. Jamie is the name of my most troublesome grandson).

Cheers, Slieve.

Slieve thanks,

I really appreciate your insight.

This is fully in line with my vague thoughts - it’s not worth it making new jibs unless the change compared to the existing ones is big enough. I was thinking maybe 1000mm chord, but 1100 is probably better to go for - the effort is essentially the same.

If I sew new jibs, and get what we want from them,  this missed shot with too small jibs has at least proved the theory of the essential SJR jib size in practice. Maybe it will help others in this community.

The part of the jib camber shape and the arc of a circle -idea is something I can’t understand, though. Could you shine more light on this?

Me too. In the meantime, here’s a little bit more food for thought:

Jamie wrote: …should I make new jibs. I might need 30cm extra on the luff in order to make any real change worth the effort. And this might turn the boat into a lee-helm monster…

Now you are talking about a different issue -just to keep the terminology clear: a change in the nature of the sail, or its placement, may affect helm balance. Whether the boat has lee helm or weather helm etc.

Wheras, the previous discussion was about the dynamic balance of the sail – sail balance – the ability of the sail itself to rotate and “weathercock” when the sheets are let fly.

Helm balance is affected by a lot of things, CLR and CoE (which are static, geometric points) are a guide but not the whole story. On a small boat like yours, trim and angle of heel are likely to affect the helm, and also provide ways of balancing the helm. My little boat has a little lee hem in light conditions, but as wind increases and angle of heel increases, weather helm develops and becomes increasingly heavy – if I don’t reduce sail it eventually becomes quite “hard nosed”. This is not unusual, and for me is acceptable. It is something which depends quite a lot on hull shape. I have found shifting my weight, moving forward or aft in the cockpit, greatly mitigates these effects. My Pelorus has heavy weather helm when pressed hard - and I can mitigate that by reducing the area of the stay sail (which, theoretically, moves the C of E aft. Think about that for a moment). The point being, helm balance is not just a simple matter of balancing a couple of geometric centres on a sail plan drawing. (How much "lead" of CoE over CLR - that's the real question, and it seems to be a bit of a magic art !)  It is a great deal to do with the shape of the hull, not just its lateral profile. And rudder too. (I'm sure your improved rudder will be an asset in any case).

Making new jibs, shifting the luff forward 30cm and extending the battens accordingly will move the so-called Centre of Effort (actually the geometric centre of the sail) forward 15cm if nothing else is changed. I would be interested in the comments of people like David, Slieve or Arne who know more about these things than I do – on what would be effectively shifting the rig forward by 15cm.

Making the jibs more powerful – now, this is something that needs a little thought, and maybe Slieve’s knowledge of aerodynamics might help here. If the jibs on a SJR are made more powerful, would this result in the actual centre of effort of the sail moving forward? Maybe it is more important to consider that, than the relatively minor effect of effectively shifting the rig forward by 15cm.

(It’s a pity you can’t really make adjustments to mast rake, which is a convenient way of tuning the helm in some cases. One of the few small disadvantages of Slieve's SJR design, with its “self-draping” sail geometry, is that you are pretty much stuck with a zero-rake mast.)

I still think that in a small boat there are so many factors that can affect helm balance, my gut feeling is, making new jibs would not turn the boat into a “lee helm monster” – unless it already has lee helm. Anyway, you know your own boat, its probably a judgement call only you can make, after getting good advice from Slieve.

...I'd rather ... find me a cheap Albin Vega... Well, I'd rather consider a ittle riding sail, if the worst came to the worst. I don't think it will, and I don't think it would be necessary to shift the mast for the sake of 15cm. I'll say one thing though, since adjusting mast rake is not a practical option with SJR of the type designed and rigged  by Slieve -  if I were building a split junk from scratch (which in fact I am) I will make elongated partners and allow for a little movement fore and aft of the mast position which would not involve major structural changes,  should that later prove necessary.

Hi Guys,

All being well I'll get a post in here in the next two days, which might help.

Cheers, Slieve.

Hi Jami. At one time I began collecting numbers, wherever I could find them, relating to the split junk rig. I didn't get very far, but here is what I have at present: (you can click on it to enlarge)

(As you know, these shelf foot sails have a designed camber (which would be the camber if they were made from tin plate) and useful enough when comparing shelf foot sails with shelf foot sails. However the actual camber of the soft cloth sail will be greater, and if comparing with conventionally cut sails, the camber needs to be measured when the panels are inflated, the way Arne does it.

No-one seems to have done it yet with a SJR panel. I just need to add that disclaimer).

I am very happy with the way Serendipity sails, including to windward, but we don't yet have much in the way of objective comparisons. Emmelene looks to be sailing beautifully in the photo in the current home page "featured boat" and Chris is happy. Amiina (the model for most of the subsequent SJR rigs) has proved successful. The parameters for your sail would seem to be within the "successful" range, so if you are a little unsatisfied, maybe we need to look for other parameters which are not shown here.

The one which comes first to my mind is "balance". I have a feeling that when going to windward the lion's share of the work is being done  by the jibs, and the wash from the jibs may be (not sure) may be slightly degrading the performance of the mains panels, because on my boat it is the mains which begin to lose that pressure difference first, when pinching up to windward. It seems to me the important point about the SJR is that to get the best from it, the jibs need to be as large and powerful as possible (the limiting factor being the sail's dynamic balance point). This notion is reinforced, in my mind, by a high performing bermuda-rigged trailer boat I once owned (a Noelex 22) which went to windward like a witch, in any decent sort of breeze, on staysail alone - and with beautiful helm balance too. The stay sail in this case was a 3/4 rig, but with fairly long foot - not quite a genoa, but it was a powerful little staysail and it pulled like a mule. (The hull was superbly proportioned too, of course).

Therefore, it seems important to have as great a jib chord as possible. So, the balance ought not to be too much less than the proven 33%. (And for those who are contemplating a SJR for the first time, not too much more either! Be sure you understand the balance calculation for SJR - it is not quite intuitive, and a mistake here could have consequences as 33% is probably not too far from the limit of what is workable. Sorry Jami, you will know all this this, its just another disclaimer which needs to be put in here.)

As you will know, if you were to increase the jib cord on your sail, this may affect the balance calculation, depending on where the extra cloth is placed. If the extra cloth of a new jib were to be added to the leech (reducing the slot width) then the balance calculation won't be affected. If the extra cloth is added to the luff and the battens extended forward accordingly, then the calculated balance will be increased, and the proven limit, to date, is 33% - though we are all waiting for someone to try 35%, which Slieve has found successful on model swingwing racing boats. If there is room to move, within the envelope of your sail plan, then new jibs and extended battens might be worth the shot. It would be interesting to know what is the current calculated balance of your sail, and the slot width - better still to see a diagram of your sail with all these measurements. A diagram would save a lot of words.

A "thought experiment" suggests to me that if the jibs are made ridiculously small, (say, a balance of 5%) they would merely interfere with the main, and in that case the boat might sail better without them. I am not saying for one moment that your jibs are "ridiculously" small, but they may be a tad too small, and it would indeed be interesting to follow your suggestion, and before the season is over, just for fun, try your boat for sailing without them. I expect a SJR with no jibs would be at least as good as any other cat-rigged sloop, sail area for sail area.

My uninformed suspicion is, maybe the SJR really needs that 33% balance to perform at its best, and maybe if there is only room in the design for 25% balance, or less, then a contiguous sail might be a better choice. And it would not surprise me if you find, with larger jibs chord, that your boat would be more powerful to windward.

Its one of those many things that we won't know until someone tries. We all still have much to learn. 

(PS the main panels collapsing first, when coming up into the wind, is normal I think. The two-part sheeting for this type of sail works very well on my boat, not really any extra hassle and perfect for controlling twist).

(PPS for those contemplating SJR for the first time, all this talk about balance is fairly important, because the sheeting forces are quite light on a SJR, and if you go over the limit with balance there is a risk that in a high wind it might not be possible to luff up or to let fly the sail! A runaway junk, helmsman with eyes on stalks, scrambling to the windward side to stack out for dear life while at the same time trying to put the helm down, would be an impressive sight - I wouldn't want it to be me!

There has been a suggestion also, that if the battens start bending, this might exacerbate the problem - so we don't want to be right on the limit with balance.)