Sail “balance” – Arne refers to it as “mast balance” and this is good terminology in my opinion as it is less likely to be confused with other uses of the term “balance”. Anyway, whatever you want to call it: strictly speaking it is the percentage of sail area which is ahead of the mast centre line.

With some sails, chord length varies at different points on the sail, and sometimes batten lengths do too. 

However, unless the sail has a very simple shape it can be tiresome to calculate this ratio in terms of area. Using chord (or batten length) is much simpler and gives a result which is close enough. Most people just look at the chord of the lower panels, or the lower battens, and calculate the percentage length which is ahead of the mast centre line. Much easier than calculating area.

Just a word on SJR – it has been repeated and can’t be repeated too often – when you are close to the 33% balance, as one probably would be with that type of sail, you are near the safe limit and need to be careful not to under-estimate this ratio. Here is what is sometimes overlooked: In the case of SJR the “sail area” also needs to include the area of the slot as if it were part of the sail -  not just the cloth part. Otherwise the calculation will be incorrect and the sail might be “over balanced” and unsafe. If there is any doubt it is much safer and far easier to use the chord length or batten length as the basis for calculation. Although it will normally be very slightly inaccurate to do so, because the sail won't be a simple rectangle - the error is on the safe side as it will slightly over-estimate the actual ratio of sail area ahead of mast/total sail area.

In summary: using batten length or chord length of the lower panels as explained by Paul and Arne is the safest and most practical way to estimate this ratio of areas.

Paul's diagram showing how this "sail balance" (or "mast balance") ratio is related to yard angle (for a given sling point) is an excellent explanation for that question, in my opinion. Very clever to express it with such clarity. You could move the sling point, of course - and very minor adjustments can sometimes help - but if you get too far away from the geometry which Paul's diagram demonstrates, then much greater parrel hauling forces are necessary to make the sail sit where you want it to sit, to the point where it might not be at all satisfactory.

I believe greatly in doing things for myself (DIY) as much as possible, but when it comes to sail design I would always prefer to closely follow the advice of people like Arne or Paul (or, in the case of SJR, Slieve) who have built up a sound body of experience, and not try to do that part myself.

Anonymous wrote:

Hi All,
I have two questions that no doubt been covered elsewhere but either I have missed them or forgotten:
1. Is the balance a percentage of the length of the batten (assuming all battens are the same length) or a percentage of the total sail area?
2. Is there a reason for reducing the angle of the yard as the balance increases? Other than the reducing the amount the height of the mast has to increase as the sling point moves further up the yard.
Thanks in advance,
Adrian 

Adrian,
I guess the answer on question #1 varies a bit.
For my part; I reckon the percentage of the length of each batten forward of the mast. Since I generally operate with parallel battens (the boom and 4 lowest battens), this makes some sense on the sails that I design.

The angle of the yard has to be lowered as the mast balance is increased, or else the halyard's slingpoint on the yard will hit the mast, or even pass it.

Arne

PS, 20240718:
This diagram, showing four sails with identical lower sections, but with different top sections, shows how different yard angles suit for different mast balance.
Better look up the full size diagram on my...

Member’s album, Arne’s sketches, section 6-44

There are some great photos of junks sailing down wind, but this was the one which first excited me about the junk rig. Mind you, any lug sail is superior to these other things,  in this particular situation. Not much of a photo but worth a second look - it's rather striking in its own way, is it not?

I rather prefer the higher balance sails, in the single-sail category, at least for an inshore cruiser - but it's for particular reasons, and not a primary requirement.

A couple of comments. Asmat (if I understood your comment correctly) considering that "split junk rig" seems to have become standard terminology for a particular type of junk rig, it might be better to refer to ketches, schooners, yawls etc as "divided" rigs (as they used to be called) rather than "split" - just to reduce the potential for mis-understanding. If that was what you meant. The other point which is worth repeating - Arne: as you know, 35% balance is theoretically possible and Slieve has "proved" it within the domain of model racing yachts. But considering the chances of spars bending or other unforeseen factors, including just plain miscalculation or measurement error, for anyone considering SJR, until someone brave enough has actually tried it, 35%  mast balance should be regarded as being on the brink of disaster in my opinion. While I would not go less than 33% for a SJR (otherwise what's the point?), I would also not go higher than a fairly carefully (and correctly) calculated 33% either. A runaway junk would be quite  a spectacle (from the shore!) and it has happened. Not trying to teach Arne anything, but for readers out there who might be thinking of this or reading about it for the first time....

I agree the jury is still out on whether the somewhat specialised SJR has any measurable performance advantages over the other modern cambered rigs, but I do know it is very good if properly designed, and if some proper match racing could ever be organised, I would enjoy to place a modest bet on the SJR (but not too much!). If for some reason the high mast balance is required (either for personal preference, or to match some other requirements such as weight distribution or internal layout etc), the SJR does provide a valid choice. It may well be that Arne and Paul are on the right track too, as they continue to push that mast-balance/yard-angle boundary. That incremental type of development being discussed here is praiseworthy and very interesting I think.

Anonymous wrote:

Conclusion, so far:
If these high-balance sails (up to 30%, so far) do not show any snags in handling or performance, I think I will advocate the use of them, in particular on sails above 30sqm. 

So - the UNsplit junk rig?