Hi Graeme,

Sounds like we're on the same page. Thanks for your patience with my fumbling attempts to express things I'm not quite clear on myself!

BTW, I didn't mention it in the write up, but that large balance was one of the things we hoped the prototype would proof. We were thinking that we'd reduce it if necessary.

But we were thinking more of the fact that its forward end is overboard by about 1.5ft. We can reach the luff while standing on a small platform between anchor rollers, but it's spooky out there in heavy going. Of course, we've never had to touch the luff in all these years of sailing.

Either way, it seems to have worked out just fine.

Dave Z

Greame Kenyon wrote (in red):

At 34.4%  – you seem to have been the first to push the boundary above 33% and found it OK. This is worth noting. Slieve has always stated, on the basis of his observation of model sail boats, it should be good for up to 35%.

We certainly had no suggestion of a problem in any winds.

Problems would certainly be present at 50% chord balance and above. My guess is that backwinding problems of the forward area would begin somewhere around 40% and rapidly increase to intolerable. So 35% seems a reasonable upper rule-of-thumb.

Myself, I'm more of an 'about yea' kindofa guy. Things look fine at about a third and spooky much above about 40%. The percentages don't say much to me.

Thank you for pointing out my error in using “chord balance” as a lazy way of estimating “McGalliard balance.” (The error of using a ratio of linear dimensions to estimate a ratio of square dimensions.) I had overlooked that. However I think (leaving the irregular-shaped top panel out of the calculation) it is still a valid calculation when analysing the balance of the lower, regular-shaped sail. The vertical dimension is constant and can be factored out in that case.

I think chord balance is the method Slieve is generally using (rather than area balance), and, given the vertical parallelogram of SJR, that shortcut seems justified. So I'll stick with that, for now.

Both yield an 'about a third' result. That seems well within bounds. I'd consider Slieve's 35% as on the long side of about a third, but not yet having left it.

In general, I feel there is a limitation and danger in numeric analysis... the numbers feel authoritative and definite, rather than one instance within a fuzzy threshold embodied within a rule of thumb. I for one, get lost in numbers and sometimes forget to look up. They have their place, but caveat emptor!

In the case of sail balance - essentially one of a teeter-totter with opposed CEs on either end - I note that, as sail balance increases (comparing alternative sails), the CE of the jibs (having less area) moves aft more slowly than does that of the after panels. So the problem of over-balance is approaching critical at an accelerating pace. The first increase has less effect than later increases.

In terms of a teeter totter, we're moving the fulcrum aft, and directing both kids to sit at the mid-point between the fulcrum and their respective end. They'll approach balance more quickly than if sitting at their respective ends. So a small adjustment, made unawares, can tip the sail into the fail-dangerous zone. Easy does it!

Whatever. The important point is: anyone calculating the balance of a proposed SJR needs to be mindful that there is a limit, and the rigs we are talking about are fairly close to it. 

Oi!

Dave Z

Graeme Kenyon wrote (in red):

One of the advantages of my SJR is, I think, that the sail stays put and there is neither the option nor the need to haul the sail forwards or aft in order to ease the helm, according to whether close hauled or eased sheets. There are a number of other good things about this configuration which I would like to retain, but my question is: are they possible without the split?

What I was wondering was: would it be possible to make another sail, identical size and in over-all shape, fixed to the mast with short parrels in the same position as my existing sail – with everything the same, except without the split.

Yes, you can use the same approach. It would hang by the roping at the (unsplit) luff and leach, but still fair from that uppermost batten.

The only caveat is that the mast 'cut' when it's to leeward of the sail will be much deeper in the aerofoil with a larger balance, unless reduced by hauling the sail aft. For close hauled sailing, that is likely to be an issue.

Arne? is this important? I'm guessing it is.

Conventional wisdom says the “chord balance” (33%) would be too great, but I don’t know why, since over-balance demonstrably does not occur when the sail is split. I realise it might be asking for speculation but I would still value your guess.

One of the issues with twisty sails is the reverse twist forward of the mast. Flat cut sails (I believe) develop camber through the amount of twist allowed. Once battens extend too far forward of the mast, though, there's a lot of anti twist to windward... gets to be a lot of weird shaped sail up forward there. It may be that keeping sail balance relatively low holds that weirdness in check?

Shaped sails prefer a straight leach, which implies a straighter luff, so balance proportions seem less important, at least for aerodynamic (vs. mechanical) considerations.

Also, if you are adjusting sail position on the mast, larger balance will mean that sail's CE has a lot of travel. If it stays in one position, not an issue.

So... my speculation is that conventional wisdom is referring mostly to conventional flat cut and/or adjustable (relative to the mast) sails?

That was why I asked Dave what was the balance of his mizzen (though unfortunately Dave misread the question and referred instead to camber.)

Sorry about the mis-read... our mizzen is 1:4 (20%).

I should say that mine are lazy numbers... they are essentially sail foot length proportions, not area proportions. I'm too lazy (busy would be more charitable) to factor in the triangular panel.

Can a low-yard-angle unsplit sail be carried with as much as 33% balance and still be willing to weathercock when sheets are eased, as the split sail demonstrably does? The old junk in the photograph below, seems to suggest it might work – a question which has interested me for some time.

I'd say yes. Equal parts (two of three thirds) essentially balance, leaving the aft third of the total sail area with good leverage to weathercock. No problems for our main.

People will recognize this photo from the JRA slideshow – it is the junk Amoy in 1922, Captain Waard, on arrival in Victoria having crossed the Pacific in 87 days. The sail shape is, in outline, remarkably similar to the SJR sails we are discussing – and the chord balance looks close to 30% or more.

To my taste, maybe the most beautiful boat ever!

(As an aside, if you enlarge the photo and examine the top panels carefully, there is evidence, on both sails, of a previous upper batten, which would have intersected the yard at the forward end – an early version of an adaptation Dave considered but also did not adopt?)

That's just what they would have looked like! I suppose they could be a line for an emergency deep reef?

Finally, hoping this has not become too tedious altogether, and for those who can tolerate a little bit of arithmetic: a comment on the terminology. There is a certain amount of questionable terminology which has historically entered the junk rig lexicon, and become permanent. The term “battens” for the horizontal spars is a harmless example, but the term “balance” in reference to a SJR sail is ambiguous and potentially dangerous. As recent history has shown.

I am sticking to my term “chord balance” by which I mean the ratio in which the lower battens are intersected by the centreline of the mast.

This is easy to calculate and much closer in meaning to the way Slieve defines the balance of a SJR sail, in which the luff of the main is located in theory along the centreline of the mast, and the slot, the missing bit of sail, is considered to be part of the sail area forward of that line. This parameter is not the ratio “jib to after panel balance” which Dave refers to, unless I have misunderstood by taking it literally. This “jib to after panel balance” will definitely under-estimate the parameter as defined by Slieve, and I suppose this is worth pointing out to others like me, making their first sail. (see forum page https://www.junkrigassociation.org/technical_forum/4793670?tpg=8&mlpg=3 for Slieve’s explanation of what he means by “balance” when he quotes a figure of 33% – and prior to that, a narrative of what can happen if it is misunderstood.)

Okay... found Slieve's explanation, but on this page. 

I agree, he's applying the term chord to the total of jib + slot + afterpanel, so I'll quit my quibbling.  8)

Slieve wrote (in blue):

When I draw up a rig, I take the total rig outline, including the slot and guestimate the centre of area at 50% of the total chord, jib luff to main leech, and position it on the centre of area line of the original Bermudan rig. This is not strictly accurate, as the aft part of the sail is taller than the fore part, but experience has shown that it is not that critical, and this is the KISS way to do it. I have then drawn the 33% chord line for the mast and drawn a minimum slot which I suppose is subtracted from the 33% area. All these simplifications move towards a more stable set up. I have drawn rigs up to 35% balance with these simplifications and they have been perfectly stable in use.

By this method, as I understand it, we used 5.5ft / 16ft = 34.37% (linear measurements along the foot). Also perfectly stable in use (not even clear on what "overbalanced" might mean in practice... prone to backwinding?).

To illustrate the point: In the case of the Amiina Mk2 sail plan which I have in front of me, the 3260mm chord (batten length, actually, but near enough) is divided into 940mm to the jib leech, 150mm for the slot and 2170mm for the main. The chord ratio is (940+150)/(940+150+2170)=33.4%   This is pretty close to the 33% sail area balance you would get if the slightly disproportionate top panel were included in the calculation.

However a “jib to after panel balance” of 33.4% (again leaving out the top panel for simplicity) would mean, say, jib area 4 sqm and after panel area say 8 sqm, so 4/(8+4) = 33.3%  However, when the area of the slot is taken into account (say 150mm x 4.2m = .6 sq m) the balance as defined by Slieve is now (4+.6)/(4+.6+8)= 36.5% which, if my understanding is correct, is getting into serious pioneering territory. These two parameters would be the same only if the slot width were zero. The small difference nicely reflects the extra moment of jib forces as they are moved out along the battens. Even if the “jib to after panel balance” remained constant, the wider the slot, the greater the “chord balance” would be – to the point where over balance might occur and letting fly the sheets would fail to depower the sail.

Dave: please tell me I am wrong and that in fact your “jib area” does actually include the area of the slot. Or, alternatively, that you have just established a new threshold of greater than 36% McGalliard balance!

Umm... the 33% reported in the write up does not include the slot, but the 34.37% from above does. If I'm doing it right.

But my reading of Slieve's method is that he's using % of total chord (not % of area) as a shorthand. This churns out different percentages as one is a ratio of linear values, while the other is a ratio of square values.

I'm not really an avid numbers guy, and get lost easily. I apologize for any misleadings.

I did work up the CEs for main, mizzen and combined then placed the OCBoards by eye. This seems to have worked out in good balance. We haven't had to adjust the boards or panels to correct imbalance. Slight weather helm on and off the wind, that picks up with wind strength, but quite manageable if reefed to suit.

So if it's new territory, it seems okay.

Boldly going...  8)

Dave Z

Three months later, and picking up on a discussion which was commenced in the forum at https://www.junkrigassociation.org/technical_forum/3399298#8128564

One of the advantages of my SJR is, I think, that the sail stays put and there is neither the option nor the need to haul the sail forwards or aft in order to ease the helm, according to whether close hauled or eased sheets. There are a number of other good things about this configuration which I would like to retain, but my question is: are they possible without the split?

What I was wondering was: would it be possible to make another sail, identical size and in over-all shape, fixed to the mast with short parrels in the same position as my existing sail – with everything the same, except without the split. Conventional wisdom says the “chord balance” (33%) would be too great, but I don’t know why, since over-balance demonstrably does not occur when the sail is split. I realise it might be asking for speculation but I would still value your guess – or an answer from anyone else who might contribute. And of course, there may be reasons other than over-balance why such a configuration is no good – if so, I would like to know. So far, I have not seen discussion of any figure more than 25%).

That was why I asked Dave what was the balance of his mizzen (though unfortunately Dave misread the question and referred instead to camber.)

Can a low-yard-angle unsplit sail be carried with as much as 33% balance and still be willing to weathercock when sheets are eased, as the split sail demonstrably does? The old junk in the photograph below, seems to suggest it might work – a question which has interested me for some time.

People will recognize this photo from the JRA slideshow – it is the junk Amoy in 1922, Captain Waard, on arrival in Victoria having crossed the Pacific in 87 days. The sail shape is, in outline, remarkably similar to the SJR sails we are discussing – and the chord balance looks close to 30% or more. (As an aside, if you enlarge the photo and examine the top panels carefully, there is evidence, on both sails, of a previous upper batten, which would have intersected the yard at the forward end – an early version of an adaptation Dave considered but also did not adopt?)

Finally, hoping this has not become too tedious altogether, and for those who can tolerate a little bit of arithmetic: a comment on the terminology. There is a certain amount of questionable terminology which has historically entered the junk rig lexicon, and become permanent. The term “battens” for the horizontal spars is a harmless example, but the term “balance” in reference to a SJR sail is ambiguous and potentially dangerous. As recent history has shown. I am sticking to my term “chord balance” by which I mean the ratio in which the lower battens are intersected by the centreline of the mast. This is easy to calculate and much closer in meaning to the way Slieve defines the balance of a SJR sail, in which the luff of the main is located in theory along the centreline of the mast, and the slot, the missing bit of sail, is considered to be part of the sail area forward of that line. This parameter is not the ratio “jib to after panel balance” which Dave refers to, unless I have misunderstood by taking it literally. This “jib to after panel balance” will definitely under-estimate the parameter as defined by Slieve, and I suppose this is worth pointing out to others like me, making their first sail. (see forum page https://www.junkrigassociation.org/technical_forum/4793670?tpg=8&mlpg=3 for Slieve’s explanation of what he means by “balance” when he quotes a figure of 33% – and prior to that, a narrative of what can happen if it is misunderstood.)

To illustrate the point: In the case of the Amiina Mk2 sail plan which I have in front of me, the 3260mm chord (batten length, actually, but near enough) is divided into 940mm to the jib leech, 150mm for the slot and 2170mm for the main. The chord ratio is (940+150)/(940+150+2170)=33.4%   This is pretty close to the 33% sail area balance you would get if the slightly disproportionate top panel were included in the calculation.

However a “jib to after panel balance” of 33.4% (again leaving out the top panel for simplicity) would mean, say, jib area 4 sqm and after panel area say 8 sqm, so 4/(8+4) = 33.3%  However, when the area of the slot is taken into account (say 150mm x 4.2m = .6 sq m) the balance as defined by Slieve is now (4+.6)/(4+.6+8)= 36.5% which, if my understanding is correct, is getting into serious pioneering territory. These two parameters would be the same only if the slot width were zero. The small difference nicely reflects the extra moment of jib forces as they are moved out along the battens. Even if the “jib to after panel balance” remained constant, the wider the slot, the greater the “chord balance” would be – to the point where over balance might occur and letting fly the sheets would fail to depower the sail.

Dave: please tell me I am wrong and that in fact your “jib area” does actually include the area of the slot. Or, alternatively, that you have just established a new threshold of greater than 36% McGalliard balance!

Hi Rob,

it looks as if you have installed the new mast vertically. Is there any possibility of adjusting it to give some forward rake? This would move the center of effort of the sail forward and would mean you would need less balance area. How tall is the mast? Can you make a higher aspect ratio sail that effectively increases the balance by reducing the area aft of the mast and consequently moves the center of effort further forward.

All the best with the project and I hope to see her sailing in the Tall Ships in January!!

David.