A Question of Balance

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  • 20 Nov 2019 01:50
    Reply # 8130151 on 7843011

    One more thing... I'm noticing that our 'crab claw' upper panels are likely quite forceful in weathercocking the sail as a whole, since their CE is well aft of the mast. Even if the lower panels were approaching over-balance (which I still doubt), the CCs might well be decisive.

    Lower yard angles on 'standard' SJR upper panels would have less of this effect. Accordingly, we may not be as good a confirmation case as I'd been thinking.

    Dave Z

  • 20 Nov 2019 01:43
    Reply # 8130147 on 7843011

    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

  • 20 Nov 2019 00:42
    Reply # 8130068 on 7843011

    Dave you are right when you say "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 am not trying to suggest these percentages (and decimal points) are authoritative.

    But you are also right when you point out later that "... a small adjustment, made unawares, can tip the sail into the fail-dangerous zone..." 

    That's really what I was trying to say, but you have put it better.

    Also we crossed posts as I went back and modified mine with a reminder that over-balance occurs when the sail will no linger freely weathercock when sheets are let fly - "teeter-totter" as you more eloquently put it - and in practical terms this also means overcoming frictional forces - especially the binding of spans which might be a bit on the short side (halyards and parrel/downhauls) and anything else which might oppose the sail freely coming up into the wind when sheets are let fly. And other factors such as the cut of the respective sails - etc etc. I would expect this to be well before the "50%"!  Clearly there is more to it than just numbers such as 33%, 34.4% etc. and at the end of the day, the proof of the pudding is only going to be in the eating. But once that limit is reached we quickly enter the territory of real inconvenience in the case of a large scow - and downright menace in the case of a small open boat. So, for anyone drawing up a SJR for themselves and relying on guidelines such as "33% balance" it is best to be very clear about exactly how that is calculated. I think a good example of what can quickly go wrong may be the narrative in the "Sail Balance - Position Relative to Mast" forum as recounted by Bert. 

    So, I think we should leave it with your words of sound wisdom: "Easy does it!" Thanks.


    Last modified: 20 Nov 2019 01:13 | Anonymous member
  • 20 Nov 2019 00:10
    Reply # 8130045 on 7843011


    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


  • 19 Nov 2019 21:52
    Reply # 8129810 on 7843011

    Dave, thanks so much for taking the trouble to go through those questions so carefully and you have helped to straighten out a few half-formed ideas I had been toying with. Much appreciated.

    From your post (together with Arne and Paul) on this forum we can say that a low yard angle contiguous flat cut sail has been shown to work satisfactorily with a balance of up to 20% and cambered: up to 25% . We don’t know the numbers for the Amoy but her (presumably flat) sails seem to be balanced as high as 30% at least. As you point out, a possible argument against too high a balance for a cambered panel might be the press of the mast into the cambered part, though Paul dismisses this argument (at least up to 25%) and Arne seems comfortable also, at this figure. Perhaps going beyond 25% balance with a contiguous sail is not a good idea, and in any case there is always the issue of “over balance”. (By that I meant the situation where the sail does  not "weathercock" when sheets are let fly).

    Which, 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, that the SJR should be good for up to 35%. I suggest the balance limit has been exceeded at the point when the sail will no longer weathercock when sheets are let fly - and in practice this means overcoming friction, binding of spans which are a  bit short, and any other forces which might act against the sail freely and immediately coming up into the wind.

    Thank you for pointing out my error in using “chord balance” as a lazy way of estimating sail 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 Slieve does it the same way - we are talking about these simple parallelogram panels and I think its valid. Leaving the top panel out of the chord balance calculation results in a slight over-estimation of the sail balance - a conservative error. Using the concept of "jib to after panel balance" will under-estimate the balance as Slieve intended it to mean, which is the opposite of conservative and why I raised 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 might be fairly close to it. The safest way is to include the area of the slot in that part of the SJR sail area which is ahead of the mast centerline (the axis of rotation) when calculating balance. I don’t think that is a quibble.


    By the way, I think the Amoy is lovely too – in fact I did a little write-up about this boat a couple of months ago with some links to further information. It is part of a series I did on “Junks with Chinese Names” which might see the light of day, eventually. I can’t find a way to link to it, so I will just give the links to some source material, including Woodenboat forum which has a couple of high definition photographs of the vessel. There is quite a bit in the JRA archives too. The hi-res photos show a row of double lashing points on each of the upper sails, which suggest batten-lashings rather than reef points. Evidently those battens had been removed.



    Links are: wooden boat forum here,

    maritime museum here

    "Misunderstood Junks" PhD dissertation here

    and JRA Library "Across-the-Pacific-in-a-junk.PDF"


    Last modified: 20 Nov 2019 00:12 | Anonymous member
  • 19 Nov 2019 12:13
    Reply # 8128933 on 7843011

    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

  • 19 Nov 2019 09:38
    Reply # 8128762 on 7843011
    Anonymous wrote:
    I note that you have managed to make good use of a fairly large balance point on the mizzen. I am guessing that caused no problems as it wasn't mentioned and gives the same benefits as of hanging balance as on the main... so fewer lines.

    Hi Len,

    The mizzen panels are 10ft, with the balance 1:4 (20%). We haven't had any problems with it.

    More balance makes for a better halyard to sling point lead, all things being equal, so less load at the short, uppermost batten parrel. I think that makes for less friction raising and lowering. 

    In triangular upper panel(s), a larger balance increases the spread between the yard and uppermost batten parrels, which gives better leverage for fixing those panels in place. So less sloppy.

    Other than mechanical considerations, I doubt more or less balance makes a lot of difference in how the sail would hang, so long as you can get it vertical and keep it there.

    Dave Z


  • 19 Nov 2019 05:55
    Reply # 8128582 on 7843011

    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!

    Last modified: 19 Nov 2019 09:41 | Anonymous member
  • 25 Aug 2019 06:53
    Reply # 7847920 on 7843011

    Thank you Arne and Paul for your suggestions. I think we might be onto something here based on your ideas, Arne. At the moment I'm flat out getting the boat in the water - been antifouling all day between the showers. I would like to draw a sail plan up in a few days time and come back to you on the forum.


    Thanks David for your suggestion about the mast rake. I am reluctant to do this now - crane costs, more spartite etc. but it could be done in the future, if required. We hope to be at the Tall Ships and look forward to seeing you there.



  • 24 Aug 2019 22:21
    Reply # 7847595 on 7843011

    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.

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