Sail Balance - Position Relative to Mast

  • 08 Dec 2019 10:13
    Reply # 8212997 on 4793670
    Anonymous member (Administrator)

    Graeme,
    you surely are right about the boat, it was a boat in the ‘coin’ series and had nothing to do with the Tom Thumb design.

    Since I don’t have hands-on experience with the SJR, I certainly don’t claim to know much about it. However, I notice that Bert made two changes to his misbehaving sail. He both reduced the size of the jiblets, and he widened the sheeting angle of them. I bet that the dominating factor when taming that sail, was the size reduction. My (armchair) idea was just that if the jiblets grow until the sail approches the point of instability, the sheeting angle and camber of the jiblets may play in.

    You mention that the jiblets are backwinding the main a little when the jiblets are correctly set. That may not be such a big problem. Many Bermuda rig sailors let the mainsail be a little distorted like that by the airflow of the jib, when fully close-hauled.

    As for your question about the maximum tolerable balance of a one-piece sail, I have never been near the point of instability. My original concern when fitting a JR with cambered (bulging) panels was about the mast distorting the camber on one tack, so at first I aimed for just 10% balance. Later experience gained by Paul Thompson (18 and 22% balance in La Chica) has made me relax that “10-percent rule”.  My sails, with 70° yard tolerate up to 17% balance, and for me, that is enough. I suggest you read Asmat Downey’s posting from 11. October ’18. It appears that the performance of the sail on the mast-to-leeward tack drop when the balance is twenty-some percent.

    So I’ll stick with my practice of rigging my sails with less than twenty percent balance. This allows me to set the yard at 70°, which lets me set the biggest area (and have the longest luff) with the shortest mast. That 3-panel top section has proven to be quite efficient on several boats now.

    Question:
    When you gradually head up into the wind with Serendipity, which part of the sail appears to be luffing first  -  the jiblets or the mainlets?

    Arne


    Last modified: 08 Dec 2019 13:12 | Anonymous member (Administrator)
  • 08 Dec 2019 03:48
    Reply # 8210435 on 4793670

    Hi Arne I have probably already gone on about it too much, and I am sorry to keep on, but I must comment on your SJR suggestions, because I am not sure if you are correct there, and I think not.

    You may be right on the one hand by saying that these are "other factors" which may impede the sails ability to weathercock, but I do not think they were the important factors in the case you are are referring to.

    I THINK the boat you were referring to was one of the "coin" series - a "farthing" if I recall, the owner was Bert. The entire conversation can still be found many pages back, near the start of this thread and I have gone back and checked the following: There were two problems (1) lee helm (2) a refusal of the sail to "weathercock". A pretty bad combination. A lot of theories and suggestions were floating around, but Slieve got to the heart of (2) by diagnosing the balance problem as a misunderstanding of his way of calculating balance, and ending up with too much sail area in the jibs. Slieve wrote at the time "Looking at the photo in Bert's photo album it is difficult to see how much camber has been built into the rig, but by taking simple horizontal measurements from the photo it would appear that in the split panels the jibs have 33% of the total area but have been moved forward by the width of the wide slot so that the mast/ pivot appears to be at over 38% of the total jib luff to main leech chord. It is not surprising that this is unstable. It would be interesting to see a copy of the rig and hull drawings to see exactly what is happening. " Bert had thought that his balance was 33% but admitted in a later post that when doing the calculation properly it was 37% so if Bert was correct about that, then Slieve wasn't far wrong. 

    In this thread Slieve also made a lengthy post in which he outlined how he had intended "balance" to be calculated in regard to SJR. If you take the JRA definition of balance literally, and calculate a 33% balance purely on actual sail area, you will get more area in the jibs than Slieve intended, and to make matters worse, those large jibs are then placed further forward, by the width of the slot. 

    Bert said at the time that his jib sheeting angle was 8 degrees, which is probably inefficiently low, but I doubt if that was any part of the problem, I think Poppy's was about 8% if I recall. (Slieve advised me later to use 12 degrees as they did successfully on Amiina's  second iteration, and it is possible that more might be even better. But 8 degrees should not have been the problem.) Nor IMHO was the problem primarily caused by the mains luffing before the jibs did, as I will explain later. Perhaps these things might exacerbate the problem, in that way you might be right by raising it.

    As a follow up, Bert posted later and said that he had made a complete new set of jibs, said that his new jib sheeting angle was 11% (I presume he meant 11 degrees) that he had reduced the area of his jibs by 2 sq m (probably more of a reduction than needed) but anyway he was able to report "Balance is right now, no leehelm anymore, no backwinding of the main. Jib sheeting angle is 11°. " And that is where it ended.

    In my SJR (33% lower panel chord balance) even with 12 degrees of sheeting angle and the jibs having more camber than the mains (deliberate), the jibs still seem to back-wind the mains slightly, at times. And close on the wind, the mains always seem to collapse a little at the luffs, just as the tell-tales on the jibs float up and indicate the jibs are in the sweet spot. Obviously something not quite right and maybe backwinding - but anyway it all works well and despite the mains seeming to luff before the jibs do, the sail always weathercocks reliably when I let fly the sheets, which is why I am sure that in Bert's case camber and sheeting angle were not the main problem and that it was mainly a miscalculation of how much area to give the jibs (with probably other little things adding to the mix).

    It  must have been a bad experience for Bert and it is greatly to his credit that he was willing to share it openly and let us all learn from it.


    Arne, just out of interest, I would be curious to know what you think the maximum tolerable "balance" would be in the case of a contiguous (unsplit) sail, and what would happen if the limit is exceeded. Is it an issue of weather-cocking, or difficulty in getting the sail to set nicely, or simply a matter of mast too near the point of maximum camber and affecting windward performance - or some other problem?

    And if I am wrong about the boat, and you were referring to some other SJR problem, I would be very keen to know about it.

    Last modified: 08 Dec 2019 08:53 | Anonymous member
  • 07 Dec 2019 10:28
    Reply # 8203396 on 4793670
    Anonymous member (Administrator)

    Weather-cocking SJR, another factor.
    This matter of balance percentage in the SJR was up a few years back, when a German gentleman found that his SJR refused to weathercock on his home-built day-sailer (little sister of the Tom Thumb?). The essence of it (..if I got it right...) was that there was another factor playing in, in addition to the size of the jiblets, namely their camber and sheeting angle compared to the mainsail. If the sail were cut so that the main part luffed before the jiblets, a high-balance SJR was likely to misbehave. If, on the other hand, the jiblets luffed before the main part, the sail would weathercock all right.

    Balance, mast rake and sheet forces.
    I can only speak from experience with my own sails. These have been rigged with only 10-15% balance in the lower section and with the mast being vertical or raking a degree aft. In spite of that, the sail swings out in the lightest wind. I think that is thanks to the low friction in the 3-part Johanna-sheeting (..see anniversary magazine, p-30...). In light winds, the combination of moderate balance and the 3-part sheet is thus quite an advantage. In stronger winds, the sheet is harder to handle, but it certainly is not a showstopper with sails below 50sqm.

    Arne


    Last modified: 07 Dec 2019 13:56 | Anonymous member (Administrator)
  • 06 Dec 2019 21:13
    Reply # 8198199 on 4793670

    The more this subject is discussed and theorized the more confusion seems to be generated.

    Oscar is sort of on the right track in his conclusions, and I think what he is trying to say is correct, but Oscar has made a couple of statements which add to the confusion and I think need to be corrected. Oscar wrote: If de-powering the sail is done only ever before entering harbour and starting the engine, then I guess it's not a huge concern to not be able to completely weathercock the sail but if you were to sail engineless you will need to be able to de-power the sails completely in most situations. 

    At all times (except, of course, in a flat calm) the sail must weathercock when the sheets are let fly. If it does not, then the rig is hopelessly unmanageable and possibly dangerous.

    This ability to “weathercock” is affected by (among other things) the amount of “balance”.

    The JRA definition of “balance” is out of date because when applied to the split rig it is ambiguous.

    To clarify: The axis of rotation of the sail is assumed to be the centre line of the mast (and as it happens, in a “conventional” SJR or "split junk rig", the mast is assumed to be vertical and the luff of the “main” panels is assumed to be on that line.)

    The area of all that part of the sail in front of that axis of rotation (including the area of the slot) is expressed as a percentage of the total sail area (including the area of the slot.)

    In other words, the calculation is done in the normal way except that when we talk about “sail area” we must be sure to understand that the slot is included as part of the area of the sail.

    As long as this convention is followed, then comparisons are valid and we all mean the same thing when we use that pesky term “balance.”

    …one could argue that you could make it work to have a 50% balance split sail, as long as you only have 5% etc etc… Just forget about that,  Oscar, maybe you are right, but it is confusing. Lets just keep it simple.

    Most people can calculate the area of the parallelogram panels without too much difficulty but the fanned upper panel (or panels) becomes a complicating factor and therefore most people use a short cut when doing this calculation, and simply base the calculation on the lower parallelogram panels. If the shape of the sail is fairly simple, then all that is necessary is to find the ratio of that part of the chord which is in front of the axis, to the total chord. This might not be 100% accurate, but the error is conservative – the balance calculated simply on the basis of chord length will be a slightly on the high side because of the lower-balanced upper panels being ignored. In other words, the true figure will be slightly less than calculated by the chord method, so any error is conservative, or “safe.”

    Split junk rigs with a balance of 33% - 35% calculated simply by considering the chord of the regular-shaped lower panels have been found to weathercock satisfactorily when sheets are let fly and at this point in the development of the rig, this might perhaps be regarded as about the limit of what is practical. We can take 33% as a safe “rule of thumb” for a conventionally shaped and normally cut split junk sail. Slieve is comfortable with up to 35%. Others taker a more conservative approach and there is nothing wrong with being on the safe side.  I found 33% to be fine - Dave Z seems to have pushed it closer to 35% and found it OK, but as he explains, the chord calculation he used means his actual balance may be a little less than calculated. Personally, I would take 33 - 35% based on chord as a limit, not a target.

    (Of course, anyone experimenting with extreme amounts of camber, or unusual sheeting angles, or sail shapes which are far away from what is conventional, or non-vertical mast-rake etc will be aware that this 33% rule of thumb for split junks may need to be re-assessed.)

    Other factors which may affect the sail’s ability to weathercock (though none have been reported as far as I know) could include excess friction in the sheeting system, and binding of the sail due to insufficient halyard span or insufficient span for running parrel/downhauls. An aft-raking mast wouldn't help things either, though I am sure no junkie will be doing that.

    The above remarks apply to the split junk rig. I am not sure why the balance of non-split or “contiguous” junk sails seems to be limited to 20% or less – whether the issue is weathercocking, or simply the windward performance of the sail, or difficulties in getting it to set properly,  I do not know. I am sure someone else can clarify that.

    Problem is then how you would properly establish the CE/CP in order to get the correct lead (another fuzzy concept) for the rig. There is nothing difficult or “fuzzy” about this, other than the need to realise that there is no such thing as an actual centre of effort or centre of pressure. The so-called CoE is a purely geometric measure which is static and not real, but is a useful substitute and is conventionally used to help decide where the sail should be placed. It is calculated in the normal geometric way, the slot being considered part of the sail. With a conventional, simply-shaped SJR sail, it is near enough to simply use the mid-point of the chord. There is no point in being paranoid about accuracy here – the actual centre of effort is a moving target – the geometric centre is merely a useful convention. Slieve has suggested that with a SJR conversion it is sufficient to align the geometric centre of the sail (approximately the chord mid-point) with the so-called centre of effort of the original bermudan rig. That makes sense, and it worked for me. Again, this assumes conventional shapes and cut. A severely hooking leech, for example, will move the actual centre aft of where it should be. This placement of the sail may not apply to a contiguous (un-split) sail, I do not know about that, and would prefer to follow Arne's recommendations in that case.

    I'm considering converting my current boat to split junk due to the split (or aero) allowing the most balance and thus the best mast position without having to make considerable modifications to the interior. From some quick sketches I would need something around 33-35% (including the slot).

    Now, that makes sense and I think you are on the right track. Assuming you are converting from bermudan rig to any of the well-proven junk variants, I am sure you will be delighted with the result, best wishes.


    PS  Without wanting to add to the confusion, but for the sake of completeness, people with conventional contiguous junk sails have reported that a forward-raking mast is desirable, among other reasons because when sailing off the wind in light airs the sail has a tendency to "sleep" in the sheeted out position - a kind of built-in "weathercocking" tendency. A small disadvantage of the SJR rig is that, so far at least, they have all been designed around an assumed vertical mast, for obvious reasons of simplicity - so with the split rig we don't get that extra, little benefit.

    Last modified: 07 Dec 2019 09:05 | Anonymous member
  • 06 Dec 2019 18:31
    Reply # 8197131 on 6719175
    Anonymous wrote:

    If the Weaverbird rig has 25% sail area in front of the mast and the Split Junk or Aerojunk has 25% sail area in front of the mast, I think the actual balancing effect would be different due to the split and thus increased chord length.     

    So, in the interests of clarity and help in ending confusion among us newbies, I think it is important to clarify whether you are talking about actual "sail area" in front of the mast, or "chord length and sail area" in front of the mast.  


    regards, Dave D 


    Having just read through the thread this seems to be what David and Slieve have been arguing about - apples and oranges.

    The way "balance" is calculated/defined is (or should be) different with a split vs a "one-piece" sail.

    Without making a distinction one could argue that you could make it work to have a 50% balance split sail, as long as you only have 5% (or some other small arbitrary number) of the total sail area in the jibs. Amount of camber/drive of the jib vs. the main will also affect the overall balance. Problem is then how you would properly establish the CE/CP in order to get the correct lead (another fuzzy concept) for the rig.

    That said I do share Davids concerns about a sail that doesn't readily and completely weathercock in all situations. This has proven to be the case at least with some wrongly designed/built split sails where the actual balance has ended up being larger than intended (due to misinterpretation nonetheless). If de-powering the sail is done only ever before entering harbour and starting the engine, then I guess it's not a huge concern to not be able to completely weathercock the sail but if you were to sail engineless you will need to be able to de-power the sails completely in most situations. And although I agree with Slieve that innovation is achieved by challenging the established limits my personal rig would rather be on the safe side. ;)

    All of this is a bit tricky as I found this thread because I'm considering converting my current boat to split junk due to the split (or aero) allowing the most balance and thus the best mast position without having to make considerable modifications to the interior. From some quick sketches I would need something around 33-35% (including the slot).

  • 22 Nov 2019 01:47
    Reply # 8133902 on 4793670

    Hi All,

    I'm a johhny-come-lately, having only recently become aware that there is some concern regarding overbalance in SJR sails with larger balances, especially approaching the top end of what Slieve McGalliard considers possible (35% of sail chord). Indeed, I had no idea that any balance 'reasonably' well under 50% might be a problem.

    At 34.4% our SJR balance is very near that top top end. We sailed extensively in inland waters (no engine) this last year (write up here).

    We can report absolutely no hesitation in weathercocking when sheets were let run beyond normal friction from the blocks. We noticed no difference in sheet responsiveness from our previous (flat cut) main, which had considerably less balance (20%), but was otherwise rigged identically.

    However, there are some relevant differences between typical SJR and our prototype, include the following:

    1. The higher peaked yard moves the CE of the uppermost panel aft. This would enhance weathercocking.

    2. Our sheets are six part vs. the more usual three,  which induces more friction. This would impede weathercocking.

    3. Opened by Thai style lashings, our after panels had gaps running along each batten. As their maximum point of camber (and therefore of gap) was 40% of panel chord, I suspect the after sail’s CE was moved aft to some small extent. This would enhance weathercocking.

    We are planning to build final sails as prototyped this winter.

    So... umm... data points for discussion.

    Dave Z



    Last modified: 22 Nov 2019 01:48 | Anonymous member
  • 12 Oct 2018 15:11
    Reply # 6719362 on 4793670

    Hi--See jpg below. I just did a check in QCAD comparing the chord-wise balance computation and the area-wise balance computation for the parallelograms and got no difference (unless the 5th decimal place is of importance).

    Perhaps a minor point but I've also found the centroids or center of area of each of the individual panels and found that the balance of the 2 highest fan-shaped panels, taken together, has a 24% balance versus the lower 5 panels which have 33% balance. 24% being more inline with conservative thought....meaning that as you reef the balance of a similarly-shaped split rig decreases.

    On the issue of speed of weathercocking I've found, even in the F3-F4 wind regime that I've had to reef, that gybes ends with much more of a "bang" then the full sail gybe in F1-F2. My explanation is that the 3:1 sheeting mechanical advantage doesn't change when reefed and becomes a 3:1 frictional 'disadvantage' that stops the sail abruptly (bends battens?). I'm wondering if the bang would be even louder if I had substantially less balance, say in the 10% range, resulting in an even greater speed of weathercocking? Forced to wear ship?

    robert self

    1 file
    Last modified: 12 Oct 2018 15:58 | Anonymous member
  • 12 Oct 2018 10:18
    Reply # 6719175 on 4793670

    Just a point I'd like to make about when talking about balance.  

    When talking about balance in front of the mast,  I think that it is important that the posters clarify whether they are talking about "sail area" in front of the mast, or "chord length".  

    When I made the Split Junk Rig for the Wayfarer, I both misread Slieve's files and in reading discussions on the forum, got it into my head that there was 33% sail area in front of the mast plus the split and built the jiblets accordingly. This would have resulted in my sail then having the luff of the jiblets approx 40% of the chord length in front of the mast.  

    Luckily before I set sail, some postings earlier in this thread , warned that this was wrong and more likely correct that the leading edge of the jiblets should be no more 33% ahead of the mast.  So, not wanting to face building a complete new set of jiblets I cut a bit off the trailing edge of the jiblets, ending up with 29% sail area in front of the mast, but still 35% or so of chord length because of the split. 

    The sail does feather with this amount of sail area and chord length, but it is slower to feather than I like.  So I intend build another set of jiblets to reduce the amount of sail area in front of the mast to 25% plus the split.

    If the Weaverbird rig has 25% sail area in front of the mast and the Split Junk or Aerojunk has 25% sail area in front of the mast, I think the actual balancing effect would be different due to the split and thus increased chord length.     

    So, in the interests of clarity and help in ending confusion among us newbies, I think it is important to clarify whether you are talking about actual "sail area" in front of the mast, or "chord length and sail area" in front of the mast.  


    regards, Dave D


     

    Last modified: 12 Oct 2018 10:20 | Anonymous member
  • 12 Oct 2018 01:21
    Reply # 6718834 on 6717559
    David Tyler wrote: Slieve,
    With respect, I do wish that you could banish this particular bee from your bonnet. I am going to hold very firmly to my position that I should not wish to go to sea with a junk rig with a balance area much in excess of 20%. I am going to continue to advocate, based on practical junk rig experience over many years and many miles, moderation in the article of balance area. The greater the balance area, the greater the issues that I have encountered with ill-behaved sails.
    LC had 22% balance in her foresail and had no problems of any sort due to balance. In fact the sail set perfectly and handled very well.
  • 11 Oct 2018 16:15
    Reply # 6717938 on 4793670
    Anonymous member (Administrator)

    ..drifting limits...

    I guess my attitude to what the maximum balance in a one-ply JR should be, has drifted a bit over the years. In the early days of my cambered panel sails, I aimed for only 10% balance to minimise distortion of the camber on one tack. Fine in theory. Then Paul Thomson rigged his La Chica with 18 and 22% (?) balance, and got away with it. I still don’t dare to recommend a cambered panel sail with 25, 30 or 35% balance to others, and that is for one reason: I haven’t sailed with this much balance myself. I may well try such a hi-balance cambered sail, if I get access to a dinghy. If it turns out good, I will recommend it, but only then. I guess I am conservative...

    However, by adding a split in the sail ( see Slieve’s SJR), the equation will change, to allow much more balance (ask Slieve how much).

    Anyway, I wonder if we are discussing a not-problem here: As far as I remember, Christopher had no need for more than 20% balance.

    Arne

    Ps: A while ago I made the diagram below to help me find a what yard angle will match different amount of balance. As a result, if I were to set the sail with 20% balance, I would design it with a 65° yard.

    (PPS: Only now did I spot Asmat’s warning against 28% balance...)


    Last modified: 22 Nov 2019 16:16 | Anonymous member (Administrator)
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