Exploring a Simple way to draw Shelf Foot Cambered Panels

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  • 28 Jun 2022 04:17
    Reply # 12831013 on 12819046

    I think the reason that no one sail construction method has one out over the years is that no one method is clearly superior in every way.  You just have to choose which things you want in your sails and what you are willing to give up.  I've started sewing my four-piece practice panels this evening and there's no mystery in the appeal of a single large panel cut-out and done.  Barrel cut offers that.

    I had to order more cloth today in order to cover what I'll need for tabling, if I'd made an Arne-style webbing-roped sail, I'd probably have needed less cloth (rough estimate, I'll give more precise numbers when I finish).  I think a huge number of the folks in the JRA fit into the "best use" for a Johanna type sail with a webbing bolt-rope and they'll save some money and time if they build that.  There's lots of pictures of them without and hooked leach and smiling owners.  Granted, if I went offshore with one of Arne's sails I'd still add some tabling, but even that is a personal choice.

    I think the only thing that is certain David is that if you were to build another sail, it would be something different than what you've done before and sharing it would only improve the JRA,.


  • 28 Jun 2022 04:06
    Reply # 12831011 on 12819046
    Anonymous member (Administrator)

    Arne wrote: ... break down and confess; your first JR sail [shelf foot] panels were built over a plywood mould to get them right…

    Yes, I confess …. Well, no it wasn’t quite like that.

    I confess I knew nothing about canvas, or sewing machines, and was afraid to pick up the hot knife. So, I made a mock-up of a jib panel using some bits of old plywood I had, just to convince myself what one of these critters would look like in reality. It’s a good thing I did, because I made a mistake with the camber and had to make a second one. (Also, little did I know that a shelf foot panel in soft cloth actually doesn’t look like the rigid model, but anyway it gave me the confidence to proceed). Then I looked at the basting tape, and wondered how to apply it when I only have one pair of hands. Suddenly a brainwave – I thought, seeing I have the plywood model, why not use it as a tailer’s dummy and lay the cloth over it. Suddenly I realised I wouldn’t even need to draw the shapes out on the cloth. Just drape the cloth over, and cut around the “chine” in situ with a hot knife, with total confidence that everything must fit. It made the whole job so easy, and quick, and gave so much confidence in the finished product, that I decided to do the mains the same way, and made a mains dummy as well.


    There’s only a day’s work in lofting and making a plywood dummy, and it made sense for my Amiina Mk ll sail (which has all identical panels except for the top triangle).

    I think, for a small sail with identical panels, I'd do it again. Otherwise, if a confidence-builder is needed, just make a first one to scale, out of cardboard.

    David, I seriously considered broad seam for the mains. Maybe the only type of junk sail panel that seriously calls for shelf foot is the jibs of a SJR. I wouldn’t use any other cut but shelf foot for the jibs. But for anything else, I don’t know enough to have an opinion. I just didn’t have the confidence, after reading the text books, that I could do it right with broad seam, or just how much camber I would actually get. It’s a bit of a mystery to me. (Is “arcane” the right word? I don’t have time left to gain the necessary experience). I made the top triangle (low camber and unsplit) using, not exactly broad seam but some long “patched darts”  which have a similar effect on shape, plus a bit of rounding – and it did work out OK, so perhaps a broad seam construction is not so difficult to design. Now that I have learned that a single curved seam does not produce a "chine" (as it would do if we used plywood or tin plate) but rather it gives the whole cloth panel a fair curve – I might look at broad seam again with a little bit better understanding. If cloth is laid parallel with the leech, as it probably ought to be, then there are going to be seams there anyway, so I guess there is further economy in just tapering the seams. The thing is … how much? A beginner can’t visualise the final result. A spreadsheet table of numbers just doesn't give me a visual picture.

    So, unlike Arne, I regard shelf foot as an alternative method not unsuitable for beginners, because you can make a cardboard or plywood model first, which gives some understanding, something visual to start with, and thus confidence to proceed. Its faux-intuitive, if that’s a word – after making one out of cardboard, you can then convince (delude) yourself that you understand how a shelf foot sail takes up its shape, and then pick up the hot knife with confidence.

    (That said, I have no doubt the easiest and quickest method by far is to be found in Arne's Files. nothing I have seen yet surpasses Arne's Files as a confidence-builder, to get one up and out of the arm chair. I suspect Annie's book and Arne's Files are up there with PJR as required reading for all students of the rig).

    The shelf foot method gives you a bit more control over where the maximum camber will be, (compared with other methods?) if that matters much, but apart from that, the panel bellies out no different from any other cambered panel as far as I can see. I’ll probably use the method again because I am confident in it now, and I like the simple geometry (or should I say trigonometry) which is all that is required to get a “predictable” result - but not because I now think the finished product is a superior cut. And it has to be admitted, there is a bit more work in it, and a little more cloth wasted.

    (The other reason, for me, is because my rig of choice is SJR, and I can’t see the point of SJR unless the jibs are as large and aerodynamically “correct” as you can possibly make them, with the maximum camber exactly where you want it, in which case shelf foot is a no-brainer. For the jibs. That's for another thread).

    Apart from that I am inclined to agree with Arne that whatever works, in practice, is going to be better than good enough for most people, and looking for that elusive extra 2% of performance is probably a fool’s errand - for most people, that is. Hats off to the few who never cease to strive for perfection, nothing would progress without them. Apart from that, it probably just comes down to whatever has the most intuitive appeal. The most important thing is - don't just think about it - do it!

    Durability is probably the only other thing that matters in the end – although even then, for most people who only mess about, that’s probably not such a big deal either. I would imagine shelf foot is as durable as any other cut, depending on the details that apply to all sails?

    Except... I think it is difficult to work out what is the best thing to do on the ends of the lenses, of a shelf foot sail, to make that area of the sail with the batten ends, strong – there is a little bit too much going on there. Darren might have worked out the best solution there, where heavy duty is called for, if there is not too much cloth to punch through. We'll see, I used layers of tabling - and added gussets of short lengths of rather light webbing across the leech ends of the half-lenses, but did not sew the panels together. That's not going to last. I should have sewn the panels together or (better) used full double lenses, and I think then maybe a short strip of light webbing across the double lense might be enough on a small sail. - but then its a hard spot.  A bolt rope or a full length of webbing right down the leech seems a bit agricultural to me, but I don't know, it seems to work on Arne's sails. I dunno.

    The biggest problem in SJR is across the luff ends of the lenses, you want to keep a nice shape there. The shape won't matter so much with the luff of a contiguous sail, but its still difficult because, unlike at the leech end,  the sail is trying to build up camber quite quickly there. It will be interesting to see how that detail works out in Darren's sail. I don’t think Paul has not yet quite reached perfection at the luff ends of the lenses, and perhaps that part of Pango’s sail could be improved.

    Leech. Beautiful.


    Luff. Would it help if the lense pairs spanned a a wide centre strip? 

    So there is fertile ground for further discussion on the structural details of a shelf foot sail.


    I make these comments from the position of being a learner who has perhaps just reached level 101.



    Last modified: 28 Jun 2022 06:46 | Anonymous member (Administrator)
  • 27 Jun 2022 19:45
    Reply # 12830521 on 12819046

    Well, if I was going to make any more sails (which I’m not), I’d stick with my preferred method of vertical cloths in each panel and broad seam and/or tucks in the forward half. Much better looking and better sailmaking practice than barrel cut; easier, quicker and more economical of materials than shelves  

    Just sayin’. 

  • 27 Jun 2022 17:14
    Reply # 12830255 on 12819720
    Anonymous member (Administrator)
     It meant that going to windward in very light airs, she was fantastic... but easily overpowered in stronger winds.

    Bonjour James

    I don't realy understand the point. Under my understanding, with a junkrig to be overpowered is just a normal way of life. When overpowered just reduce a panel or two.

    For me, it is in the pointy world that the overpowerness is an issue because it is expensive in time and energy to reduce the sail area... but with a good racing crew.

    Eric

  • 27 Jun 2022 14:01
    Reply # 12830037 on 12819046

    Even if I'm successful, I don't think there's likely to be an exodus from making barrel-cut sails.  I don't think there's much more that can be done to lower the barrier to making sails than what you have done Arne.  I'll do the best to document what I do, because I think I owe that back to the JRA given how I've benefited from those before who've shared their ideas.  However, what I do isn't going to be comparable to Arne's instructions.  Hopefully, it will add another option for those inclined to do such things. 

    I'm genuinely grateful to everyone who's taken the time to comment not just on my build or posts, but on every other one that's available here on the JRA to learn from. 


  • 27 Jun 2022 10:53
    Reply # 12829923 on 12819046
    Anonymous member (Administrator)

    Graeme

    I surely salute Paul for making such fine and good sails.

    However, as for amateurs   -  there are amateurs and there are amateurs. You and Darren are robust amateur craftsmen who don’t shy away from working with any material;  plywood, concrete , steel or canvas, so you don’t really need any advice from me.

    I, on the other hand am aiming for the first-timers with little or no experience with sewing, and who are not the bravest go-ahead type of people. I want to encourage them to make their very first sail, by trying to make the first step up from the armchair as low as I can.
    And... break down and confess; your first JR sail panels were built over a plywood mould to get them right.

    Change of theme,
    over to that photo from Sep 2013, showing me measuring the actual camber of Frøken Sørensen’s sail. That photo reveals a couple more details than the max camber. When I designed that sail, I wanted 8% camber, but I decided to use my chain calculator to get 9%. Then, when rigging the sail, the camber was flattened a little by hauling the sail just a little taut along the battens. That worked well. I got my 8% camber and I got rid of just about all wrinkles along the battens, and  - not least  -  the vertical curve became pretty trapeze-like.

    This photo also reveals that odd detail; the slack bit in the space between the fore and the aft batten pockets. It doesn’t move or flutter, and there are no wrinkles or distortion of the camber around it, but it still looks a bit odd. I have decided to forget about it. It’s the same on Ingeborg’s sail and probably all barrel-cut sails made with batten pockets. If this detail makes one unbearably eye-sore, then I guess one must add broadseams, tucks  -  or step up to the shelf-foot method.

    Luckily, I make my sails for sailing, not for selling...

    Cheers,
    Arne


    Last modified: 27 Jun 2022 11:51 | Anonymous member (Administrator)
  • 27 Jun 2022 02:07
    Reply # 12829692 on 12819046
    Anonymous member (Administrator)

    Arne: "...that [shelf foot] method has proven to be great when executed by Tuchwerkstatt in Germany or by Paul Thompson, and possibly other pros as well. My scepticism is against promoting the shelf-foot method for DIY beginners to JR sailmaking.

    I can't let that go past.

    I am not qualified to promote anything, and I'm not doing it here. But on the subject of being a beginner: I am one, don't have to rely on memory, so I can say this much, with confidence:

    Anyone can understand and successfully make a shelf foot sail.

    Arne’s great contribution, apart from being a pioneer in the evolution of the modern cambered sail, is in the area of encouraging beginners like me to have a go, developing, proving and providing practical pathways which make that seem possible. I can't say "thank you" enough for that.

    If I were building a contiguous sail for coastal cruising I think I would like to follow one of Arne's pathways. And I can understand that there would be near enough to the same return for less effort. That's good.

    It would seem a pity then, to discourage anyone from making a shelf foot sail if the idea appeals. I am sure that is not what Arne intends. There might be a little more work in shelf foot, but I can't see where it is any more difficult. There are enough successfully amateur built shelf-foot sails already on the water to demonstrate that. A curve is a curve, and a seam is a seam. 

    And building camber into the sail is only part of the task anyway.

    The reason Paul’s sails are so good is not because there is any mystique about shelf foot, it is simply that his workmanship and his use of excellent details stand out, as would be expected of a professional and a craftsman. We should all aspire to that and try to learn from it, whether using shelf foot, broad-seam, barrel cut or even macrame style.

    I am sure Arne is as interested and as pleased as all of us, in following Darren's future progress as he works his way through and documents the making of sails for Leeway, sparing no effort.



    Darren:  "...a lower shelf angle lets you put the effective camber across a greater portion of the panel height..." 

    That is true in the tin plate model. In the case of soft cloth reality, it remains to be seen. I think you might get a surprise. I found, somewhat to my surprise, that the shelf panels tend to merge with the centre panel and once fully inflated, they take up together a continuous curve (bulge) in the vertical section. Maybe the 30 degree shelf will be a bit better in this respect (distributing the camber vertically) than 45 - I look forward to learning if it does.

    At present, I doubt if a 30 degree shelf foot sail will spread camber across the height of the panel any better than this barrel cut sail...

    We'll see.

    Last modified: 27 Jun 2022 10:14 | Anonymous member (Administrator)
  • 26 Jun 2022 23:18
    Reply # 12829576 on 12829572
    Anonymous member (Administrator)
    Graeme wrote:

    Hi Arne. I don’t recall anywhere saying that I thought shelf foot sails are “better” than barrel cut.


    Nonono, Graeme, I don’t accuse you for saying that.

    As for my attitude to the shelf-foot method; that method has proven to be great when executed by Tuchwerkstatt in Germany or by Paul Thompson, and possibly other pros as well. My scepticism is against promoting the shelf-foot method for DIY beginners to JR sailmaking.

    There is another factor: The sail has to be rigged correctly and handled correctly to look good. Not even the best sail will look good if incorrectly rigged and trimmed  -  and I dare say that even the best junksails will from time to time look untidy.

    Arne

    PS:
    I ‘grew up’ with the slide-rule on high school and the first year of technical school (1973). It had a little advantage  -  it could only deal with about 2½  to 3 digits resolution. That is all you can handle when making sails anyway...

    PPS:
    I admit that I often gives the dimensions in my sails to the nearest millimetre, but I expect that people are happy enough if they can get as accurate as one centimetre (cm  = 10mm).


    Last modified: 26 Jun 2022 23:56 | Anonymous member (Administrator)
  • 26 Jun 2022 22:56
    Reply # 12829572 on 12819046
    Anonymous member (Administrator)

    Hi Arne. I don’t recall anywhere saying that I thought shelf foot sails are “better” than barrel cut.

    This is just a conversation about how to do the lofting, and how the lofted shapes translate into the final result - and to express a little joy in the process.

    Arne is quite right to pounce on the use of high precision scale factors “1.4142 instead of the 1.1547” – those “accurate” figures are ridiculous, of course, when it comes to cutting the cloth.  That was only to make a point in the discussion with Darren. However, when doing calculations some of us use computers these days, not slide rules or long multiplication, so it’s just as easy and it is actually better practice not to round decimal figures down until the time comes to actually pick up the scissors.

    I have to admit, it was a revelation to discover how much the shelf foot panel bulged into a vertically curved camber, virtually the same shape as Arne portrayed so well at the bottom of his most recent diagram. The result I got from shelf foot was very similar in vertical section to the result Arne is getting from his barrel cut sails. Closer in fact, I would say, to the left-hand of Arne’s pair of examples. In that respect, Darren might be in for a surprise.

    I think the barrel cut sails in Arne’s photographs look magnificent and I could be easily persuaded to use Arne’s methods and the barrel cut if I were building a contiguous sail for coastal cruising.

    I can also appreciate Darren’s perfectionist approach to what looks like a heavy-duty, long-distance explorer type of vessel. I wouldn’t say Darren is “over egging the pudding”.

    I think a better metaphor is “going the extra mile”.

    ………………………

    [  However, for the record, and to change the subject slightly, I would venture to suggest that for the type of sail I decided to make, shelf foot is worth the extra effort, at least when making the jibs.

    Arne’s reasoning that a well-formed camber shape is “will be thoroughly crushed on that other tack. does not apply to Split Junk Rig.

    I am less certain about whether it is worth it for the mains of a SJR, but I like the look of the result I got, as the mains, too, set near identically and near perfectly on both tacks. I just found it aesthetically pleasing to take the advantage of being able to build camber further forward along the chord and to know that I had done the very best I could. It was a first time effort and I was enjoying to explore the geometry of a sail, and learning to use a sewing machine.  ]



    PS So far it looks like "one nill" in favour of adopting Darren's preference for reckoning shelf angle with respect to the sail's orthogonal plane. Looks like I might have to change what was evidently just a "mind set".

    Last modified: 26 Jun 2022 23:04 | Anonymous member (Administrator)
  • 26 Jun 2022 18:33
    Reply # 12829408 on 12819046

    Your point is well taken Arne, but it is worth considering how the camber is spread across each panel as well, a single measurement of maximum camber in the middle (vertically = 1/2P) doesn't tell the whole story.  Panel height, max camber and in the case of a shelf-foot sail, the foot angle all change how the built-in camber is spread across the sail.

    Only Sebastian could answer why he changed to shelf foot sails, but there are two reasons that come readily to mind.  First is that paying customers may not want wrinkly sails, if you look at the sails Sebastian makes they not only set well, but also look great (scroll to bottom of page here).  I completely agree that this does not matter to everyone and that effort building the sail and difficulty are also considerations.  With a rounding-only, you have to decide if you want to stretch the sail along the battens to get rid of the wrinkles, or let it a bit slack to get the designed camber and have some wrinkles.  This is totally fine, it is just the way barrel-cut sails work and they clearly work very well.

    The other reason that makes shelf-foot sails more interesting is that a sailmaker has more control over the shape of the sail, you can vary not just camber, but also the angle of the shelf foot.  This is never going to be case of WYSIWYG, but having more control over shaping the sail opens new opportunities not present with barrel cut. I don't think anyone has said barrel-cut isn't good, but surely there is room for other methods of making sails?

    I haven't played around with each of the methods enough to convince myself I fully understand how things are work, but here are my thoughts:

    I suspect there are sweet spots for each kind of sail making.  8 -10% camber with barrel cut sails seems to work very well with the sailplans you use.  If you look at a shelf foot sail you can see that a steep (45o) shelf foot on a narrow panel sail yields an odd shape (very narrow-waisted hourglass panel).  So, it seems to me that the more camber you want, the shallower the shelf angle should be although the panel height matters.  At the extreme, a flat-shelf foot and high camber will lead to a sail that is baggy until there is sufficient wind to fill it.  Pango's sail while not extreme, is a nice example of this where Paul used a flat shelf foot, but lighter sailcloth to help the sail fill easily.  Similarly, if you want less camber (6%), then you have less "excess cloth" in the sail and a lower shelf angle lets you put the effective camber across a greater portion of the panel height.  At low (4%, 2%) camber barrel cut sails make a lot of sense because there isn't much rounding to try and force onto a straight batten.  I drew the top panels of Leeway's sail with just rounding, I think this makes sense for the low-camber triangular upper panels.

    Whether any of this turns out better than barrel cut, who knows.  I'm trying to make a shelf-foot sail that I hope won't be much more difficult to build than a barrel-cut sail.  Maybe, I won't succeed, we'll see.  The point isn't to make hyper-accurate sails, it is to explore which techniques make the most sense for a given mission.  I'll post the actual camber when the sails set on the boat (which will be a joyous day).

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