Exploring a Simple way to draw Shelf Foot Cambered Panels

  • 26 Jun 2022 18:33
    Reply # 12829408 on 12819046
    Deleted user

    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).

  • 26 Jun 2022 15:00
    Reply # 12829255 on 12819046
    Anonymous member (Administrator)

    Add a correction factor to the shelf-foot camber.

    Although Graeme does his best to remind one that the actual REAL camber of the sail at work is not the same as the tin plate camber (calculated with 5-digit resolution), most of you still appear to stick to the latter. Trigonometry is such a wonderfully exact science...

    I think it is about time you measure the actual camber in your sails. This may give a firm enough clue to establish a correction factor. This factor may well be 1.1 or 1.2, which means that a sail with a calculated tin-foil camber of 8%  can have a REAL camber of 9.6 or 10%.

    On the attached diagram I show the real camber I got in Ingeborg’s sail for a given Round (25cm) in my barrel-cut panel. At first, when I measured the REAL camber, it came out at only 7% of the chord. Then I eased the stretch in the sail with about 3-4cm along each battens. The camber was measured again to around 8%, without too many wrinkles popping up along the battens.

    Hollow at luff and leech
    When I introduced the barrel cut method in the nineties, it was assumed by everyone that the success of the method depended on a soft and stretchy cloth. I bought that. However, when Sebastian Hentschel visited Stavanger in 2008, I noticed that his sail stood fine even with a stiff, real sailcloth (Sebastian only later changed to using the shelf-foot method). How come?  The warp of Ingeborg’s sail is running along the battens, so even the Odyssey III  material is not much willing to stretch in that direction. Even so, the panels of my plain, barrel-cut sail inflate well enough. The answer came when watching the luff and leech of Ingeborg’s sail with the sail squared out to about right angle to me. The horizontal stretch in each panel clearly results in a gentle hollow at luff and leech. No need for an elastic cloth for sure...

    Finally...
    It may well  be that the resulting 8, 10 or 12% camber you find in your sails is good, and better than the 8% I usually make.
    Good luck!

    Arne


    Last modified: 26 Jun 2022 15:16 | Anonymous member (Administrator)
  • 26 Jun 2022 07:44
    Reply # 12829130 on 12829049
    Darren wrote:

    Either reference can work if it is made explicit.  I'm not the first to make a 30o shelf foot sail, although the search function has frustrated me in showing another example.  Given that flat and 45 don't change as a result of the reference point, I'd suggest using a horizontal reference as creating less confusion with the already existing posts in the JRA.


    I agree. To be pedantic, orthogonal to the panel, not horizontal.

    A shelf that is at 30˚ to the panel isn't worth taking the trouble to make, as it is not very different from a simple barrel cut. 

  • 26 Jun 2022 03:16
    Reply # 12829049 on 12819046
    Deleted user

    Either reference can work if it is made explicit.  I'm not the first to make a 30o shelf foot sail, although the search function has frustrated me in showing another example.  Given that flat and 45 don't change as a result of the reference point, I'd suggest using a horizontal reference as creating less confusion with the already existing posts in the JRA.


  • 26 Jun 2022 01:16
    Reply # 12828970 on 12819046
    Anonymous member (Administrator)

    Darren wrote: If we are talking about 30o and 45o shelf feet, I think the reference plane has to be horizontal (orthogonal to the sail).  If you change your plane of reference to vertical (parallel to the sail), then a 30o shelf foot no longer makes sense.  Given that the only references on the JRA are to Flat, 30 or 45 degree shelf feet.  I think we have already settled on horizontal as the plane of reference? 

    I don’t think the reference has to be horizontal. If you wanted to design your sail using the vertical plane as a reference, you would simply choose 60 degrees as the parameter instead of 30, and get exactly the same result.


    I suspect that Slieve had the vertical plane in mind when he wrote his notes some years ago on shelf foot sails (but it was never entirely clear, as his shelf angle of choice was 45 degrees, so the question did not arise).

    You are breaking new ground, Darren. (Unless there is a body of literature somewhere I have missed). The term "shelf angle" has now become ambiguous.

    I am faintly in favour of referring to the vertical plane when discussing shelf angle – rather than the horizontal plane which you have preferred, but it’s an arbitrary choice. I don’t think it has been settled on at all . That is the issue, and why I was asking if the established junk sail designers who have used the shelf foot method (in particular Paul and Slieve – and I think David) – could perhaps agree on a standard.

    As you state, 45o shelf feet are simpler to understand in that they don't change depending on the plane of reference,

    That is not quite what I said. I don’t think there is anything simpler about 45 degrees, the trigonometry is exactly the same, its just a matter of choice of sail characteristics you want.

    With 45 degrees, the scale factor between camber-of-cloth-shelf  and  designed-camber-of-sail is 1.4142 instead of the 1.1547 which you used. That's the only difference. Your sail will be a little baggier (or “fuller” if you like) than if you had used 45 degrees (for the same amount of camber).

    The simple case is horizontal shelf, in which case the scale factor is 1.0000  (in other words, no scaling required). eg Pango’s sail. For a given amount of designed camber, this is the greatest volume you can get, when fully inflated. Beautifully made by Paul, this sail looks magnificent when filled with a puff of wind. I happen to think the less “bulgy” and perhaps slightly easier to fully inflate 45 degree shelf is a good choice too, and I am happy with it, intend using it again.

    In shelf angle, your sail will be somewhere in between mine (45) and Pango’s (horizontal) – perhaps an even better compromise – I look forward to seeing the result of all your careful thought and attention to quality.

    (I am not sure if the differences between 45 degrees, or your proposed shelf angle, or the horizontal, are all that earth-shatteringly great, actually. We are only talking about “tin plate” models anyway. The reality is, they will all be baggy, some a little baggier/saggier than others, some a little easier to inflate in light airs). 


    I would have thought that for off-shore work the least baggy would be the better choice, but that's outside my experience, and anyway, as well as shelf angle, the amount of camber is also part of the mix.

    As I think more about it, intuitively, the more baggy should have the higher number, so I will now advocate for the vertical plane as being the reference plane when discussing shelf angle. In that case, 30 degrees would be a fairly shallow shelf angle (the least baggy), 45 would be half way, 60 would be a steeper angle (like your proposed sail) and the maximum volume  (horizontal shelf) would be 90. That would be my choice of nomenclature.

    But the other way round works too – as long as we don’t get mixed up which is which.

    The most important thing is, we should all be speaking the same language.

    So, Paul, Slieve and David: which way do you want it, the horizontal plane or the vertical plane as the reference for “shelf angle"? Arne, you can have the “casting vote” if needed!


    [PS Thanks Darren for pointing out that some sail-makers have full-size plotting tables - I never thought of that - I'll look into it.]


    Below (Pango) shows the massive volume you get in a horizontal shelf, fairly heavily cambered sail, when fully inflated. I still prefer 45 degrees for my next sail - but... perhaps I might follow Darren and go for a little more volume...

    (that's Marcus up there...)

    Last modified: 26 Jun 2022 03:37 | Anonymous member (Administrator)
  • 25 Jun 2022 22:50
    Reply # 12828920 on 12819046
    Deleted user

    I think you have it Graeme.

    I deliberately skipped over the Trig, as I was attempting an Arne-ish approach of keeping it simple.  As you've shown, within hours of getting QCAD, anyone can build a 30o shelf foot panel just using the scaling provided.

    If we are talking about 30o and 45o shelf feet, I think the reference plane has to be horizontal (orthogonal to the sail).  If you change your plane of reference to vertical (parallel to the sail), then a 30o shelf foot no longer makes sense.  Given that the only references on the JRA are to Flat, 30 or 45 degree shelf feet.  I think we have already settled on horizontal as the plane of reference? 

    As you state, 45o shelf feet are simpler to understand in that they don't change depending on the plane of reference, but if you have shorter panels, 45 may end up with the lenses coming quit close together in the middle of the panel.

    I think you'll like QCAD, it's wonderful to be able to create a host of boat projects virtually before committing any materials.  Smaller projects can be printed on regular paper with reference marks and taped together for templates.

    Rather than looking for a large format printer, I'd look for a friendly sail loft.  Some sail lofts have a plotting table, this table will not only print the outlines for cutting, but I think you can also label all your pieces in CAD and have the labels printed directly on the cloth as well (labels best put in seam allowance).  I'm not doing this, but I know Erik is.  I'm in a bit of rush to get sails made, and can loft a small number of templates onto plywood easy enough, so I'm going with what I know.  The costs Erik mentioned for plotting directly on the cloth seemed comparable to what large-format printing for paper templates would cost at a print shop.   You do have to give up a bit of cloth on the edges as they don't plot right to the edge of the cloth and you have to make your virtual cloth panels match the length of the plotting table.


    Last modified: 25 Jun 2022 22:56 | Deleted user
  • 25 Jun 2022 01:30
    Reply # 12828342 on 12819046
    Anonymous member (Administrator)

    I would like to thank Darren for creating this thread,  and giving us a PDF of his step-by-step instructions in the use of QCAD to loft the panels of a shelf foot sail.

    It inspired me to make the effort to get started and learn how to use QCAD myself, something which has been on my bucket list for some time.

    I was able to follow the QCAD manual and to understand how QCAD is used to make 2D and isometric drawings of objects. Though I do not yet understand how Arne and David have been able to design hull shapes using QCAD (perhaps someone can provide some pointers here), I was able to understand Darren's instructions which show how QCAD can give us the patterns required to make a shelf foot cambered sail.

    If I could find a print shop with large enough equipment to print full size patterns, I think this would make an easier and more interesting way of lofting my next sail. (So far I haven’t been able to find one in Auckland, but hopefully there is one somewhere?)

    Thank you Darren.

    I did have one serious problem in my understanding of this article, and to save the same mis-understanding occurring to other readers in the future, I would like to clarify something here.

    In the “Instructions” PDF Darren wrote: “…since we’re using a 30o shelf-foot the airfoil needs to be taller, as running at 30o it has to cover a longer distance than a straight line…”

    Well, er… I understand what is meant anyway.


    The cloth aerofoil which makes the shelf, will need to be taller than the desired aerofoil shape in the sail, as it is on the hypotenuse of the above triangle.

    You can work through the trigonometry if you like (given you know your camber height you can calculate the 30o shelf (hypotenuse) with b/cos a), but I was really pleased to find out that it all reduces to just scaling the airfoil vertically by 1.1547 for a 30o shelf foot…”

    Here is where I wasted a few hours in desperation, struggling to know where I was mistaken in this very basic piece of trigonometry. I was certain, for all money, that the scale factor should be b/sina giving a scale factor of 2.000 – not 1.1547   - a discrepancy of potentially disastrous proportions.

    Eventually it dawned on me: Following an earlier exposition of shelf foot development by Slieve McGalliard, I had a mind set and was intuitively following Slieve's diagram and his nomenclature.


    The scale factor is b/sin45 here, and of course b/sin45 has the same value as b/cos45 anyway, so there is no possibility of confusion leading to a mistake.

    But for a shelf foot angle of anything other than 45 degrees it is vital to be clear how shelf angle is defined.

    Intuitively, to me, shelf angle is the angle shown in Slieve’s diagram, the angle in relation to the plane of the vertical sail. In Darren’s exposition it is implicit (though there is no diagram which makes this clear) that Darren defines shelf angle in relation to the "horizontal" plane, the plane which is orthogonal to the sail.

    Going back to the first diagram above, Darren defines shelf angle as angle “a”, whereas I had intuitively followed what I thought was Slieve’s definition and was using angle “c”.

    I actually prefer Slieve’s implicit definition (though I don’t know why, it probably doesn’t matter - though it is consistent with a reference I have seen to "horizontal shelf foot" as being "90 degree shelf foot") but the really important need is this: Would the experts please confer, decide and define unambiguously, what is meant by “shelf angle”. Is it in relation to the vertical plane of the sail, or to the horizontal or orthogonal plane of the sail?

    The definition should then be added to the JRA "Vocabulary" of technical terms, and thereby prevent possible disastrous mis-understandings on the part of future DIY designers of shelf foot sails. (And, if I may suggest, perhaps Darren ought to include a clarifying diagram with his instructions).

    As for me, I'm, sticking with 45 degrees!

    ........................................................................

    [PS I would be extremely interested if a QCAD aficionado could give some instructions on the use of this software for designing or drawing boat hulls. Arne seems to have designed his dinghies using QCAD, including lofting the cambered plywood panels. The QCAD manual does not give instructions on how to do this, apart from the use of splines. A how-to PDF similar to Darren’s would be of great interest.

    (I understand that nesting the panels on a plywood sheet can not be done with QCAD, but drawing the plywood hull shape in 2D and developing the conic sections? Striking waterlines and calculating displacement? Centres of area?)]

    .......................................................................

    A second matter, which has been pretty well canvassed now so should be understood by all, but again for future readers, perhaps should be laid down clearly in the JRA "Vocabulary": the term "camber" is ambiguous. A shelf foot sail with 8% of camber should NOT be be compared with a 8% camber sail of some other cut (such as Arne's barrel cut). They are not the same, and since the first question often asked by DIY sail makers is for advice on "how much camber" the following diagrams should be understood. The designed or theoretical camber which is built into a shelf foot sail results in an actual camber which is more - as James G., David T. and others have pointed out. (The same ambiguity possibly occurs when barrel cut sails are designed to have a certain camber, but the actual camber might not be the same?) Anyway, one thing is for sure, you can only very loosely compare the "tin plate" or designed camber of a shelf foot sail sail with the actual camber of a barrel-cut sail.

    This diagram is attempting to show a comparison between the theoretical (tin plate) camber of a shelf foot sail, with the actual camber of a shelf foot sail (which bulges in the way tin plate will not.)


    A good project would be to try to measure and find the relationship between "tin plate" camber and actual camber. It would then be possible to generalise with more validity as to what is the best camber to give to a shelf foot sail.

    It seems to me that at the present time, the only valid way to make comparisons in the camber of various sails is to measure the REAL camber, in the way Arne is doing here.



    (By the way, I built 10% tin plate camber into the jibs and 8% tin plate camber into the mains of my SJR shelf-cut sail. I don't know what the real cambers are, but the sail works just fine. I would be tempted even to push the numbers up a little, for a coastal (sheltered waters) cruiser. James's sail looks good to me. I can't help wondering if River Rat is simply slightly over-canvassed, or perhaps burdened by a slightly heavy mast, rather than considering the camber as being too great).

    Last modified: 25 Jun 2022 11:40 | Anonymous member (Administrator)
  • 18 Jun 2022 04:49
    Reply # 12820665 on 12819719
    Deleted user
    Anonymous wrote:

    Darren,
    I do not want to discourage you, quite the opposite. However, it appears to me that you with all the pondering over details are about to ‘overegg the pudding’  -  simply trying too hard. I fear that all this accuracy drawn into the curves will slow down your hands-on construction work a lot. Remember, those drop-dead perfect camber curves will be thoroughly crushed on that other tack. Even so, my and others’ experience is that our boats sail just as well with the mast on the leeside, where the fore section of the aerofoil is destroyed by the mast.

    I concluded a long time ago that the actual shape of the camber is less important. The important thing is to let the air pass over, and then be deflected by the curved sail, and with the airstream attached to the lee side of it (indicated by the leech telltales standing). To me those standing telltales are the proof that the sheeting of the sail is right.

    Good luck!

    Arne

    Arne,

    There's a sail for everyone, some like pointy mylar sails, some couldn't care about the wrinkles, I can't help but look at Pango's sail and go Wowwweeeeee!   I'm not sure if I'm capable of the artistry in cloth that Paul Thompson achieves, but I'm willing to give it a go (probably with a bit less camber).

    Barrel cut sails are great in a lot of ways, but I don't think they suit every use case.  Like all sails they have their strengths and shortcomings.  For example, I think Barrel Cut does better with stretchier cloth because the stretch absorbs the excess cloth inherent in the method by stretching as you tension the sail along the batten.  The stretchier cloths seem to have held up well for many years on boats that do coastal sailing, but as Annie and David  have pointed out, the hours and wear are going to be much greater on a cruising boat.  I think this makes cloths that are closer to sailcloth (in terms of lower  stretch and high thread counts) more interesting and barrel cut less suitable for my interest.  I could do Barrel (Round) with Broadseaming, but I'm not sure that would be any less work than a Shelf-Foot sail.  There are even differences in construction, I like the idea that the lenses of two shelf-foot panels can be combined into one piece and the batten pocket sewn on when all you have is small piece airfoil shaped cloth to deal with.  I think that will be even easier then Amateur Method B for a Barrel-Cut sail.

    If someone wants the quickest way to get a sail, then I think the Barrel Cut method is the way to go (in large part because of the work you've put into development and documentation).  I'll carefully track my time with the shelf-foot sail and maybe we could find some folks who tracked their time on a barrel cut sail to compare with.

  • 17 Jun 2022 11:02
    Reply # 12819720 on 12819267
    David wrote:

    Oh, and another point: a 60˚/30˚ triangle has sides in the ratio 1:2:✓3 so that a shelf width of 2 units, sewn together with a panel concavity of 1 unit, gives a theoretical panel camber of 1.732 units. As the cloth isn't stiff like tinplate, but will be blown into a curve by the wind, the actual camber is something in the same neck of the woods as 2 units - this is sailmaking we're talking about, not precision engineering ;-)

    So cut a shelf width equal to the camber you're aiming for, and cut the panel concave by half of that amount, and you won't be far wrong.

    I found that out the hard way!  I calculated the panels for River Rat's sails using a complex spreadsheet, but didn't allow for the floppiness of cloth.  The result was that the panels had quite a lot more camber than I was aiming for.  It meant that going to windward in very light airs, she was fantastic... but easily overpowered in stronger winds.
  • 17 Jun 2022 10:56
    Reply # 12819719 on 12819046
    Anonymous member (Administrator)

    Darren,
    I do not want to discourage you, quite the opposite. However, it appears to me that you with all the pondering over details are about to ‘overegg the pudding’  -  simply trying too hard. I fear that all this accuracy drawn into the curves will slow down your hands-on construction work a lot. Remember, those drop-dead perfect camber curves will be thoroughly crushed on that other tack. Even so, my and others’ experience is that our boats sail just as well with the mast on the leeside, where the fore section of the aerofoil is destroyed by the mast.

    I concluded a long time ago that the actual shape of the camber is less important. The important thing is to let the air pass over, and then be deflected by the curved sail, and with the airstream attached to the lee side of it (indicated by the leech telltales standing). To me those standing telltales are the proof that the sheeting of the sail is right.

    Good luck!

    Arne


    Ingeborg's sail showing its anything but perfect camber. Ingeborg still sails well close-hauled, with the leech telltales flying.

    Last modified: 17 Jun 2022 11:43 | Anonymous member (Administrator)
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