Polytarp lugsail 3D shaping

Discussion in 'Sailboats' started by laukejas, Jan 15, 2015.

  1. laukejas
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    laukejas Senior Member

    Hello,

    For a little project I'm planning to do this summer (I have a thread about it in Hydrodynamics section), I've chosen a standing lug sail rig. Since right now I don't have enough money to make this 7.3m^2 sail at sailmaker, I want to make the first sail out of polytarp and test it, so that I can make adjustments in design and one day invest in a real sail.

    Since polytarp comes in cheap, very large sheets, but the sticky tape is rather expensive, and my sailmaking skills are amateur at best, I won't mess with broadseaming (or broadsticking, in this case), but revert only to darts in this sail. I am now extensively researching theory behind dart shaping. There isn't much info, as many sailmakers rely on their experience and gut feeling, things I lack. There is one good article I found by Jim Machalak, and it's here:

    http://www.jimsboats.com/webarchives/2008/1nov08.htm#Lugsails From Polytarp 1

    I think I understand most of what he wrote, but what concerns me is what he didn't write, and some inconsistencies along the way. For draft, he chose 10% "from leech to luff" (quite confusing, given that these two are not parallel). Then, he chose a line (E-C) which is *almost* vertical (yet not exactly vertical for some reason).

    At this point, several things are unclear to me:
    1) Why z coordinate for the point C is 2 inches deeper than same coordinate for point E?
    2) Why E-C is not exactly vertical? Is this small angle result of some relationship between some of the lines?
    3) Why E-D is not horizontal, but again, almost horizontal?
    4) Most importantly, how did he choose the length and horizontal distance from luff of the E-D line? Why not closer to the luff, why not farther?
    5) Anyway, why did he chose 10% draft? Is this a lot? What are normal limits for lug sail of such size, considering that polytarp is already stretchy material?

    (I deleted some of questions that were in the original post because I figured them out)


    Maybe someone could help me out with this homework? Also, if I'm barking up the wrong tree, maybe there is a better article on dart shaping in 4 sided sails? :)

    Thanks in advance!
     
  2. laukejas
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    laukejas Senior Member

    I went ahead, and made a pattern with darts for this sail (standing lug sail from my project):

    [​IMG]

    The pattern looks like this:

    [​IMG]

    These darts go to the E-C line like in Jim's article, and this line is 1/3 between luff and leech. Draft is 10%.

    Then I printed 1:20 scale pattern on A4, cut carefully and overlapped those darts with some sticky tape.

    Here are results:

    [​IMG]

    [​IMG]

    [​IMG]

    I can't say I am happy with this shape. I am not sure if you can see from these poor-quality images, but the resulting sail has a sharp transition at maximum draft line, almost breaking there. It is not as smooth as I expected. Maybe this is because printing paper is stiff and inflexible, unlike polytarp, maybe I should have used less sticky tape on those seams, or maybe my darts are just wrong.

    I have no way of knowing. Does anybody have any good info on dart shape? Jim's article is great for producing patterns, but he says nothing about initial dart choice.
     
  3. Skyak
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    Skyak Senior Member

    I don't know why he chose the dimensions he did and I think he is the only one who can answer with certainty. I suspect you will wait a long time and only get guesses so here are mine.

    -The shape you are trying to create is to be vertical on the boat sailing -it has no parallel or perpendicular relationship to any edge.
    -The shape you are trying for likely has a depth or camber that is proportional to the horizontal length of the sail at any given point along a vertical line.

    That is the scientific portion of my answer.

    My opinions are:
    -Polytarp is so stretchy and elastic there is not much value in trying to shape it by cutting and joining and much to loose in strength. I would skip the darts altogether and just shape the sleeve on the yard and boom. He calls it a 1" round but I would say you want to reach the deepest point in the first 20 to 30% of the sail and tapper straight back from there.
    -making a flat leech out of the baggy material is difficult and worthwhile -a leach line in a hollow cut leech and even a couple battens might achieve this though I have never seen battens on a lug -just a Junk.

    About expensive double sided tape -have you considered 'contact cement'? It's a crazy strong glue that you apply to both surfaces and let dry. Then you carefully put them together with a little pressure. The glue does not stick very well to anything but itself when dry -but it it sticks to itself stronger than your tarp base material. It's the glue Yost uses to skin his kayaks and the Russians use for their inflatables. It's commonly used to stick laminates on kitchen counter tops.
     
  4. laukejas
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    laukejas Senior Member

    I'll try to make that shape vertical as Jim did, but what exactly did you mean by "likely has a depth or camber that is proportional to the horizontal length of the sail at any given point along a vertical line"? I have red that a dozen times and still scratching my head.

    P.S. Oh wait. I figured it out.

    Well, I wanted to go for loose-footed... As for yard, I already expect it will bend, so i have to compensate for that too. A lot of guesswork. I'm afraid I might end up with flat sail. My design calls for decent performance in ultra-light winds, so a decent draft is required.

    I'll consider your advice to forget darts, but if I make them, what shaping would you recommend? 20-30% from the luff, line between deepest points of darts - vertical,that much is clear. But what about the height and length of this line? For example, in my current attempt, the tack corner dart had it's deepest point on a line which extends from tack corner to the leech, and is perpendicular to it. Same for throat corner. Is that the right idea?
    Also, should these darts be a straight V, or curved V?

    As for leech hollow, I'll definitely add it, just haven't got to it yet!

    I know that one, vinyl cement HH-66. Haven't seen anything like that in our hardware stores, but I could look again. Only that Yost uses heat gun to reactivate this glue before joining, and polytarp can't stand heat at all (I tried). Have you attempted this yourself?
     
  5. Skyak
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    Skyak Senior Member

    The only answer I am sure of is 'curved darts' not straight. You don't have to cut to make a dart. You can just fold and tape. That way you can pull it apart and try again if you don't like the result.

    I am not sure what to tell you about a loose footed lug sail because so much will depend on the angle and tension of the main sheet. I tend to think you need either a boom or a traveler to control shape. Otherwise you have no 'outhaul' control and trouble controlling twist.

    As I told you before I don't have experience with lug or polytarp sails. This calls for both.

    Contact cement is plenty strong without heat but there may be compatibility issues with polytarp -I have no idea what plastic they are made of. I don't think anyone knows.
     
  6. rcnesneg
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    rcnesneg Senior Member

    Darts should look like this:
    [​IMG]

    Curved, and they should give you the shape you're after. Like they said, don't cut, just fold and tape or glue. That way, if for some reason your seam comes out, it doesn't tear the sail, it just deforms it and you can still fix it real easy. If you cut it, and the seam comes out, you've got a big gaping hole in your sail.

    Good luck!
     
  7. PAR
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    PAR Yacht Designer/Builder

    Getting a good shape out of poly tarp material is a crap shoot at best. I've never seen a poly sail set anywhere near a reasonable shape. Yeah, you can get it to billow with some wind on it, but usually, even built by the best poly tarp seamstresses, they suck wholesale in comparison to a real, well cut sail of the same dimensions. I've seen this in action with real sails on one designs against the finest home made poly sails and the polys get their butts kicked on every point of sail. The only real advantage of these sails is you can get a sail like bag cheap, but don't exspect it to last very long, tolerate high winds or set very well.

    As far as understanding how to cut one, why you might want to include rounding and darts, well some research into how sails are built might help, which will cover the basics and looking at polysail.com might offer some insight to how the better (relative term) poly ones are made.
     
  8. gdavis
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    gdavis Junior Member

    Try using Tyvek building paper instead of polytarp, it has nearly 0 stretch. 3M makes a spray contact cement called 77 spray, wicked good stuff. With the sail layed out on the floor you can run your bolt ropes and then fold and glue the tyvek over them. I've also sewed them after the edges are glued with a cheapo sewing machine from W.M. Pretty easy to sew thru 5-6 layers of corner patches.After using your polytarp sail a few times it will start to lose it's shape very quickly tyvek won't.....................have fun.....g
     
  9. PAR
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    PAR Yacht Designer/Builder

    Tyvek works, but most find they can only get the perforated house wrap stuff (type 14), which is a bit self defeating for a sail. There's a huge difference between the types, with the preferred stuff being the type 10, which is fairly stiff, though the type 14 isn't comparatively and is perforated if house wrap. The worst part is the racket these sails make.

    The bottom line is there's no short cut to a good setting sail. You either learn how to shape one, or you Mickey Mouse it with darts and duct tape. Guess which will set well.
     
  10. laukejas
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    laukejas Senior Member

    You misunderstood me there. By "loose footed" I meant loose footed, not boomless. There will be a boom, but the sail will attach only at tack and clew.

    Thank you, good advice. But you said "curved", and didn't explain it. Could you, please? Which direction should the curve go, and how is this curve defined?

    I can see you are pretty angry about polytarp sails... But I understand your reasoning. I only made 1 polytarp sail, and although I wasn't happy with performance, it was better than nothing. At high winds, the shape made me shudder in horror, but it survived gale winds (35+ knots) and brought me home. Of course, if there is better option than polytarp, I'm all for it, it's just that I can't afford professionally made sail.

    I have used Tyvek in the past. Although it is not easy to find tape that sticks to it, it is much more pleasant to work with than polytarp, mostly because you can draw on it with pencil just like on paper, whereas polytarp only accepts dry soap, and that provides error margin of several centimeters at best.
    I am not sure what do these types correspond to what we have here, but here, 3 types of Tyvek are sold: Soft, Solid and Pro. Soft is really weak, one layer stuff, definitely not good choice for a sail. Solid is 2 layers, I haven't used it, but a friend of mine made a junk sail and it lasted. Pro has one additional layer to a total of 3, but this last layer is green, feels like a carpet, and de-laminates very easily. I have used it for 3 sails in total (can be seen here and here), and I really must say that this additional layer makes sail very ugly, and de-lamination requires constant attention.
    Oh, and I forgot - no sticky tape sticks to this additional layer. Well, it kind of does at first, but after a couple of days, it goes off by itself, no force needed. My guess is some chemical incompatibility.

    So, if I'm to use Tyvek, I'll go for Solid. I don't know what type does it correspond to, but maybe you know.

    I agree, but what if I can't afford a visit to sailmaker? What would you say is the next best solution? I can spend 50-100$ on this sail at most.

    And by "learn to shape one" you probably mean that instead of darts, I should go with broadseaming? It can be done with Tyvek, I believe.
     
  11. laukejas
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    laukejas Senior Member

    I attempted 3 more designs that I cut out of paper with various dart widths, lengths and angles, keeping the imaginary line between ends of darts vertical. Out of these 4 sails I attempted, the last one is pretty good, compared to others.

    Here is the pattern:

    [​IMG]

    And here are some photos:

    [​IMG]

    [​IMG]

    [​IMG]

    As you can see, the shape is a big improvement over what I did yesterday. Much smoother and pleasing to the eye.

    The before-mentioned imaginary line between ends of darts (let's call it E-C, like in Jim's article) is vertical, and is 1/3 forward from the middle of the sail. The ends of E-C line, however, is defined by half-angles running from tack and throat corners. Therefore, for example, tack corner dart extends at a half-angle between luff and foot, and throat corner dart extends at half-angle between head and luff.

    This gave the best result out of all attempts, as the ends of these darts are no longer "breaking" the sail as much as before.

    Yet, as you can see, these darts are not curved. Maybe curves could improve shape even further. If anybody could give any info on that, it would be great.

    I am also considering to try my hand at broadseaming, but again, with paper and tape. Only that I don't know how to shape those broadseams. Even Emiliano Marino on his famous "Sailmaker's Apprentice" doesn't go into detail on theory behind broadseam shaping. If anybody could shed any light, I could take this sail to a whole new level.

    And thanks for the help so far!
     
  12. PAR
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    PAR Yacht Designer/Builder

    I've made about a dozen tarp and Tyvek sails over the years, usually to test a rig conversion or something. The only way to shape a sail other then mold it is to broad seam. With material as wide as tarps and Tyvek, you can have pretty big panels, but you still need to know what to do with them. You can play around with "SailCut" software, but again, you do need to know what makes a good sail shape, before you can ask the panels to conform.

    I don't hate tarps or Tyvek, but I do put them well below other materials. It's a bit like using hemp standing rigging after enjoying stainless, it's hard to go backwards, once you've enjoyed far superior stuff. Maybe a bit like remarrying an ex-wife - didn't you learn the first time . . .
     
  13. laukejas
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    laukejas Senior Member

    I don't hate tarps or Tyvek, but I do put them well below other materials. It's a bit like using hemp standing rigging after enjoying stainless, it's hard to go backwards, once you've enjoyed far superior stuff. Maybe a bit like remarrying an ex-wife - didn't you learn the first time . . .[/QUOTE]

    Well... That's a convincing comparison. I'm almost glad I haven't tried professionally built sail :D

    P.S. I'm surprised that apart from Dacron, Tyvek and polytarp, no one suggests traditional cotton. After all, it has been used for hundreds, if not thousands of years. What's the deal with it nowadays? Is it even worse than polytarp? Why is no one talking about making cheap sails with cotton?
     
  14. gdavis
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    gdavis Junior Member

    So over here we have tyvek and typar. the typar is probably equal to your soft and is not good for sails. There are also company's that make tyvek like fabric that is much tougher. I've seen it used for a shipping wrap for modular homes, big machinery and wind mill blades. I think I would use it for large junk sails and the like. You could ask about contact cement at your local cabinet or woodworking shop if your having trouble finding it, most cabinet shops use it for c-tops and some veneer work. I say if all you can afford or you just don't want to spring for sail cloth use poly tarps, tyvek or whatever and go sailing! Learn about sail making and have fun. As for cotton sails the panels were always quite narrow to keep the stretch at a minimum. And after a break in period there was a lot of rework to keep the desired shape. For a quick and dirty sail cotton would be much more work and at that point you might as well go to Dacron. Also, trying to use just a large single piece of cotton will result in a whole lot of stretch out, much more than tyvek. If you can avoid the really cheap poly tarps and find some of the better ones that are made a bit heavier you'll get less stretch. I made a fairly large lateen sail with it and used for for a summer and it seemed much better than the almost see thru discount store blue tarps. the blue ones are 8x7 weave, the white is 12x12 and silver is 14x14 weave. Do you have the heavier ones over yonder? Okay, hope some of this helps...................................peace....g
     

  15. Pericles
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    Pericles Senior Member

    Because a cotton fabric strong enough to perform as a sail would cost more than Polytarp. They also need to be protected from the elements & dried after every voyage.

    These links should help you.

    http://en.wikipedia.org/wiki/Sailcloth

    http://en.wikipedia.org/wiki/Sail#Sail_construction

    Make your standing lugsail flat without darts. Sew in a rope to reinforce the luff & allow the leach to shape itself. That will soon happen & eventually you'll have to make another sail. Meanwhile you'll be sailing & not agonising over your present lack of sailmaking skills.

    I favour lateen sails as you sew the sail to a spar & haul it up an shorter unstayed mast. With the right rigging, sheets & spar proportions you could change tack without having to go forward.

    http://en.wikipedia.org/wiki/Lateen

    You could even revive the Artemoon & set a rectangular sail on a yard.

    "Quinquireme of Nineveh from distant Ophir,
    Rowing home to haven in sunny Palestine,
    With a cargo of ivory, And apes and peacocks,
    Sandalwood, cedarwood, and sweet white wine."

    http://i.ytimg.com/vi/LHgXYZZNEnE/hqdefault.jpg

    http://allpoetry.com/Cargoes

    Have fun,

    Perry
     
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