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Thread: Testing stitch & glue plans with cheap plywood

  1. #1
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    Default Testing stitch & glue plans with cheap plywood

    Hi all,

    A year and a half ago, my wife and I used Freeship to design the panels for a stitch and glue El Toro. We didn't test the design and went straight to building it out of 6mm Hydrotek, and amazingly it came out on spec! We later realized that our spec wasn't quite right -- we'd built the 7' 10" min-length design shown in the construction guide, and not the 8' version everyone else uses. Oops. That's okay, this boat wasn't for racing and is for our daughter to grow into (we didn't quite get it done before she was born, but had it done before she turned 6 months, and can barely squeeze 3 of us into it to circle the harbor).

    Our daughter is one now, and we're wanting to build a second El Toro so we can all go out at once. We spent a lot of time test fitting panels for the first boat, and I've become a bit obsessed with the idea of pre-designing all the panels for the boat beforehand. Sort of like a build-your-own-kit. So we'd cut out every panel before assembling anything. I don't want to use Freeship for this - we didn't have fun with Freeship for the first boat (I don't use Windows, so that was a hinderance in itself), so we're starting from scratch. I write software by day, so I've been cobbling together a programatic CAD program (like OpenSCAD, but for developing panels). I have each major panel figured out, with a list of coordinates and a diagram nested for 4x8 plywood.

    So here's my question - I'd like to test these panel shapes before cutting into nice plywood. If I use cheap plywood, can I expect it to bend similarly to Hydrotek, or will it crack (or not bend in both dimensions the same)? I was thinking I'd use the cheapest .25" 4x8 sheet I could get my hands on, and stitch a test boat out that way. No epoxy, only wire stitches, and it'd all be scrapped after. I'd stitch the outer panels, put spreaders in, then stitch in the seat tanks (we have full length tanks; I'll try to attach some pictures). I'd then have our friend with a measuring jig measure the test hull to make sure it's all class legal. If it checks out, then I'd start building the real one out of Hydrotek.

    If you all think that cheap plywood is a bad way to test this, then I'll go ahead and figure out how to import my 3D lines into Freeship, then export the panels, and compare to what my program came up with. I just don't have a Windows machine handy to do that easily, and plywood sounds more fun. I also like the added benefit of using a measuring jig on the real thing to add confidence.

    I've been reading the forum pretty seriously since we started the first El Toro, and it's been hugely informative! Here goes my first question!

  2. #2
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    Default Re: Testing stitch & glue plans with cheap plywood

    For more context, here's the first El Toro. Everything but the outer 4 panels were done with trial and error, which is what I want to avoid with the next build.



    Here it is, plus tiny skipper, after much more finishing work:



    And here's the perspective view for the one in question which requires testing. The main difference between this and the original (besides that it's 8' and not 7' 10"), is that the forward bulkhead has a cutout so that the mast step doesn't get stored full of water, and the seat tanks extend fully to the bow. Shape of the tanks and shearwaters is slightly different too.


  3. #3
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    Default Re: Testing stitch & glue plans with cheap plywood

    Wouldn't it be easier to make a model? Instant Boatbuilding With Dynamite Payson (LINK) has a comprehensive chapter on the whys & hows-to.

    The aircraft plywood used in model making is bound to be cheaper & better than the crap wood you're thinking of using; and when you're done you've worked out the building kinks and have a nice model to display.

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    “What use is a house if you haven’t got a tolerable planet to put it on?”


    ~~~ Henry David Thoreau

  4. #4
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    Default Re: Testing stitch & glue plans with cheap plywood

    I have used cheap plywood or even a 4 mm board variety that looked like it was made from sawdust, cellulose fibers and resin glue to validate stitch & glue shapes, and I liked that very much. The best thing about that procedure was that I could use these as templates, stapled to good okoume plywood, to duplicate the shapes with a copy router in less than no time.
    You have easy bends in your boat. Should work without problems. Axel

  5. #5
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    Hyannis, MA, USA
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    Default Re: Testing stitch & glue plans with cheap plywood

    Could the LOA error be due to taking the plan lines, normally to the inside, as true LOA outside? If so, was there also a small error in the beam and/or molded depth?

  6. #6
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    Default Re: Testing stitch & glue plans with cheap plywood

    Quote Originally Posted by sharpiefan View Post
    Wouldn't it be easier to make a model? Instant Boatbuilding With Dynamite Payson (LINK) has a comprehensive chapter on the whys & hows-to.

    The aircraft plywood used in model making is bound to be cheaper & better than the crap wood you're thinking of using; and when you're done you've worked out the building kinks and have a nice model to display.
    I think this would have been useful before the first boat, to get an idea of all the parts involved. But I've already built one (just without it spec'd out), and have a pretty good idea of the process (and an especially good idea of all the parts, since I've designed all the panels on the computer). What I want to test is how the exported shapes fit together, to make sure that the algorithms I came up with actually develop the panels correctly. I'm 99% sure they do; I've printed them out on paper and taped them together, and it works perfectly at that. El Toros have a building tolerance of 1/4", and I'm not sure I'm skilled enough to build a scale model and scale down the tolerances as well, so there's some benefit in doing it full sized there.

    Also, this may have been way more work than it is worth, but my inside panels are all calculated for the actual plywood thickness of the pieces they butt up to. So that's part of what I want to test here, is if I did those calculations correctly.

    Thanks for the book reference; I will likely use that for the next boat that's not an El Toro!

  7. #7
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    Default Re: Testing stitch & glue plans with cheap plywood

    Quote Originally Posted by canoe_sailor View Post
    I have used cheap plywood or even a 4 mm board variety that looked like it was made from sawdust, cellulose fibers and resin glue to validate stitch & glue shapes, and I liked that very much. The best thing about that procedure was that I could use these as templates, stapled to good okoume plywood, to duplicate the shapes with a copy router in less than no time.
    You have easy bends in your boat. Should work without problems. Axel
    Wow, you can use 4mm as a template? I had thought of that, but didn't think it'd have enough height for the router bearing. That's definitely something to look forward to! Copying the coordinates to the plywood isn't my favorite task.
    Last edited by yarpa; 10-12-2017 at 11:06 AM. Reason: fix quote

  8. #8
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    Default Re: Testing stitch & glue plans with cheap plywood

    Quote Originally Posted by Ian McColgin View Post
    Could the LOA error be due to taking the plan lines, normally to the inside, as true LOA outside? If so, was there also a small error in the beam and/or molded depth?
    Nothing as fancy as that. The construction guide from the class association has the measurements marked out on stations for a 7' 10" boat by design. It mentions you can stretch them to be the max 8', which we just didn't do for that boat.

    I *think* the reason they show a 7' 10" and not an 8' is that it claims you can't build an 8' without scarfing a panel. For this next boat, I nested the panels on a slight diagonal, and they fit on 1 sheet without scarfs. So I'm not really sure why they don't just say to do that.

  9. #9

    Default Re: Testing stitch & glue plans with cheap plywood

    Quote Originally Posted by sharpiefan View Post
    Wouldn't it be easier to make a model? .

    1/4 scale model worked for me.


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