Epoxy over XPS method

Discussion in 'Boatbuilding' started by mvoltin, Nov 16, 2018.

  1. rxcomposite
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    rxcomposite Senior Member

    Somebody has beaten you to it. There is a high density foam core commercially available (the brand escapes me for a moment). This a foam core with fiberglass embedded in the core and is usually used in transom of powerboat. If you want optimum shear strength, the fibers should be running at +-45 degree angle to the base, not 90 degree. At 90 degree, you are only increasing vertical shear strength, not in plane shear. Better use balsa, it is lighter.

    If you want to improve the core shear, you have to understand the physics. Shear is greatest at the center and diminishes towards the surface. That is the first principle. Therefore, in order to optimize, use high density/high shear strength in the middle and laminate low shear/low density foam on both sides. Laminated foam anyone?
     
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  2. fallguy
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    fallguy Senior Member

    The amas are based on the work of the much respected Gary Dierking. They are really only canoe stabilizers for me. So not even subject to sailing speeds. The amas can be used in sailing craft. They are typically made from glue ups of xps and wrapped in glass. In forming; they are shaped and rough sanded which does increase their surface area. A nice coating of thickened resin can also create an impact shell, although not needed.

    this is some really outstanding and fun stuff for anyone who loves boats..

    Foam and Fiberglass Ama Construction https://outriggersailingcanoes.blogspot.com/2021/02/foam-and-fiberglass-ama-construction.html?m=1
     
  3. fallguy
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    fallguy Senior Member

    coosa
     
  4. Prettypicturegirl
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    Prettypicturegirl Junior Member

    Thanks guys, just looked up Coosa, but a) its pretty heavy and b) its pretty expensive.

    Looking at rxcomposite comment regarding angled beeing better than 90 degree "connectors", that points me a little towards the kind of strip planking method I was thinking about, but with the board sides at 45 degrees angles along the edges.
    So both skins are longitudinally connected by glass bands every say 10-15cm which are at 45 degrees in crossection.
    A lot of work too. Hmmm.

    Any alternative low budget materials? Wonder fiber "W" = Wood springs to mind, but I am not a big fan of it as it can rot.
     
  5. rxcomposite
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    rxcomposite Senior Member

    Right. Sharp you are.
     
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  6. fallguy
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    fallguy Senior Member

    Plywood sheathed in glass is a pretty perfect material afa low budget goes. Tabbing is generally not bulky and fairing is relatively simple. Ply is strong and stiff on its own. Properly decored, ingress risk can be greatly reduced; especially if the builder is the owner.
     
  7. missinginaction
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    missinginaction Senior Member

    I don't believe I ever commented on this and I should have. Back over a decade ago I removed a pair of rotted sagging stringers from my old Silverton as I restored her. I used Formular 250 foam insulation as the core material. Right out of Home Depot. If my memory serves me I covered them with 1708 biaxle stitchmat. I can't recall exactly how many layers but I used the scantling rules out of Dave Gerr's book "The Elements of Boat Strength".

    Those stringers have been holding up my main engine/trans/v-drive for over 10 years with no issues at all. I'll see if I can find a photo or two. I did add some wood to the assembly where the engine beds attach for compressive strength.
    IMG_0778.JPG IMG_0242.JPG
    123490-029b012bad5cccc5f897360b3ad60e66.jpg
     
  8. Dejay
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    Dejay Senior Newbie

    I haven't posted here for a while but I'm still dreaming of the perfect solution haha.
    Fallguy is of course right from a commercial boat building perspective it's a solution looking for a problem. Just buy the damn overpriced PVC foam!

    But the "Truss Core Sandwich" could solve these problems: 1) how to build dirt cheap and light weight 2) how to have internal flow channels to reduce waste material like external flow medium 3) how to vacuum infuse thick insulated walls

    The development of a "foam sewing machine" would be expensive, lots of time to design and test something like that. Maybe a year for an engineer. But the actual material costs would be low using motors and electronics from 3D printing, most special parts could be 3D printed. So once such a design exists open source you could get the parts, build it and let it run for a few weeks to get your panels. You'd feed a stack of XPS panels on rollers through a gantry where the stitching occurs, panel after panel. It only costs time.

    If you'd for example wanted to fabricate large scale wall panels for something like RVs or tiny houses you could build reusable two part molds where you just throw in the ready stitched XPS panels in and can use thinner fiberglass on the outside. For maximum stiffness you could use carbon roving. Again lots of development and uncertainty if it works as advertised. But if it does it could be superior to honeycomb (someone would have to do the math) and be insulated as well. Cheap and easy to transport container house kits could help in the future for reacting to disasters and refugee crises.

    Prettypicturegirl the 45 degree idea sounds like what was suggested earlier to basically create "corrugated" sandwich structure. I'd think that should work just more manual labor.
     
  9. fallguy
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    fallguy Senior Member

    What about strip planking and alternating polystyrene and wood?

    say, 1/2-2 inch thick core and strips of wood 6 mm thick and strips of foam 25mm wide

    then strip plank it; the wood acts as a shear web and longitudinal stiffener

    cheap, lotsa labor; if the foam crushes or delams; it only delams to the web

    still not sure I like the hull with 400 horsepower and flying off 6' swells...but maybe for a canoe, a modification to say 6mm foam thickness could work and be an ultralight..
     
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  10. Dejay
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    Dejay Senior Newbie

    Hmm, now that and the other things and reading the harry proa build log again gave me an idea.

    For the cargo ferry prototype Rob Denney is experimenting with trapezoidal stringers on solid fiberglass. In the link above you can see the pictures. He's using XPS foam as formers. This is apparently cheaper, lightweight and easy to lay out and infuse. He's still experimenting and getting help from a university testing this.

    But you could also add XPS foam for insulation "in between". So not a "fully corrugated core" or stitched core like suggested earlier, but a few strategic stringers to add stiffness. This reduces the "unsupported span" for the XPS which isn't really supposed to provide shear strength between the layers. Because that is what the stringers are supposed to do even without insulation and an interior skin. Hope that makes sense.

    Not sure if you'd need to perforate the XPS or if some kind of scoring would be enough to make sure the resin flows.
     
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