Plastic "skin" instead of epoxy

Discussion in 'Materials' started by cac, Dec 27, 2006.

  1. cac
    Joined: Oct 2006
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    Location: Stockbridge, GA

    cac Junior Member

    It seems like this has been knocked around sometime before, but I can't find it now...

    Does anybody want to chime in on the feasibility of using Kydex/HDPE/XXXX as a "skin" on the outside of a plywood boat? What I mean is, build the boat with the normal framing and skin, fasten it all together good, coat the outside with "glue" of whatever sort is deemed appropriate for a wood/plastic bond, then fasten on sheets of the plastic.

    Most of these materials can be either heat or chemically welded together so that the edges are fastened and it becomes one big (hopefully waterproof) skin.

    The wood still provides the structural strength, but the plastic provides the impermability and impact resistance.

    The immediate problems look (to me) to be:

    - different expansion rates between the wood and the plastic
    - UV protection for the plastic (there are marine and outdoor grades availalbe)
    - Welding it so that its truly waterproof

    Thoughts?
     
  2. JR-Shine
    Joined: May 2004
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    Location: Vero Beach, FL

    JR-Shine SHINE

    the first two that come to mind (there are more):

    The boat will be much heavier. You get no stiffness from the plastic as you might from the glass, not to mention the plastic itself is heavy.

    I would worry about delamination, bonding plastic to wood is not easy

    We have a builder of our XF20 (plywood) skiffs who runs over oysters and rocks. His bottom is several layers of biax glass in epoxy. He also has a final coat of graphite powder/epoxy for extra slippers surface.
     
  3. cac
    Joined: Oct 2006
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    cac Junior Member

    Good point on the weight, although fiberglass and epoxy has to weigh something, but probably not as much as even a very thin skin.

    On the delamination... not trying to be argumentative, but why would we care? Obviously I don't want the entire skin to fall off, but if its all welded together and is essentially one piece, if there is a "bubble" under it, what would happen?

    Now if water got in there (and I'm sure it would, somehow), then you've got a rot problem, but...

    Just tossing some ideas around.

    Thanks.
     
  4. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Here in the NW there are boats with a UHMW shoe (skin) on them. The boats are designed for going over rocks in shallow rivers, The UHMW slides easily over the rocks with little or no damage. Not many people use a shoe on their boat, but they can be put on aluminum, wood and fiberglass boats.

    The shoe only covers the bottom of the boat and is not water proof in any way, attatching it can be a bit tricky because glues don't stick well to UHMWPE, that means you need to mechanically fix it in place.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    HDPE and UHMW shoes have been around for quite a while and the are not a "skin" but exactly as their use suggests a "shoe", which is short for grounding shoe. These are strips of plastic bedded and typically screwed to the keel and maybe the chines. They're also seen on trailers to limit damage as the boat slides on. I use the stuff all the time, but it has limitations like everything else.

    There are several difficulties that can be associated with this type of sheathing. Because it is a sheet goods product, the hull will have to be "developed", though with the use of heat, you may have some luck draping the material over compound curves. They would have to be installed as planking as no adhesive is very good at bonding this stuff. Some adhesives do a reasonable job of stick it down to non-porous surfaces, but the grip is questionable. It can be welded, but this would likely cause issues with the bonding adhesive. Any separation between the sheathing and the substrate will permit moisture (found in the air) to have a nice comfortable place to rot out things. It offers no structural component, in spite of the healthy weight penalty paid for it's being there. It can't be painted, so you'd better like the colors you've chosen. This is the short list.

    If you're willing to design the additional displacement into the boat, to support the HDPE (or what ever) skin, it would be nice if it did something better then other, lighter, less costly materials.
     
  6. cac
    Joined: Oct 2006
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    Location: Stockbridge, GA

    cac Junior Member

    Thanks... that's some points I hadn't thought of, and all boiled down in one easy paragraph :)

    Bruce
     
  7. Ike
    Joined: Apr 2006
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    Ike Senior Member

    My first question would be, why?

    Wood is a very tough, resilient material.

    Wood is easily repaired.

    Wood can be easily protected with, paint, sealers, expoxy, and a variety of other coatings.

    Most mahogany plywoods are relatively light.

    If you want to build compound curves rather than developed panels you can cold mold the hull with veneers that are epoxy saturated.

    So why go to all the trouble to use a really good material and then cover it with plastic?
     
  8. cac
    Joined: Oct 2006
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    cac Junior Member

    Rational...

    Well, my thoughts/rational was:
    • Bottom protection... its tough
    • No bottom paint (potentially... I don't know how well critters latch hold to some of this plastic, but might be worth a shot
    • Use cheap wood for the structure (not worrying about the finish) with the plastic being the "pretty" layer (providing you like the color and finish of the plastic, which I kind of like)
    • No epoxy... would depend on if the "other" glues and welding were worse to deal with or not. Epoxy just "bugs" me to think about using in large quantities... small amounts are a big enough pain
    • Right now we take (usually very nice) wood and cover it in epoxy and/or paint... I was brainstorming other things we could cover it in and maybe get the same effect... maybe for less work/cost/??? and maybe for more effectiveness.

    Anyway, I figure ideas are sometimes worth tossing out and seeing if they float or not :)
     

  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Nothing new ever gets done without tossing around some harebrained goo, just to see if it sticks. Keep thinking out of the box, if for no other reason then you can only have so much beer on hand, when inside the box.
     
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