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Thread: Would this fiberglass/plywood method work?

  1. #1
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    Default Would this fiberglass/plywood method work?

    Hi

    I recently finished a fiberglassing job on a small 3.6m plywood boat I am building (Selway Fisher TAW). after an excruciatingly painful learning curve, I've managed to get a workable surface thanks to long hours with the long board sander and fairing compound.

    I will be Looking at building an 8x4 meter catamaran next and would like to ask if the following method would be possible.

    The idea would be to cut out all the panels from plywood, laminate what will be the outer face of each (while still horizontal and separate) with fiberglass and epoxy, leaving a 5cm margin around each panel, to later be glass taped together at the seams (10cm tape), thereby reducing sanding and fiberglassing on large uncomfortable areas to only the seams. Small radius curves would have to be glassed later.

    My main question is, would it be okay to bend these ply/glass panels or will it cause problems ie delamination etc

    Many Thanks,

    Ashley.

  2. #2
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    Default Re: Would this fiberglass/plywood method work?

    Should be OK. With the glass on the outside of the curves delamination would be unlikely even if you use crappy resin.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Would this fiberglass/plywood method work?

    Just keep in mind that the 'glass layer will increase the stiffness of the plywood to a surprising degree for such a thin layer. For the gentle curves of a cat hull it should not be a problem, but you might want to do a test run on a scrap of ply and bend it to the most extreme curve of the new boat shape before committing to a large amount of new, expensive plywood.
    Hope for the best, but plan for the worst.

  4. #4
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    Default Re: Would this fiberglass/plywood method work?

    Ashley, You might have the same issues as your first project if you use the same methods and tools for applying the fabric.

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    Default Re: Would this fiberglass/plywood method work?

    Thanks for your input guys!

    Navydog, yeah I think I've learned my lessons! After sanding an entire hull of badly laid fibreglass, I did multiple tests on scrap ply, and finally reattempted. A few areas were not so pretty but workable and the rest were fine. I basically over drenched on the wetting out stage, allowing epoxy to get under the fabric and ripple. Defo learned the hard way!

  6. #6
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    Default Re: Would this fiberglass/plywood method work?

    You can roughly pre-bend the panels on sawhorses before glassing to get around the stiffness issue.

    If you're glassing the inside to add shock resistance you will be much better off doing this. If you glass the inside with the panel flat then bend it in place the glass will be compressed and lose a lot of it's usefulness.

    Either way it's not likely that it will delaminate.

  7. #7
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    Default Re: Would this fiberglass/plywood method work?

    Hi Gib,

    Thanks for the tips! I'm not sure if the design I'm looking at requires glass inside and out yet but useful anyway!

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    Default Re: Would this fiberglass/plywood method work?

    It all depends on the bend amount. First of all, understand that it will make the panels significantly harder to bend if their outside is pre-glassed flat. Secondly, bending a panel which was glassed flat just a tad too much will fracture the glass down inside the weave - generally not enough to damage it significantly (unless you want a clear finish) but there is no question that fractures of any sort do weaken the laminate to at least some extent. They will show up as whitish, with an obvious weave pattern where there was none before. They may also telegraph cloth texture to previously filled and flat-sanded areas.

    As for the inside glassing, the compression is so slight because the glass is on the inside of the curve that you aren't really going to lose any significant strength. These strip panels were glassed flat on their insides, bent to shape and seamed together. It worked just fine. The outside was glassed after the sides, bottom and transom had been assembled.

    I'd suggest that if you do pre-glass panels, do it all the way to the edges and then overlap your seam tapes onto those edges, Two or three quick coats of resin over those spots will blend the seam tape in well enough to fair it smooth without cutting into the cloth.
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    Default Re: Would this fiberglass/plywood method work?

    Thanks Todd! I think I will take a vague measurement of the curvature of each panel and try and replicate that curve on the work table. If the curve looks too small of a radius I will glass after fitting the panel. Thanks for the info and photos!

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    Default Re: Would this fiberglass/plywood method work?

    I wouldn't try glassing the ply before shaping the hull-just in case.Just be careful that the resin doesn't drain when applied to the hull panels.

  11. #11
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    Default Re: Would this fiberglass/plywood method work?

    Hi John, I just found on my last project that it was so difficult to get an equal covering of epoxy on the vertical panels. If I can at least get the side panels of the cat preglassed on the outer surface by mimicking the curve on a table, I can see it saving a lot of headaches!

  12. #12
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    Default Re: Would this fiberglass/plywood method work?

    Quote Originally Posted by Ashley View Post
    Thanks for your input guys!

    Navydog, yeah I think I've learned my lessons! After sanding an entire hull of badly laid fibreglass, I did multiple tests on scrap ply, and finally reattempted. A few areas were not so pretty but workable and the rest were fine. I basically over drenched on the wetting out stage, allowing epoxy to get under the fabric and ripple. Defo learned the hard way!
    I was having similar problems wetting out but tried Peel Ply, marvellous stuff, the excess epoxy comes off with the peel ply leaving an even surface with a very fine texture.

    A lot will depend on the type and weight of the fibreglass. If it is stretchy and lightweight just for waterproofing and some abrasion resistance may not be a problem, different if the glass is structural.

    A few years ago I experimented with some 4mm with the following results:-


    "I did some crude experiments some time ago, with sheathing the plywood I am using to build my boat.

    My plywood is 4mm 3-ply, the outer layers are about 1mm each and the core about 2mm. It was described as Far Eastern marine ply, stamped BS1088.

    When I got it, it was obvious that it was very much stiffer parallel to the face grain than perpendicular.

    I was wanting to assess how to improve stiffness at least weight/cost.

    I cut strips parallel and perpendicular to the face grain and used various treatments one side only and also on both sides giving 16 strips in total.

    They were:-
    a) untreated
    b) 3 thin coats of unthickened epoxy
    c) 1 layer 81grm/m^2 plain weave one side only
    d) 2 layers of 81grm/m^2 plain weave one side only
    e) 1 layer of 162grm/m^2 open weave one side only
    f) 1 layer 81grm/m^2 plain weave both sides
    g) 2 layers of 81grm/m^2 plain weave both sides
    h) 1 layer of 162grm/m^2 open weave both sides

    all the cloths were finished with peel ply which was removed prior to testing. Where only one side was clothed the strip was tested with the cloth in tension.

    I tested the strips for stiffness before adding the treatments and the average stiffness parallel to the face grain was approximately 7 times the stiffness perpendicular to the face grain. The strength was about 2.75 greater parallel to perpendicular.

    The results for the improvement parallel to the face grain were


    treatment _______strength____________stiffness
    a)__________________ 1 _________________ 1
    b)__________________ 1 _________________ 1
    c)__________________ 1.1 _______________ 1.2
    d)__________________ 1.1 _______________ 1.2
    e)__________________ 1 _________________ 1.2
    f)___________________ 1 ________________ 1.75
    g)__________________ 1.3 _______________ 2.15
    h)__________________ 1.1 _______________ 2.35

    The results for the improvement perpendicular to the face grain were

    a)_________________ 1 _____________ 1
    b)_________________ 1 _____________ 1.1
    c)_________________ 1.7 ___________ 2.1
    d)_________________ 2.3 ___________ 2.5
    e)_________________ 2.4 ___________ 2.45
    f)_________________ 2.8 ___________ 2.7
    g)________________ 4.35 __________ 4.4
    h)________________ 4.3 ___________ 5



    Given I only used 1 sample per spec. it is subject to random variations that a proper testing system would resolve but I think the results are reasonably consistent and do give an idea of what is happening.

    Where plywood spans in 2 directions the spacing of its supports will determine which direction is critical for failure.

    My conclusions are
    1) that epoxy coating on its own provided negligible benefit in terms of strength/stiffness.
    2)Where the stress parallel to the face grain is critical, sheathing one side is ineffective and sheathing both only improves stiffness with not much better strength.
    3)Where the stress perpendicular to the face grain governs e.g. a multi-chine canoe where the ply tends to span between the chines, significant improvements in strength and stiffness can be achieved.

    Once you go to thicker plywood with 5-ply the difference between parallel and perpendicular will be greatly reduced so the above analysis probably would not apply. Similarly for 3-ply with thinner outer layers and a thicker core.

    It was comforting to see how far you had to bend ply before it went bang."


  13. #13
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    Default Re: Would this fiberglass/plywood method work?

    I just found on my last project that it was so difficult to get an equal covering of epoxy on the vertical panels
    This would indicate that you are putting too much resin on at once. Use less resin and more coats of it. If you are getting a lot of run-off on vertical surfaces use less and roll or squeegee it out better. Do be aware though that moving a roller too fast or pushing down too hard on either a roller or a squeegee tends to foam the resin by creating a lot of tiny bubbles which get trapped down in the weave, so smooth and steady with moderate speed and pressure is the way to go.

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    Default Re: Would this fiberglass/plywood method work?

    Ashley, me thinks more experience with glass and epoxy is in order. most of us use way more then needed in just about everything when building boats. wetting out glass is difficult because it needs to be saturated then the excess needs to squeegeed away or off. it only take a few minutes for the glass to get "wet". if we leave it wet. it will start to float off the substrate. it's amazing how much epoxy can be squeezed off
    Denise, Bristol PA, Oday30, Anchor Yacht Club, On tidal Delaware River. my current project; http://forum.woodenboat.com/showthre...0-Ducker-Resto

  15. #15
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    Default

    I'm just glad to hear someone else has trouble with the glass rippling and doing other odd things. The instructional you tube videos make it look easy. Glassing is not one of things where you can just proceed slowly and carefully to get a good result. As with many things practice seems to be the key.

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    Default Re: Would this fiberglass/plywood method work?

    Move it around with a slanted squeegee and remove excess with a more vertical squeegee.

    Vertical panels of glass are easily saturated on plastic sheet rolled onto a mandrel, then rolled onto the surface desired. Additional resin can be rolled or brushed as needed, then squeegeed.

    Peel ply or plastic sheet help get a smooth surface with good saturation and not too much resin.

    Practice, practice, practice...

    Peace,
    Robert

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