Sailing plywood and epoxy cabin roof - compound curves

Discussion in 'Boat Design' started by TomBlake, Aug 24, 2018.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This is contour scrim. This price is rather high and in Australia; hope you would find it cheaper. This brand comes in 3/4" thickness or full inch.

    Jamestown Distributors https://www.jamestowndistributors.com/mobileportal/show_product.do?pid=1586
     
  2. TomBlake
    Joined: Aug 2018
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    Location: Qld Australia

    TomBlake Junior Member

    I've found a similar product called thermo-lite. Very strong. They are getting back to me re layup
    Supposed much cheaper as divinycell which has their outlets very closely tied up

    www.thermo-lite.com.au
     
  3. TomBlake
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    TomBlake Junior Member

    What type of scrim? The same they use in car upholstery? Do you cut it in strips?
    What is the mixture ratio with cabosil and balloons?

    Could I lay down say a 3mm layer of plywood first so that the scrim doesn't stick to the mould? If so, do I lay down some wet expoxy/glass before laying down the scrim?
     
  4. fallguy
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    fallguy Senior Member

    So, the size of your project poses some issues. You would need to make a walking scaffold on the edges of the mould/jig.

    I would build a couple of sawhorses the shorter dimension of the jig and get a couple or three or four 2x12s to walk on. Think sawhorses like 8' wide with planking atop.

    If you try to layup the entire job as primary bonds; you might fail, so you need to talk things over with staff/help and plan the work.

    I would put down plastic or wax the entire mould 3-4 times and then insert the beams and then glass the mould surface and finish the day by laying down the scrim panels. You will probably need to put some thickened resin over the beam tops for the best bonding. A thin coat like 1/8" would work well. Resin will run into your beam area, so you must use plastic or release agents.

    You need the planking because you can't walk on the greasy surfaces. Use good materials for planking to avoid the planking putting lots of garbage into your layup. It will be a nuisance.

    So order of business.

    Finish plug mould for a suitable release surface.

    Dryfit and label scrim and glass to fit and label them well.
    Wax mould.
    Apply release agent with a spray gun to the beam slot and entire mould.

    Next day.
    Layup inside glass to specs.
    Laydown scrim.
    Allow cure.

    Next day.
    Dryfit glass or know layup.
    Fill scrim with manufacturer of scrim recommendation. This can be a variety of things, but you need to make sure to fill it well and avoid products that will shrink and too much resin will exotherm and heat.

    For my rooftop, I am on the fence on filler still. I had considered making my own filler with cabosil and microballoons, but I am also considering using a Gurit recommendation for a Spabond.

    Once you fill all the openings; your day might be shot and you might end up secondary bonding the exterior glass. If so, you'll need to sand the surface with a 50 grit paper and then apply another thickened coating before glasswork. This would also help with filler shrinkage issues and reduce staffing needs. That layer would be cabosil and resin about 2.25-2.5 parts cabosil to epoxy. Too much cabosil will be hard to trowel in and too little will be runny. Use a 4-6 inch trowel and work in resin batches of a pint or so at a time. Mix the cabosil in a gallon jug and finish mixing it in a hawk or board. The hawk allows the resin to be spread thin to avoid exotherm. The starting batch might be curing when you start laying glass. Not an issue, keep going.

    Lay the glass.

    This is a multi day project. If you have a staff of three and use the manufacturer of the scrims filler; you might be able to primary bond the entire job, but it will be tough. The cure rates should always be slow for this job.

    Are you doing it in half or whole?

    If you are doing half sections; you'll want to put relief into the scrim on the exterior for the join. This would be done during your dryfit with an electric planer.

    It may seem daunting, but won't be too bad. Buy an extra piece of the scrim and test the filler plan. You can't afford trouble there.
     
  5. fallguy
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    fallguy Senior Member

    This will need to be cut on one side. But cut both ways for your compounding on about 1-2" spacings.

    You could also use the pvc product. They show some with contour cuts.
     
  6. fallguy
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    fallguy Senior Member

    No plywood. Please see my long explanation. If you have a good enough mould surface; you can apply wax and spray release agent and do you inside glass layup and then apply the scrim foam with the cuts facing up for filling another day.
     
  7. TomBlake
    Joined: Aug 2018
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    Location: Qld Australia

    TomBlake Junior Member

    Thanks V much. That is the way I will go. Just have to source the scrim at a reasonable price.
    What is the PVC product you're talking about?
    You input is much appreciated
     
  8. groper
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    groper Senior Member

    Hi Tom - where in australia are you? Have you seen this website which mostly aussie guys building boats? DIY-Yachts.com
    A friend of mine here locally in Cairns, Qld is building a 40ft cat entirely from polycore.
    See this link for his compound roof construction - Finally Does It - Page 2 - DIY Yachts https://diy-yachts.com/forum/viewtopic.php?f=23&t=1021&start=10
    Theres a bunch more info youll find on that site aswell - many boats being built there.

    The amount of compound curve youve given is quite modest - you wont need the scrim type material which is what you need for significantly compounded curvature. If you want to do this cheap - use plastic honeycomb core like polycore etc - cheaper than divinycell or PVC foam etc. Full sheets will bend over that curvature easy. If it needs a little persuasion - just run a few 3/4 depth cuts thru it with a circular saw and it will squeeze up a bit more.
    Hold it down onto a former, like a frame and batten grid, whilst you glass it. Once you glass 1 side - it will hold its shape pretty well due to the compound curve in the cured glass. Then flip it over and glass the other side. Bog it, sand and fair it then paint it - done :)

    You could do the same with thin ply also if you dont wanna use composites. If the material wont take the curvature directly - just cut it into smaller strips and stagger the joins as you glue the layers together...
    You could also do it with ply skins and XPS foam core - like from bunnings in the form of 25mm knauf insulation board. I made a table like this once and it was stiff as concrete. Glue the ply and foam together with thickened epoxy. Apply it with a tile glue trowel - the ones with small V notches (also bunnings) so you can spread the glue evenly and quickly before pressing the foam down into the glue lines and weighting it down until cure... soooo many ways of skinning a cat :)
     
  9. TomBlake
    Joined: Aug 2018
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    Location: Qld Australia

    TomBlake Junior Member

    Thanks v much. I'll have a good look. I'm having difficulty getting Thermo-lite to confirm layup and ability of thermo-lite re compound curves. At $100 per m2 it is alot cheaper than others on the market but still a significant cost.
    I'm based behind the Gold Coast. I need to be able to stand on the roof. Is polycore fit for this? How does it compare with thermo-lite or divinycell foam?(strength/weight). Polycore and Knauf's panels are certainly much cheaper.

    What do I use to fill the kerf cuts, thickened epoxy(eg. 1:1 using q-cells) and epoxy base for the fibreglass?
     
  10. groper
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    groper Senior Member

    Your using thermolite for the whole roof!? As in that really tough ply substitute like coosa board? Woah thats crazy... use that stuff for transoms and winch bases or other high localised stress areas which need to resist high compression- like inside the deck core for pads for the windlass... or rope cleats that type of thing...

    Avoid kerfs unless absolutely necessary as you will be left with a slight chine from them which needs fairing and tries to keep the panel in simple curvature- but yes fill them with thickened resin just prior to laminating- wet on wet.

    Yes Polycore is fit to stand on- my friends entire boat is made of it save below waterline!
     
  11. TomBlake
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    TomBlake Junior Member

    Apologise for my v basic Q. For thickened resin, what ratios would u suggest; to maintain strength and to keep weight down?
     
  12. groper
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    groper Senior Member

    Thickened resin is normal laminating resin mixed to correct ratio. Then keep stirring in 25% cabosil and 75% qcell mix, bit by bit until its the consistency you like it- i like a mayonnaise to soft peanut butter consistency.
    But check out diy yachts - youll learn heaps about how things are done :)
     
  13. TomBlake
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    TomBlake Junior Member

    Thanks. I have a little project at home where I am practising my fibreglassing. I found that using around 1.5-2.0 to 1 for Q cells gets around that soft peanut butter consistency. haven't used cabosil.
    Who are the best to buy from for these products?
     
  14. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    G - the top is compound curves
    I told him to use scrim.
    those are the kerfs he refers to I'd say
     

  15. Mike Inman
    Joined: Oct 2018
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    Location: Jacksonville, Florida

    Mike Inman Junior Member

    I feel late to the party, and definitely low on the experience totem pole, but I'd like to ask: why not considering vacuum bagging? Would seem to be a relatively cheap/easy way to keep weight down while also ensuring full wetting of the glass and ply...
     
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