Sailing plywood and epoxy cabin roof - compound curves

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

  1. TomBlake
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    Location: Qld Australia

    TomBlake Junior Member

    30 foot sailing cat. Roof is to be 4metres across and 4.5 fore and aft. Covers cabin, cockpit and gantry for dinghy. 4 laminated 45mm beams across the boat. Roof also supported by cabin walls.
    There will be a steel structure holding up the roof where the mainsheet traveller is going and where the gantry for dinghy is going.

    Have a mold made up. see attached photo(this is half the mold). Due to various reasons, i want to go down the following track. Was going to get professionals to have a foam core fibreglass roof but the cost is really blowing out

    Layer of fibreglass cloth followed by whole sheets(2400*1200mm) of 4mm marine plywood across the boat(Athwartship) followed by 300mm wide strips of 9mm marine plywood fore and aft followed by 6mm foam core followed by 4mm sheets across the boat, another layer(???) of cloth and then gelcoat.

    Going to use lengths of timber to batten down the plywood for each layer

    Maximum curving fore and aft is 140mm over 2000mm (stern half is nearly flat) and across the boat, 115mm over 2 metres

    Doable? Have read lots of threads on compound curving but as a novice I am confused as ever.
    Read the gougeon book on compound curve. Couldn't really understand the data on figure 25-29. Longitudal bending seemed very little.

    Any advice on how to go about it? Is there a difference is bending across the plywood sheet as compared to along the sheet? Alot of forums mentioned 3mm as the first layers

    Someone mentioned not to soak the plywood before fitting as it will stiffen the board too much.

    Also use ot epoxy and thicknesser when putting on the boards?


    Thanks
     

    Attached Files:

  2. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the sheeting material on the mould ?
     
  3. TomBlake
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    TomBlake Junior Member

    It is 6mm MDF painted with a sandable primer. We're looking at putting down the sticky plastic so we can pull the roof away.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like a lot of trouble, all this, why didn't you just have a timber frame, screw down ply sheeting to it, and epoxy/dynel or glass over the top ? Alternatively, you could just use your mould as a former, glass followed by small square foam, more glass on top.
     
  5. TomBlake
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    TomBlake Junior Member

    That is what I was going to do originally. However the cost quoted to me for the mould and finished product has blown out my budget and I was advised by someone else that the above was a doable DIY and under half the cost. Quote was based on the following to allow standing on:
    GelCoat
    • TY layer 450gm Commercial Grade Chopped Strand Mat with VE resin
    • 2 x 600 Commercial grade Double Bias Mat
    • Includes 15mm core with adhesive
    • 450gm Commercial grade Chopped Strand Mat
    • 2x 600gm Commercial grade Double Bias Mat
    • Resin Coat
    • FlowCoat – White
    • Approx Thickness of Hardtop: 23mm
    • Approx Size of Hardtop: 4250 mm x 4400 mm
     
  6. TomBlake
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    TomBlake Junior Member

    I got an estimate to do the mould of around $2500 by a recommended boat builder and to-date it is $6000 with me putting in an unforeseen 5 days assisting.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Starting to sound expensive ! Were it not for compound curvature, life could have been simpler.
     
  8. fallguy
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    fallguy Senior Member

    I am building a rounded rooftop 2m wide by 4m long using scrim.

    I am going to be painting it in the end.

    12mm scrim-will fill with balloons n cabosil then lightly sand and fill again right before glassing the top

    750g triax both sides

    After a good cure; plan to flip and support it with a corecell jig and laminate the inside on the table. All hand lam.

    Scrim foam is the way to go.

    I don't plan to vac mine; just hand laminate, but vac bagging would be possible.

    I will probably rebate the scrim for a glass overlap to reduce fairing on the outside. I use a $50 electric planer with for reliefs. The doubled glass adds strength where the scrim thickness is reduced.

    My scrim costs were about $500. Total costs for roof about $1000 or a bit less.

    Gelcoat is going to be the harder part. I gotta disqualify there.
     
  9. TomBlake
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    TomBlake Junior Member

    compound curvature was recommended to provide structural integrity (to allow walking on)
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Hardly likely to be an issue if it had a good crown, and with sandwich construction.
     
  11. fallguy
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    fallguy Senior Member

    If you itemize the cost; is it really the compounding adding the cost or the finishing?
     
  12. TomBlake
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    TomBlake Junior Member

    I've costed both methods. Material cost almost the same. Pointing towards foam core fibreglass construction. My stepson has done plenty of fibreglass from moulds/plugs. Need to do some more work on the mould. Can you do a non-gloss gelcoat finishing as the first coat. The catamaran currently has very little glossy gelcoat. I'm not worried about having a glossy gelcoat underneath and the top is going to be mostly non-slip or covered by solar panels
     
  13. fallguy
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    fallguy Senior Member

    I wish I could help more, but I am not a gelcoat or VE guy. The top looks heavy as designed with 900g of csm, bit I could be wrong.

    I also think when you pencil it out and itemize cost; you ought to do a weight budget. The csm layers are 64# at 100% resin. And if you are traction painting the top; you have no need for gelcoat unless you need it for fairing?

    There are quite a few wiser guys than me here. Give them time to respond. It usually takes about a week to get a well balanced response.

    Ondarvr is the best guy for this sort of thing.
     
  14. upchurchmr
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    upchurchmr Senior Member

    how much compound curvature?
    You can get "some" just by bending ply over a form.
    You would need to use multiple ply layers or an interior strip planked layer.

    Have you looked at "Constant Camber"?
    Still requires a mold.
     

  15. TomBlake
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    TomBlake Junior Member

    Maximum curving fore and aft is 140mm over 2000mm (stern half is nearly flat) and across the boat, 115mm over 2 metres
     
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