Fibreglass layup advise

Discussion in 'Boatbuilding' started by Theo van der Toorn, Oct 2, 2022.

  1. Theo van der Toorn
    Joined: Oct 2022
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    Theo van der Toorn Junior Member

    Hi all
    I need some input on fibreglass schedule.
    Hoping to use Combi mat.
    I am looking into replacing the plywood skin of a design with fibreglass .
    I would use the plywood bulkheads and stringer's as designed but am thinking about a glass skin (cost of plywood is crazy)
    The skin would be 9mm in Ply.
    This is on a wharram cat 42 feet.
    Constructive input please
    Thanks
     
  2. fallguy
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    fallguy Senior Member

    Replacing plywood with solid glass would require testing the glass and resin system to exceed the engineering attributes of the plywood while also still being able to torture the panels. And then losses to trimming the panels need to be included. The likelihood money is saved is not high. The only way to do it well is on a vacuum table. Most ply builders don't have that either.

    So, it is wholly impractical to even consider for most ply builders of a single boat.
     
  3. wet feet
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    wet feet Senior Member

    If the cost of plywood is too great,the cost of glass will be much more.... Have you built a boat of either previously? I'm guessing that the plan is to lay up flat sheets and then to wrap then around the bulkheads.Holding large panels in place to clad the bottom is a pain with any material too.
     
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  4. gonzo
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    gonzo Senior Member

    It will be heavier and more expensive. Also, plywood is stiffer by weight than fiberglass, so you need to re-engineer the whole structure. Wharram designs for plywood are based on flat panels that develop fair curves. If you want to build with fiberglass, and preferably with a core, then get one of Wharram designs that are for that method and material. However, it will also take a lot more time.
     
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  5. Theo van der Toorn
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    Theo van der Toorn Junior Member

    I'm just looking for a layup schedule.
    I have a plan for a simple mold with the correct curvature and allow for joining panels.
    The cost of decent plywood in Europe is extremely high.
    The overall cost of a glass skin would be substantially lower by my calculations.Plywood needs a lot of finish work where as a glass skin would be almost complete when it goes on.
    The weight should be similar to plywood.
    Yes,I have built several boats in ply.
     
  6. gonzo
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    gonzo Senior Member

    You would have to make a mold out of plywood and then finish it. That would be the equivalent of one of your hulls. The weight will be much higher. Fiberglass is a lot heavier for the same stiffness. Also, the whole structure needs to be redesigned. It is not simply a matter of a layup. Can you post your calculations? A mold needs to be sturdy enough so it doesn't distort with heat and the weight of the hull plus whoever is inside while laminating.
     
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  7. fallguy
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    fallguy Senior Member

    He thinks he can build flat panels in glass and torture over a jig. But he is forgetting all the details are problematic.

    A jig is made for both.

    For plywood, the panels can all be cut and atitched in one week.

    For fiberglass, one needs to test and he thinks someone has done a line comparison and made it public that x mm of plywood is x mm of fiberglass. This, of course, is not simple because even whether he uses vac or hand lays and each variation of glass and resin affects the strength and bending character of the panel.

    And then, as mentioned, the hull weight will be off...

    The entire time, forgetting building a laminating table, dedicating the space, and two to three days to make each fiberglass panel are involved. So a boat using 10 panels takes maybe 30 working days or two months to get a stack of panels versus a week to have the hull assembled in ply.
     
  8. gonzo
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    gonzo Senior Member

    Building a flat panel is not easy. Unless he is vacuum bagging, they will distort as they cure.
     
  9. Theo van der Toorn
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    Theo van der Toorn Junior Member

    Guys,you are all making a lot of assumptions about how this will be achieved.
    Not one is correct.
    I get it if you don't know, feel free to speculate amongst yourselves......
     
  10. gonzo
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    gonzo Senior Member

    We do know. How about explaining what you know and what the methods will be? Everybody is trying to help, so please don't make snide remarks. Thanks
     
  11. Rumars
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    Rumars Senior Member

    Sorry, but what advice are you looking for exactly? If you keep the weight the same it's absolutely clear how much glass you can use. Same thing for cost, you can't know it unless you know the layup schedule. The only variables are lamination method (hand layup, wet bagging, infusion), and glass type, and you have told us only you want to use a combi mat.

    Example:
    9mm okoume ply = 4.5kg/sqm
    2 layers 660gr/sqm combi mat, polyester resin, hand layup = 4.6kg/sqm, 3.5mm thick

    You have already been told you can't match stiffness and weight for the same panel span without a core.
     
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  12. Theo van der Toorn
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    Theo van der Toorn Junior Member

    Thanks
    The design allows for heavier ply.The ply is also saturated with epoxy,covered in two layers of 200 gram cloth ,with a substantial layup under the water line,so if I went with 3 X combi mat and resin,there won't be much in it weight wise.
    I would bag it over a former so no bending flat panels.
    At 5mm thickness,do you think the strength would be similar.?
    There are stringers every 200 mm,so a lot of support for the glass panel.
     
  13. Theo van der Toorn
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    Theo van der Toorn Junior Member

    I calculate the Ply with all the epoxy ect would weigh in at more 7 or 8 kg per mt2
     
  14. gonzo
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    gonzo Senior Member

    You are missing the fundamentals of materials. Stiffness varies with the cube of the thickness. For the same E value, a skin that is 3 times thicker will be 9 times stiffer. You will not match the stiffness of the plywood with a fiberglass lamination of the same weight. It is simply impossible. Strength does not have any meaning in engineering unless you qualify it. For example, there is tensile strength or shear strength. There are other properties that are just as important, like hardness, stiffness and toughness. A laminate as you describe will be a complete failure. The hull structure has to be completely re-engineered, which you keep on disputing.
     
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  15. Rumars
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    Rumars Senior Member

    The issue is stiffness not strenght. Stiffness depends on thickness, and that depends on the technology and fiber type. In my previous example I used the bulkiest combination, woven rowing with CSM hand laid and resin rich. If you bag the same stack, the finished thickness will be 2.xmm, if you use a biaxial stitched combi the thickness goes down further to around 1.6mm. If you use no CSM the thickness goes down further to around 1.2. The weight will stay roughly the same but the stiffness difference will be obvious.

    If 9mm ply was specified you don't want to go lower on thickness, you should use a 9mm FRP panel. There are plenty of laminate calculators on the web, just don't be shocked when you see the weights, wich will be around 14-17kg/sqm.

    A heavier plywood at 650kg/cum (sapeli, meranti) plus epoxy and glass can come out at 7kg/sqm, but you can't do a FRP panel of the same thickness at that weight, you are still dealing with a minimum density of 1400kg/cum, or, to put it differently, if you keep the weight you must half the thickness.

    Testing is easy, make 10x10cm coupons. Then you will know exactly how thick and heavy the fiberglass you want to use comes out with your chosen method and can plan the stack.
     
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