Need lightweight design advice

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Apr 23, 2023.

  1. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I built sections of interior benchtops using 3/4" marine plywood.

    They are roughly
    2'x6'
    2'x3'
    2'x3'

    The unsupported dimensions are maybe 2" less or so on the 2'.

    I want to rebuild them to match the 3/4" thickness, but be ultralight while still not cracking if someone 300# sits on them.

    If I laminate 3 pieces of 4# core together; would it be good enough with say a couple layers of db1700 on each side? Or is the first big person gonna crack it? I need to make whatever I build about 3/4" to fit the existing hardware profile and hinging. But I really want to shave some weight here. These are 72#/32sqft or 2.25#/sqft, or about 55 pounds total. It seems like I could easily shave 20 pounds here.

    Thanks for any advice.
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Have a vacuum forming rig? This is one of those things that only makes sense with industrial processes; i.e. net shape forming.
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    These are just flat benchtops 3/4" thick by sizes stated.

    I have a vac table, but not vac former for stiffeners and open bottom..
     
  4. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    Simple enough project.but perhaps daunting if you've never done it before.A flat surface is the first thing you need and you may care to add boundary strips to give a neat moulded edge.they can be held down with double sided tape and you will need a small plasticene radius in the corners.Process: lay up the first skin and if you are confident,add the core material-which you vacuum bag into place.When cured,add the second skin.don't get too agitated about the fibres from the side not contacting the laminate on the underside as a wide enough bearing surface will take care of it.To be extra sure of resisting deflection you can take a slice of foam-say 30mm thick and cut it at 45 degrees to add an X shaped section pointing at the corners of the panel and stopping short of the bearers,which you they add a ply of glass to for additional stiffness.A laminate of 450gsm might be a little light and wouldn't have a great deal of puncture resistance if a heavy tool is dropped on it,but 600gsm and the stiffeners mentioned would be a good starting point.As ever,try a small test panel first.
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I have a vacuum table.. the build process is ez for me, I have 300 gallons of epoxy experience and most of it was on the table.

    The tophats are a good idea.

    Absent any layup advice, I'll probabably start at 400gsm per side and see if that is enough.

    I have an excess of 6mm M80 core. I want to build it up to 18-20mm, but I don't want a bigger person to delam it. So the tophat is a good idea at low penalty.

    I'll try to put it together one layer of glass at a time and see what happens. I might be able to turn biax 45 degrees to make a uni-like tow on the 2' direction as well.
     
  6. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    The stiffeners don't have to be a pure top hat section,just a 45 degree slice off a block of core material would work and be easier to laminate over.

    stiffener.png
     
    fallguy likes this.
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Thanks. This helps a bunch. Any guesses on the laminates? I am going to use the 3/4" ply edges I decided for solid place to hinge to..
     

  8. Skip Johnson
    Joined: Feb 2021
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    Location: Lake Tenkiller, Ok, usa

    Skip Johnson Junior Member

    I'm assuming the tops are supported on all 4 sides, if not the 2x6' is problematical. I'd recommend 2 layers 12 oz biax on top and one on the bottom to start and test. At the other end of the spectrum I wanted 3 very lightweight planks 8" wide 72" spanning 63". On hand was some 1" xps foam, 9 oz tooling cloth and some 3" wide uni 22 oz tape. Running the numbers; planks laminated to 2" thickness with one layer of uni and 9 oz cloth almost good enough to theoretically support 250# at midspan (I weigh 185). Test panel failed with the top covering buckling. Subsequent planks with 2 -3" uni on top and one on the bottom support me bouncing up and down on a plank. Planks weigh 3.2# each but no finish yet.
     
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