best layup method if glass is to be tapped with threads?

Discussion in 'Fiberglass and Composite Boat Building' started by leaky, Dec 1, 2017.

  1. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    For a water tank for the application you describe I would have no qualms tapping your threads into a piece of 1/2" G10 and gluing that to the polyester tank top. G10 is many layers of fine glass cloth press molded with epoxy and holds threads well, much better than any other glass laminate and will bond well to the tank top. I would use a different approach for a fuel tank but whats the worst that can happen if you did get a leak out of a water tank in the keel?

    Steve.
     
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  2. leaky
    Joined: Sep 2008
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    Location: nh

    leaky Senior Member

    Thanks for that, what I think I decided on just gotta get the tank closed off in the area is this.

    I got aluminum bar in 3/4 and 1/2 inch that fits the ID of the keel. The 3/4 stuff I'll tap for the pipe threads plus will drill for mounting holes. I'll then cut the pipe holes a bit oversized to match both in the top of the tank and in the 1/2 inch bar.

    I'll set the 3/4 thick "flange" I've built down atop the tank, will drill through the top of the tank using the mounting holes as the pattern, locate the 1/2 inch "backing plate" I've built on the underside of the fiberglass top of the tank, mark the 1/2 inch stuff with the same hole pattern and tap threads for bolts (probably will tap the threads with the parts in place to assure I get any angles right).

    Then when I install it on the top I'll use a removable sealant or gasket for the flange on the top, but something like a 5200 for the backing plate on the bottom. Doing it like that I ought to be able to reasonably remove the flange on the top if I ever wanted to, however the bottom part with the threads will remain in place. There will be no concern over fiberglass threads etc.. etc.. The only caveat is since I've got aluminum and my fittings will be brass or bronze I'll want to use a plastic fitting or bushing on the NPT threads, but that is of no concern here, almost prefer plastic since it seals better.

    Jon
     
  3. redreuben
    Joined: Jan 2009
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    Location: Beaconsfield Western Australia

    redreuben redreuben

    My answer is probably redundant by now but what I would have considered is waxing the bolts and then wrapping them in glass rovings to get fibre into the threads, then laminate the rest of the lid incorporating the rovings wet on wet.
    The mould thread with good fibre orientation would be much stronger than cut threads.
    Just thinking aloud (allowed)
     
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  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Metal inlaid or tapped G-10 would be my recommendations.
     
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  5. leaky
    Joined: Sep 2008
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    Location: nh

    leaky Senior Member

    Thanks for all your help guys, after going around in circles on this just gonna go with what I know.

    Will make a flange out of metal with the threads, a backing plate with matching non-threaded holes and tapped for the bolts, use sealant to keep things out on top and to hold the bottom in place in case I ever want to remove the flange. Put a couple pics of the setup as I go along below. Still gotta do the metal machining but the seat for the flange is glassed and I got it in there nice and true so the surfaces mate perfectly.

    Chunk of 3/4 aluminum in place where it will be, checking fit which was perfect.

    flangeLined.jpeg

    Coated up and ready to go, just gotta get my flange and backing plate built, then can proceed along with glassing the rest of the top of the tank.

    flangeSeatCoated.jpeg
     
  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    What epoxy are you using with the Aluminum? I skimmed and read you are embedding?
     

  7. leaky
    Joined: Sep 2008
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    Location: nh

    leaky Senior Member

    I'm not embedding - I think if I was I'd be using bronze, something that without a doubt would stand up to corrosion.

    What I ended up deciding to do is just use 3M 5200 on the backing plate to hold it there permanently inside the tank (which has the female threads on it the bolts on the top will tap into), and on the top I'll use a less aggressive sealant to permit removal of the flange (like maybe 4200 or maybe something even a little less strong than that).

    Jon
     
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