Latest News: Forums Technical Aft buoyancy tank leaks

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  • #29480
    Nigel Waddington

    I recently acquired a much neglected GRP Mk1.  I am currently working through a number of issues with the aft buoyancy tank and have a couple of queries if anyone is able to advise.  First, a smoke test shows smoke escaping around the bolts that secure the turnbuckles to the deck.  Having dismantled one it is difficult to see how it can make a water tight seal unless it is supposed to have a rubber washer for the nut to tighten up against (which it currently doesn’t). It looks like someone has had a go at epoxying around them to create a seal but without much success.  The rubber washer would seem the logical solution but just want to check how they were supposed to work when originally manufactured.  The second issue is that the foam buoyancy block is absent.  Am I right in thinking that all GRP Mk 1’s should have a foam block attached underneath the aft deck?  If so, what sort of foam and what dimensions should I use and where can I obtain it? Grateful for any advice. Cheers, Nigel.

    Bob Harland

    Hi Nigel, I would expect a good seal with some sikaflex 291i (or equivalent), if there is a lot of play in the bolt (and if the bolt length allows) a backing pad glued in position should help. Depending on the state of the hole filling with epoxy and re-drilling might be required. If the nut does not have a metal washer – then add one.
    If you can post a picture showing the problem that would be useful. (Just copy and paste).

    The GRP boats should have some kind of permanent buoyancy to stop them sinking if the buoyancy tanks fail. But this would have varied over the years. A block of polystyrene moulded into the aft tank was indeed a normal option, usually with some under the side decks too. There is quite a good discussion here with details of what is required;

    hope that helps


    Nigel Waddington

    Thanks Bob.  That’s really helpful


    I bought some expanded polystyrene insulation board from Wickes and cut it into manageable pieces that I calculated to be 20 litres each. I stuck them together to make two blocks ( I think I used CT100 also from Wickes) then put them inside two blue plastic rubble sacks liberally Duck Taped to make them neat packages.

    Two blocks were easier to fit through the hatch than one big one.

    Regards your leakage problems, I’m with Bob.  Sikaflex 291 is the answer, now what’s the question?

    • This reply was modified 4 months, 4 weeks ago by Kez.
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