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Thread: Adding some extra support to a thru-hull fitting

  1. #1
    Join Date
    Apr 2018
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    Sweden
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    3

    Default Adding some extra support to a thru-hull fitting

    Last year then I put my boat in the water I noticed some leakage from a thru-hull fitting for the cooling water intake. A closer look showed that the fitting actually moved a bit then the ball valve attached to it was maneuvered. The valve was a bit hard to maneuvered, but the fitting should move at all any way. So now I’ve disassembled it and bought new ball valve and thru-hull fitting in bronze (as the rest of the trh-hull mounted at this boat). However I have some question how to fit the new one some the same problem won’t occur again.



    The size of the hole i a bit bigger than the fitting, about 1/8” (3,5mm) bigger than the trhu-hull, which is 1/2”. This should be ok since some say that there should be some play. The only problem is that the nut doesn’t provide enough support to prevent the fitting from moving, thus causing the leak I guess. The hull is made of mahogany planks, about 3/4” thick or so, but because there is a cutout for the strainer there the fitting is located, it’s thinner around the thru-hull (see pics). So what to do about this? I’m thinking about to put a slightly concave piece of wood/plywood at the inside as extra support. Seacocks doesn’t seem to be available for 1/2” NPT. What is best to use? A piece of plywood or solid teak or so?



    What sealant to use? Many people recommend M3 Marine Sealant 5200, which I think is similar to Sikaflex 290. Also there are people using tar-based compounds. And there bitumen-based stuff…



    Pics

    Hole from inside;
    http://www.bilddump.se/bilder/201804....122.187.2.jpg

    Hole from oustide;
    http://www.bilddump.se/bilder/201804....122.187.2.jpg

  2. #2
    Join Date
    Apr 2013
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    Default Re: Adding some extra support to a thru-hull fitting


  3. #3
    Join Date
    Apr 2018
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    Sweden
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    Default Re: Adding some extra support to a thru-hull fitting

    Quote Originally Posted by Corella View Post
    Inteeresting, but I still can't find any 1/2" seacocks, so I'll probably use a thru-hull fitting with backing plane. The link to make from fiberglass instead of wood... could work.

  4. #4
    Join Date
    Nov 2004
    Location
    Port Townsend WA
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    11,090

    Default Re: Adding some extra support to a thru-hull fitting

    A through hull fitting that is designed for a planked hull can either have a flush or a ringed flange that stands proud of the planking surface. The standard method of installing it is to bore a hole in a frame bay that has clearance for the diameter of the spigot. A backing block that is fitted to then inner planking curve is used to provide the cushion for the flange nut take up. This can be installed prior to the drilling of the spigot hole if a pilot hole is drilled first. I, personally, prefer to use Dolfinite as a bedding compound both for the flange and backing block as it will be able to be removed if necessary. 5200 and other sealants are difficult to work with if removal of a component is called for at a later date. If deemed necessary, the backing block can be screw fastened, at the corners, from the inside of the hull prior to boring.
    Jay
    Last edited by Jay Greer; 04-13-2018 at 03:18 PM.

  5. #5
    Join Date
    Apr 1999
    Location
    Hyannis, MA, USA
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    43,952

    Default Re: Adding some extra support to a thru-hull fitting

    Go with a 3/4" properly flanged seacock and an adapter for your 1/2" feed. That's far far far safer than dinking around with a 1/2" not a seacock. There's are reason for doing this right.

  6. #6
    Join Date
    Jun 2000
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    Pleasant Valley NS Canada
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    13,029

    Default Re: Adding some extra support to a thru-hull fitting

    Jay and Ian have it right.
    Hope for the best, but plan for the worst.

  7. #7
    Join Date
    Apr 2018
    Location
    Sweden
    Posts
    3

    Default Re: Adding some extra support to a thru-hull fitting

    Thanks for the replies, I'll probably do as Jay described. That way I don't need make a bigger hole in the hull and I can still fit the strainer.

    I still have some questions however.

    Quote Originally Posted by Jay Greer View Post
    A backing block that is fitted to then inner planking curve is used to provide the cushion for the flange nut take up
    Kind of what I was thinking, but what material should I use? Oak? Teak? Plywood may no be as durable, but it wont split as solid piece of wood might. I will probably end up treating it with linseed oil as the rest of the keelson.

    Quote Originally Posted by Jay Greer View Post
    If deemed necessary, the backing block can be screw fastened, at the corners, from the inside of the hull prior to boring.
    Jay
    It should be screw fastened to the hull if a solid piece of wood is used, so that if it would crack or split. It will still support the thru-hull fitting, not failing apart?

  8. #8
    Join Date
    Apr 1999
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    Now, there's a long story...
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    Default Re: Adding some extra support to a thru-hull fitting

    Oak or teak will both work. I like purpleheart for that task, too. What's easily available in Sweden? Locust, elm...

    The interlocking grain on something like locust will help prevent splits.

    Screw fasten it to the corners so that it won't shift about when you bore the hole for the through hull and the mounting bolts on the flange. You could use ply, but it would need to be rather thick, and marine grade. Its purpose is to spread the load over a wider area, similar to how you'd back a winch base or cleat base. Another option could be G-10 fiberglass. It really depends on how traditional you want to be.
    Heute ist so ein schöne Tag...

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