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Old 25-07-2008, 15:46   #1
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Seacock Backer Donut

My raw water intake's seacock (Grocco) has a rotten wooden backer donut that has to be replaced. I would like thoughts on whether I need to remove the exterior bronze through hull and inspect the hull in this area then re-bed as part of the repair or considering that the fitting has never leaked can I get away with just removing the seacock from the nipple and fabricate a new wooden donut and reassemble? (Think I know what the answer is going to be). Also some reccomendations on what species of wood (Teak) solid or plywood?

Thanks All!!

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Old 25-07-2008, 18:34   #2
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Take the whole thing out. Clean surfaces inside and out. Use marine grade plywood.

Since you are working on a hole in the bottom of your boat it is prudent not to do the job 1/2 way

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Old 25-07-2008, 18:58   #3
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What's the worst that could happen? It might!

With backing plate bad you don't have a solid foundation to fix this problem. Marine plywood is just fine and easier and stronger.
Paul Blais
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Old 25-07-2008, 18:59   #4
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If using plywood...

First I recommend cleaning the area with an appropriate degreaser (follow instructions, make sure there is plenty of ventilation), then abrade roughly all places that might be glued.

The ideal would be to make up a replacement "doughnut" using layers of fibreglass mat and epoxy resin, and then to glue this in place using epoxy glue. If it is possible to do this "in situ" (drilling holes after the resin has set ), even better.

If using ply I suggest cutting the piece to size, drilling all holes and chamfering the edges then saturating all end grain with epoxy resin thinned 10%. Then continue to saturate the rest of the ply, then the end grain again. After the epoxy has set and all solvent has evaporated sand lightly and coat again with straight epoxy resin. Glue in place with excess glue and clean up with a spatula. Redrill all small holes.
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Old 25-07-2008, 19:13   #5

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I second the thought of using something that can't rot to make the new donut. Even if that means filled epoxy, which would be sufficient all by itself.

I don't see any "must" to remove the whole thruhull, but this would be a good opportunity to inspect the whole thing, and removing it, or making sure it didn't need removal and was solidly fixed, would be a prudent thing at this time. The fact that the donut has gone bad indicates moisture, and that's never a good sign.
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Old 26-07-2008, 06:34   #6
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Because you will have a hell of a time getting the valve and the nut off the thru-hull, just take a small angle grinder and grind off the flange on the outside of the boat and drive the remains into the boat. Then replace with all new--the fittings are not that expensive.
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Old 26-07-2008, 11:09   #7
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Feel Free

Feel Free to read these articles on seacocks and take away any info you like..

Seacock & Thru-Hull Primer/Pre Information

Mismatched threads be careful:

Replacing Thru-Hulls and Seacocks

Fiberglass Backing Blocks:
Marine How To Articles
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Old 26-07-2008, 16:05   #8
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Since the primary function of the "doughnut" is to spread the loading around the perimeter of the hole, you can do another option: Assuming you can extract the "doughnut" without challenging the watertight integrity, use aforementioned hole saw to cut two new doughnuts, each one half the thickness of the original, or better yet, one larger than the original. Cut the doughnuts in half and thoroughly seal all surfaces with epoxy. Insert the first, wider doughnut in two halves around the throughhull base, using 3M 5200, followed by the second, smaller pair, rotated ninety degrees, also using 5200 as the bedding adhesive. You can then screw through the upper doughnut into the lower, not penetrating the hull. The following day, when the 5200 has set up, tighten down on the throughhull nut until it is secure against the top doughnut. You can remove the screws and fill with 5200 if you wish.

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