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Re: Tips for cutting hole for flush mount through-hull
I just did side-deck drains. There had been no such on my boat, and a bit of an engineering flub caused the boat to always hold water on the side decks, that would then get full of atmospheric crud, evaporate, and leave guck on the side decks. Hated it.
I bought 2 flush drains with 2-1/2 inch tops and 3/4 inch outlets. You'd obviously want bigger for cockpits. Though I was pretty sure there was adequate deck thickness in the installation spot, I first epoxied a 3/8" fibreglass backing plate underneath each future hole to ensure adequate strength and thickness. Luckily my side decks are not cored. If yours are, this whole narrative might be moot.
I took a 2-1/2" hole saw, and cut into the deck a little at a time, removing the material with a sharp chisel. This left me with a lot of glass fibres in my sleeves that drove me crazy for a couple of days. When I found I had had adequate depth, I switched to a 1" hole saw, using the same pilot hole to guide the cut. So I ended up with a squared, stepped hole. I masked the top of the drains and around the holes, gooped up the hole and the drains with a thick bead of 4000 (will not yellow) and dropped them in. I didn't even do up the backing nuts, just left them flat. They are now in there like concrete. I did up the backing nuts just for good measure, but I don't think they will ever come into play.
You see, I did not worry about a bevel, and I thought it would be beyond my ability and/or tools to cut one anyhow. The amount of gap in the bevel that is filled by the 4000 is relatively small, and there is a lot of surface area in there for the adhesive to do its job. The drains fit the 2-1/2' hole perfectly, there is less than a playing card gap filled with 4000 between drain and surrounding deck surface.
So now thinking about it, even if you have a cored deck or cockpit sole, you could cut out a larger hole, glass it in, and use the same method as above.
Bashing about on Lake Ontario and Beyond
"Ariel" '79 Endeavour 32
Last edited by Ritchard; 04-19-2013 at 11:38 AM.