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Old 01-05-2015, 10:37   #1
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Drilling through deck

So I have just gotten my solar installed but to do it cleanly I need to drill a hole somewhere close to the engine room to run the wires. The most convenient place would be to the starboard site of the companion way, just behind the combing. The hole will be small, 1/4 inch maybe. Will a regular drill bit do? What precautions should I consider before doing this? I've never drilled through the fiberglass before and I am a bit leery in doing so.

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Old 01-05-2015, 10:56   #2
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Re: Drilling through deck

Man, I wish the previous owners of some of my last boats had the sense to ask this question...

Deck surfaces are usually a composite made up of fiberglass laminate on the top + bottom and a "core" in the middle, usually wood, sometimes foam. Like a sandwich where the bread is fiberglass and the meat is "core".

When you drill a hole in a composite, you need to make sure you protect that "core" from future water intrusion. Too commonly people assume the thing they are sticking in that hole will be "bedded" with some junk and all will be well and watertight. A few years later the bedding fails, water starts weeping in, and slowly but surely the "Core" is rotting and getting soggy and ruining the structure.

When I drill holes in such a structure, I drill the hole, then I gouge out the "core" surrounding the hole, then fill the whole thing with epoxy, and then re-drill my hole when it cures. This way the core material is safe + dry on the other side of that epoxy even if the stupid doodad I stuck in the hole is no longer watertight. Better to have just a leak to repair than a leak and a rotten deck surface.

And probably best not to put this somewhere where water pools.

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Old 01-05-2015, 11:03   #3
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Re: Drilling through deck

Use a sharp bit. Cobalt or titanium coated bits are better as fiberglass dulls tools quickly. Make sure the hole is where you want it. Otherwise look on the backside to make sure there is a void there and nothing that you will damage like wiring.
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Old 01-05-2015, 11:05   #4
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Re: Drilling through deck

What he said, plus I always start with a smaller pilot drill, then step up to the final size.
I drill from the Gelcoat side as I believe if I drill through the rough side, the Gel coat may chip. I've seen others put masking tape down first, then drill through the tape to help prevent chipping, scratches etc. I don't but probably should.
Obviously be sure your not going to drill into something you don't want to, as a kid I helped a friend install a car stereo in his Volkswagen, he was drilling into the dash, guess where the gas tank is on a Volkswagen?
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Old 01-05-2015, 11:51   #5
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Re: Drilling through deck

Thanks Guys. It seems straight forward I just dont want to make any mistakes. The location I want to drill is perfect, it goes right into the hanging locker which has access already set to the engine room.
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Old 01-05-2015, 12:17   #6
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Re: Drilling through deck

Here are the steps I take but note I, if I feel like it, may do this from below instead of from the top:

1. drill hole of final size all the way through and verify that position is right.
(from here on you can do this from below)
2. drill hole at least 3/8" diameter but only through the first fiberglass skin and the core material, leaving the other skin intact.
3. using a Dremel with drum sander, remove all the core material and also as far from between the fiberglass skins around the hole as the little drum-sander bit allows. This is about 3/16".
4. With everything cleaned up, tape the little hole on the other side and fill the whole cavity with epoxy. From above gravity works with you and you can make it thinner but from below you need to make it thick as peanut butter. Use colloidal Silica (= Cabosil) filler.
4. After cure sand both sides and re-drill hole.


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