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Old 11-19-2012
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Cutting above waterline hole in boat

I am adding a secondary bilge pump sooo... I need to cut a 1" hole above the waterline for the exit fitting. What is the best type of drill bit for making a clean cut in gel coat / FRP?
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Re: Cutting above waterline hole in boat

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Originally Posted by kellysails View Post
I am adding a secondary bilge pump sooo... I need to cut a 1" hole above the waterline for the exit fitting. What is the best type of drill bit for making a clean cut in gel coat / FRP?

A round one.


Seriously though, a hole saw should work fine. Just make certain you have the pilot bit installed; I've seen people try to use a hole saw w/o and the results were NOT pretty. Also, when you go to make the cut you might want to make a very small (1/8") hole from the outside in, verify that everything is clear on the inside and then go back and make the actual cutout. This way if you have to relocate the hole an inch or so you can and it will still encompass the tiny check hole.
Also, after you get the pilot bit through the fiberglass but before you let the hole saw bite into the gelcoat, STOP. Reverse the drill and then CAREFULLY run the hole saw in reverse to "mark" the hole. Now drill it out normally in Forward. This trick will minimize, if not completely prevent chipping and splintering of the gelcoat. Or course the edges of the hole will be covered by the through hull, but why not make the hole as neat as possible.
Good Luck.
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Old 11-19-2012
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Re: Cutting above waterline hole in boat

I drill/ saw through the blue masking tape...prevents splinter of gelcoat
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Old 11-19-2012
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Re: Cutting above waterline hole in boat

Wow, just the advice I was looking for, perfect. Now I just need to summon the courage.

Thank you very much!

-craig
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Old 11-19-2012
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Re: Cutting above waterline hole in boat

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Old 11-19-2012
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Re: Cutting above waterline hole in boat

For a 1 inch - use a fortner bit. No worry on chip out or wandering.

Drill from outside in, and blue tape.
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Re: Cutting above waterline hole in boat

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Originally Posted by chucklesR View Post
For a 1 inch - use a fortner bit. No worry on chip out or wandering.

Drill from outside in, and blue tape.

Sorry Chuckles, I have to disagree w/ you on this. A Forstner bit chews up everything that is inside the hole limits. A Hole saw on the other hand just makes a thin slice and you're left w/ a plug that comes out of the hole and a WHOLE lot less fiberglass dust. Makes a bigger mess, ruins your nice Forstner bit and if it "grabs" and scoots across the hull... can ruin a whole lot more.

I do however, "second" the use of blue tape. Meant to put that in my first post.
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Re: Cutting above waterline hole in boat

Have to go with sailor dave on the Forstner bits.. I used several on a recent project.. they work great for clean holes and cut better than the average holesaw, IMO, but they do make a mess. For single skin or even cored glass up to an inch or so I'd go with a holesaw.. I was drilling through glass encapsulated 2" timbers so the depth of most holesaws was problematic, as would removing the cores have been. The Forstner bit did a good job despite the large amount of shavings.
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Old 11-20-2012
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Re: Cutting above waterline hole in boat

I agree with the "no Forstner" bit. They are made to bore a large diameter hole that does not go all the way through the material. Very handy when installing European syle cabinet hinges. Definately use the hole saw. Much easier and cleaner.
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Old 11-20-2012
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Re: Cutting above waterline hole in boat

Oh, come on guys. If you want a really clean hole in fiberglass, you find a local hospital using one a them DaVinci surgical robots, with a laser ablation attachment, and have them burn the hole for you. Off the books, at night.

Clean, round, neat, no fuss. Slip the OR tech a couple of bucks and he'll bring it down while the machine is out being "sterilized" and no one misses it.
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