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How to drill thru inside fibreglass partition?

Frank Langer

1984 Ericson 30+, Nanaimo, BC
I need to drill two half inch holes, one through the fibreglass partition between the engine compartment and the space under the quarterberth and the second one between the space under the quarterberth and the battery compartment under the quarterberth. It looks like the fibreglass in those areas is only about 1/8" to 3/16" thick. I would like advice on how to avoid the fibreglass splintering or cracking, such as would applying some masking tape over the planned hole, or starting with a small pilot drill before going to full size drill, etc.
Thanks for any advice!

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
IIRC my smallest hole saw (drill) is 7/8. The tape idea is good for wood surfaces, but I am not sure about frp.
Pilot hole, for sure.
I have often been told that turning the bit in reverse as it gets down to the outer edge of the bit will help, but have not tried this.


Member III
Blogs Author
I searched "how to drill through fiberglass boat" on YouTube and it appears that unless you are drilling through gel coat, it should be straightforward.

If you are drilling through gelcoat, the advice is to tape the surface and then countersink the hole a little to chamfer the gelcoat once the penetration is complete.

I think I would start with a smallish pilot hole first for the sake of alignment.


Member III
+1 on running the hole-saw in reverse at the beginning of the cut to avoid surface edge chipping (e.g., do this until you are completely through the gelcoat) - tape will not save you in this application.
You can do the same revering trick at the very end of the cut too, if you want to avoid punching out chips on the inside (if that will matter?)

Kevin A Wright

Member III
For a 1/2 hole I'd use a Forstner bit or a brad point bit if you have one. Like a hole saw they have a cutting surface at the outer edge of the hole to prevent splintering. If you can, drill from both sides. If not, drill from the side you want to stay pretty. The front is not the problem, it's the backside that blows out. The other option is to have someone hold a block of soft wood on the backside as you drill through to help reduce the tearout.

Kevin Wright
E35 Hydro Therapy

Frank Langer

1984 Ericson 30+, Nanaimo, BC
This was much easier than I had feared. It helped that the holes were between the engine compartment and the area under the quarterberth, and between that area and the battery compartment, so neither is visible or cosmetically important. I decided to try my electric drill and a 7/8" hole drill, which is all we had available. It worked perfectly, made a nice hole through about 3/16" fibreglass and gelcoat, similar to when I previously drilled hardwood.
Thanks for all the advice above!

Keith Parcells

Sustaining Member
I didn’t see this earlier, but....

when using a hole saw in brittle material like plywood or fiberglass, IF you have clear access to both sides, start cutting your hole at the desired location but cut only about half way through the wall thickness. Your pilot drill should have punched through, though. Then go to the back side and reinsert the pilot drill in the hole and cut in from that side. Cleaner cuts with fewer splinters.