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sailak 08-28-2010 11:56 PM

Drilling Hole in Fiberglass Hull
I'm putting in a second bilge pump and need to drill a hole through the transom. Anything special I need to know about drilling through the hull? I need a 1 1/8 inch hole so will probably use a hole-saw.

I'm guessing it will be best to drill from outside in to limit chipping around edge of hole.

arf145 08-29-2010 12:28 AM

Haven't done a hole saw through the hull, only a 1/4" hole for my solar line. But I will recommend that you check several times exactly what is behind where you are drilling. No, I don't have any humorous stories on this--because I checked a bunch! Then did a pilot hole and checked that.

sailak 08-29-2010 01:31 AM


Originally Posted by arf145 (Post 637783)
But I will recommend that you check several times exactly what is behind where you are drilling.

Oh yeah. The plan is to put this 2nd pump line next to the original...can use it for lineup to help make sure things are clear. Don't want to mess this up, to late in the year to be learning glass repair techniques. <G>

SlowButSteady 08-29-2010 02:18 AM

Once you figure out exactly where you want the hole (don't put it so close to any existing holes that the material between them is likely to fail), drill a pilot hole first using a bit significantly smaller than the centering bit of your hole saw (to prevent it from "grabbing" when you drill the larger hole), check from both the inside and outside to make doubly sure you have clearance. If everything looks good drill your hole. Seal the newly exposed laminate and about 1/4" of the inner and outer surfaces of the hull with epoxy (this should be hidden by the new thru-hull), letting it set up and thoroughly cure before installing the new thru-hull.

sailingdog 08-29-2010 06:43 AM

If you run the hole saw in reverse until you get through the gelcoat, you'll cut the gelcoat without much chipping. Taping the area with masking tape will help too.

If the hull is cored where you install the through-hull, you'll want to remove some of the core and pot the hole with thickened epoxy before installing the through-hull.

Also, don't put through-hulls too close to each other. Doing so can weaken the hull and lead to failure. Back the through-hull with a fiberglass backing block bedded in thickened epoxy.

sailak 08-29-2010 12:58 PM

Thanks gents!

SD, running the saw backwards was the kind of thing I was hoping to learn here.

oceanscapt 08-29-2010 03:39 PM

Make sure you take the proper safety precautions by wearing a mask, face shield, and tyvex suit. FRG is nasty stuff.

Make the hole bigger than you need so you can seal the mat/interior from water migration.

Good tip on running the saw backwards. Gotta remember that!

bluwateronly 08-29-2010 04:30 PM

I don't think you want the hole in the getcoat larger than the size of the thruhull. If the area is cored and not pure glass then you want to prepare it, think dremmel tool here, their are so many threads you can learn from here, really, that can say it much beter than I.Backing plates are easy to make since you have the epoxy and some fiberglass.I would use 4200 when installing thruhull, uv. Take care and the hardest part is drilling the hole!

mitiempo 08-29-2010 04:52 PM

The hole should be the size required for the through hull. If the area is cored you want to cut the core back a bit and replace it with thickened epoxy, leaving the inner and outer skin intact.

A face mask yes, but a tyvek suit to drill a hole?

sailingdog 08-29-2010 08:53 PM

Really don't need a tyvek suit for this job... a full face mask respirator is probably a good idea and far more protective and comfortable than a half-mask and goggles.

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