What drill bit for gelcoat/fiberglass?

I’m planning to install a pump in my Nordkapp and am getting near the scary point- having to drill a hole in my beautiful boat. The hole will need to be 3/4" or 7/8" depending on the discharge fitting I decide on.

So… in thinking about twist bits, spur point bits, etc- I’m thinking a hole saw will make the cleanest hole. I imagine I should then seal the edges of the newly cut hole with some epoxy to protect the exposed glass.

I’d appreciate any insight on the entire operation from those who have been there and done that.


– Last Updated: Mar-13-06 9:44 PM EST –

a hole saw, not a drill bit

best wishes

people always say to put tape over the area where you are going to make the hole so it doesn't chip.

I have done 5 so far and never used any tape. used a hole saw and gentle pressure and had no chipping or none I can tell...the flange on the discharge covers back quite a ways anyway. I use a little aquaseal when I tighten down the discharge even tho it doesn't really need anything. I never have felt the need to use any epoxy in the hole area. the aquaseal and the flange and gasket keep everything out ...the water never gets to the glass.

Holes in boats…

– Last Updated: Mar-13-06 11:31 PM EST –

I haven't installed anything like a pump, but I did add some padeyes at the bow and stern of my FG Caribou for spare paddle bungies. I was very nervous about drilling holes into my boat (just seems so unnatural!), but it turned out to be no problem. I don't know the name of the drill bit I used, but it was pointy at the business end. :-)

In order to make sure I would drill in the exact spots I wanted, I used a sharp awl to make a little indentation at each spot before applying the drill. Then, I drilled slowly, and all was well.

At the time, I didn't think about sealing the edges of the holes with epoxy before screwing in the padeyes, but I did apply a bit of silicone to the bolts before putting them in, then a bit more on the end of the bolt inside the boat before putting on the nylon "locking" type of nuts. It's been a few years since I did this, and I haven't noticed any water getting into the glass.

That said, when I built my S&G Arctic Hawk, I did use the "drill-fill-drill" method for drilling all the holes and sealing off the edges of the glass sheathed wood. This worked very well for all the holes I had to drill in the boat. I'm not sure if/how the drill-fill-drill method would work with a glass/gelcoat boat, but here's how it's done:



not necessary with fg
the absorbant crushable core in s&g kayak might benifit from dfd but it’s not necessary with a solid fiberglass laminate. Depending on the installation some fg through deck installations may need fender washers. With s&g usually just giving the hole a good soaking with epoxy is sufficient to make the hole solid and waterproof.

A few of those CLC shop notes hints were filler material. The dfd method is more applicable to thicker cores and mountings on bigger boats where the stresses require the technique.

only thing i would add is
to let the bit do the work, as mentioned above gentle pressure and think 5 minutes though it wont take that long


– Last Updated: Mar-13-06 11:51 PM EST –

I agree with medicineman and Fadedred, use a holesaw for that large of a hole. Tapeing is a good idea along with light pressure and a sharp bet. drilling a hole in a boat always makes me nervous but it always comes out fine

Oops! I should have…
…read more carefully about the size of hole he needed to drill. Please ignore my at least most of my previous post! :slight_smile:


For what its worth … a regular
twist bit run backwards 'till it is through the gelcoat, then set forward to cut through the glass / carbon / kevlar is all that is ness. for hole drilling … wetting out the hole w/ epoxy ( unless the boat is wood or cored ) is a time wasting folly. I would also hole saw super super slow, probably backwards first if only using a toothed saw 'till through the gelcoat before cutting through. An abrasive saw is the right tool for gelcoat cutting or ‘drilling’ as question is.

Holesaw Too…
just drilled through my composite surf boat to install a drain plug. Masking tape, light pressure, let the teeth do the work. Also helps if you can stick a piece of wood, or foam or anything under the cut to prevent splintering of the fibers from accidentally applying too much pressure.

Take a deep breath and then go at it.


I just installed a pump about a month ago. I used a hole saw which worked out fine on the kevlar. I had a devil of a time finding the thru hull fitting that I wanted. I ordered the pump with a 1" hose adapter so I used 1" ID hose. I wanted a 90, or elbow, with a 1" adapter and I wanted it very close so that it did not protrude into the boat any further than it had to. I spent hours looking on the internet and finally found a two piece ninety with a 1" hose fitting, in the UK. I was able to cut down the thread half of the fitting to provide a bend right at the inside hull, just right.

I used tape on both sides of my hole to help prevent splinters. I trimmed the kevlar fluff with a pair of needle nose and razor knive. I sealed the edges of the hole with resin and used a silicone based bedding compound on the thru hull.

Happy Paddling,


After the drill
chamfer the edge of the hole. in other words sand it smooth, don’t leave a square edge at the edge of the hole. this will help prevent future cracking of the gel-coat.

try this
1. Select a spot for the through-hull. Before drilling. A small flexible light (like those used for reading at night) shining through the hull can help relate in to out.

2. Check the location for the through-hull to make sure that the very stiff non-crimping bilge hose will bend enough to connect to the outlet of the pump. A PVC right angle coupling on the pump end can help with making the turn.

3. Using a brad-point bit, drill a ¼ inch pilot hole.

4. Using a hole saw, drill the hole for the fitting. I used a smaller (3/4”) hole saw and then enlarged to fit with the Dremel tool.

5. Apply marine caulk to the hull side of the through-hull’s flange, then insert the through-hull into the hole.

6. Thread on the nut by hand, and then tighten using a wrench. Do not over tighten.

7. Wipe off excess caulk.

Yep, use a hole saw
It will cut much cleaner than a large drill bit and there’s no chance of it catching and tearing/cracking the hull.

Closing the deal-
The hole was drilled with success and no harm came to the boat or myself in the process.

Although I have not yet finished the photo album, you can witness the drilling here-