Fiberglass Drilling

D

dbsea

Well-known member
Joined
Oct 1, 2021
Messages
1,063
Fluid Motion Model
R-23 (Outboard)
Vessel Name
HALCYON
MMSI Number
368365270
I have identified a potential location for my Pepwave 42G antenna up on my flybridge, which will keep my cable run to the modem very short. I will likely install the modem in a waterproof box under the flybridge helm with NPT outlets and run the electrical to the helm fuse box / switch panel, where there is already an un-used "accessory" switch, which I will use to power the unit. Question is - how does one drill through the fiberglass safely? Is it as simple as a diamond/carbide tipped hole saw and my drill? I need to drill a 1" diameter hole to feed all 7 wires for the antenna to the underside of the helm. The antenna will be mounted on a 6" antenna mount, connected to a deck mount, and the cabling will feed through this and through the drilled hole. Here's a link to my icloud shared album, the most recent photo has the currently targeted general area for the install. This gives me the best coverage for the antennas, and a minimally invasive install.

https://www.icloud.com/sharedalbum/#B0hG6XBubsGED9
 
dbsea":2bezirua said:
Question is - how does one drill through the fiberglass safely? Is it as simple as a diamond/carbide tipped hole saw and my drill?
Click to expand...
Diamond or carbide tipped is great. I’ve had fine results with high-speed steel bits and hole saws, though they won’t last as long. There is a trick though:

Run the bit or hole saw *backwards* until you grind down through the gelcoat. I didn’t know this the first time I tried to drill a hole in fiberglass on a previous boat. The bit grabbed in spite of my best efforts to apply only light pressure and the result was a network of spider cracks radiating out in the gelcoat. I muttered some words that should not be repeated here.

If I am using a hole saw I like to use just the mandrel (or a bit of equal size) first and make the pilot hole—running it in reverse till the entire angled face of the bit is through the gelcoat, then in forward to make a clean hole. Then after mounting the hole saw to the mandrel, same thing: backwards all the way through the gelcoat then forwards the rest of the way. Good luck! Sounds like a great project.

John
 
Probably the best way to drill a hole or cutout in gelcoated fiberglass is with a Dremel and a tungsten carbide rotozip-type bit. Dremels are (relative) cheap and a really great addition to your toolbox. Drill a small pilot hole then put the Dremel to work. The little plastic cup that keeps the cutting tool perpendicular to the work surface is also a great relatively cheap investment. The tungsten tip and the high RPM of the Dremel help to cut the fiberglass before it destroys the bit.

The other protip is that, in addition to minimizing gelcoat chips with the Dremel, you should use painters or masking tape over the entire area. I didn't think tape could solve the problem of spider cracks / chipped gelcoat but man it really helps!

If you Google "dremel to cut gelcoat" you will probably find all sorts of interesting stuff to corroborate and other related tips. Maybe even some videos!

Also, having tried to cut fiberglass with a hole saw... I would not try that again.
 
My suggestion is 100% hole saw for a 1" hole. Use a good hole saw. I have a full set of holes saws that I purchased 30 years ago and only used them for fiberglass. I always pre-drill my center hole using a 1/4" bit. The hole saw mandrel that I have uses a 1/4" drill bit for centering the hole saw. Pre-drilling allows the saw blades to contact the gel coat immediately. Reversing the cut can help from chipping the gel. I don't use this method (there is nothing wrong with doing it) I approach the blades slowly and evenly to cut through the gel and make sure the saw blades are cutting square all the way around the hole. I have made cuts in fiberglass from 1/2" to 6" in fiberglass without issue and used the same saws for years. Anything larger I use a rotary cutter.


This is a 5" hole cut in the side of my C26. This hole had to be cut exact so it did not interfere with the hull lines that was 6" below the rubbrail
gallery2.php?g2_itemId=66814&g2_imageViewsIndex=1
gallery2.php?g2_itemId=66817

A 1" will be an easy cut. Good luck with your project.
 
Thanks. What is a “good” hole saw with mandrel for fiberglass ? The choices available are dizzying…
 
Probably the most expensive ones you can find. Like most things in life you get what you pay for. Always good to stick with the name brand like Milwaukee here in the midwest. Avoid the store brand or Amazon. A contractors supply house is better than the box store. I build houses and most of my subs are now using the Milwaukee line of tools. I bought a set of phillips bit tips from the lumber yard about 20 years ago. When I got the bill a month later I about lost it they were so expensive. I still use those same bits today. Not one has worn or broken. If I lose or loan one I replace it with the same. Good tools are priceless.
 
dbsea":37pvsl6k said:
Thanks. What is a “good” hole saw with mandrel for fiberglass ? The choices available are dizzying…
Click to expand...

Take the advise of Mr Ben. That is a good choice. I'm sure there are better ones out there but I have always used Milwaukee hole saws. I purchased a large set in the 90's and added to that over the years. I have a set that I only use when working on fiberglass boats and a set for wood and metal.


Messenger":37pvsl6k said:
OMG
That must have been measured 15 times before it was cut. I'd have a panic attack putting that big of a hole in the boat.
Click to expand...

At least 5 times. I made a template with a centering hole and measured that 5 times. After years of cutting holes in boats it is less intimating. You just don't want to make a mistake!!
 
When drilling through fiberglass, always run the bit in reverse until you are all the way through the gelcoat. Someone above mentioned this as well. It's important, otherwise it may chip and crack your gelcoat. It's also a good idea to sand the edges around the hole slightly (so the hole doesn't have a sharp edge on the gelcoat) to help prevent further cracking of the gelcoat.
 
To second what others have said, and passed along from a marine service yard, drill the pilot hole for your hole saw, assuming the hole saw has solid center shank/pin , the run your hole saw "backwards" to groove through the gel coat.

on a cordless drill drill with high/low... id be on low, and backwards for "awhile".
 
Thank you all for the advice. I am rethinking the location for a number of reasons. Stay tuned for the final outcome…
 
I little off topic for your size hole, but whenever I drill a smaller hole, like for a 3/8" bolt, I always use a countersink bit to taper the gelcoat edge to keep it from spider cracking. If using a drill I run it backwards, but often I just work it with fingertips to taper the gelcoat.
 
Still unsure why one would use a hole saw for this rather than a Dremel, with (ideally) or even without a circular guide. A 3-5" hole in fiberglass is a perfect use for one!

Also, one thing that hasn't been mentioned here: when drilling that much fiberglass (regardless of how you do it), don't forget to wear a well-fitting N95 mask. It's vicious stuff if it gets in your lungs.
 
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