What is the best to cut 3" hole in FG?

What is the best tool method to cut a 3" hole in Fiberglass?

I am installing the Compac 50 Whale Bilge in my foredeck compartment bottom. Not the hull but the compartment floor, which is FG.

Is there a hole saw that would work or is there a better method?



Might try drillin’ a pilot hole…
an’ then use a jig saw wit a fine toot blade or a metal cuttin’ blade ta cut out de 3" hole. Ah’d put some maskin’ tape on de area first.


Check local tool rental stores.
They ought to have a 3" hole saw (probably part of a larger kit).


PS: You WILL make lots of dust - take safety precautions.

I think I remember reading somewhere that if the drill is run in reverse there will be less damage to the surface.

ok i am on it!
thanks mates! I am on it!

yes , a 3" hole saw …

– Last Updated: Apr-30-09 10:26 AM EST –

..... that is if you can get the drill in the place where you want cut the hole .

The type that have the seperate "mandrel" are better because the same mandrel is used for the other size hole saws available from a given manufacturer ... although if you don't think you'll be needing other sizes later on , the fixed mandrel type are less expensive .

Also there is a specific type of hole saw that has a diamond edge around it instead of regular saw teeth .These are used on ceramics , glass etc. and cost more .

go slow
and run a shop vac pointed at the hole saw teeth.



not too much pressure
as the hole is nearly thru, you can peel off the inner laminate if you’re pressing too hard. Just enough pressure to keep the saw cutting, and don’t spin it so fast that you burn the teeth - that’ll take the edge off of them right quick.

If cutting braces is any indication

– Last Updated: Apr-30-09 3:32 PM EST –

then using a high-speed dremel tool or roto-zip should work. I used rotozip (30K RPM or so) to chop off my thigh braces off a fiberglass cockpit and it did a very smooth job. Mask, gloves, and ear and eye protection are a must.

If you have an attachment for round holes for the rorozip or dremel tool, I'd think that is the best way to do it. But by hand it is OK too - the high speed on these things makes a very clean cut (with the right bit). I recently installed a small day hatch with this same rotozip tool and just used my hand to cut the 4" diameter or so round hole - it came out nearly perfect to within a mm or so of the circle I drew... Just don't let it loose - it can cut thru most things like butter -;)

I have a feeling a jig saw or a 3" "door lock" type of drill would leave jagged edges at the cut and may crack the gel coat. But I have not used one to tell for sure...

thanks to everyone. I am getting a hole saw and I do have a dremel. I will practice with the methods you all suggested. I can see how important it is to know this stuff. Could really go less than well otherwise.

Hole saw
If you use a hole saw, the best kind is one with a carbide slurry on it. The standard bimetal saw has a very aggressive tooth shape. And if possible put a piece of wood behind it. A carbide hole saw is very expensive and if it were me I would consider the carbide tip dremel.

thanks Falcon, after seeing the other posts, I began to realize what you are stating. I will likely go the dremel route.

Hit a rock just right
then patch the hull hole.

Sorry spring fever hitting.

Go with a dremel
I would concur. Whenever cutting fiberglass, I’ve found a dremel to be the best tool. It may take a (little) bit longer to cut a largish hole, but you will have a lot more control over the shape and will unlikely delaminate the fiberglass.

Shop vac
Might want to use a bag for glass work, or be very careful when dumping it out.

drill fiberglass
As a plumber, I drill fiberglass on a nearly daily basis. (Bathtub surrounds, showers, etc) We use a regular holesaw for metal or wood on the tubs. They cost about $35 and you can get em just about anywhere. I have found that a higher speed reduces tearing the glass. The dust is horrible. Don’t breathe it

Yes, they were sold on late night TV in the 90s.

Yes, it works pretty well.

Draw the circle with a sharpie or wax pencil, then trace with the RotoZip or a Dremel. I find the smaller kerf is much cleaner (albeit slower) than using an aggressive hole saw with a wide blade.

That’s true with plastic…
…and it’s common to use reversed blades on band saws and circular saws used for cutting it. With fiberglass, reversing the cutting direction would just build up a lot of heat.

Another thing to consider when cutting fiberglass is that it contains glass, which is a very hard material. It will dull steel tools in a heartbeat. Carbide tools are best and will last a long time, but heat-treated steel will get you through a small job or two.

They also make circle guides for…
… those and the dremels. Can adjust hole size. If you like buying stuff…

For one hole if (since don’t have special toys like that), I’d mark well, then cut short of line with whatever, and finish up to line with a sanding drum (drill, dremel, whatever) as it is easier to control and eliminates chip/crack problem. Respirator recommended…

deptf control
This is probably stating the obvious, but if you are drilling through a compartment floor you wnat to make sure you do not drill into the hull when you break through. Some type of index mark or stop on your bit will keep your bit where you want it.