Drilling for skin fittings

[gtmoore]

I am fitting an electric bilge pump and need to drill the stern for a suitable skin fitting. I'm not that familiar with drilling large holes in fibreglass/gellcoat. Should I use a smaller bit and enlarge with a drill file, use one of those hole cutters that you fit a ring blade into a disk with grooves or would the type of flat bit used for drilling holes for locks in doors be more appropriate.

I assume I drill from the outside in?

Thanks for any advice

 

[Guest]

Use a hole cutter the right size. Stick masking tape over the area to be drilled. Check that where you are going to drill will not meet an obstructions on the inside.
best of luck!

 

[Gaffer]

Use a proper hole cutter, the newer and sharper the better. Drill from the outside in, but if you drill a small hole from the inside first then you know you will clear any obstructions. Apply masking tape to the gel coat on the outside where the hole cutter will cut to prevent the gel coat from chipping.

 

[Robin2]

If you can use a hole saw partly from the outside and partly from the inside you will get a clean cut on both sides.

 

[SimonD]

Use a slow drill speed and keep the hole clear of dust. GRP blunts hole saws very quickly (bitter experience speaking!).

 

[vyv_cox]

That's what I do. Buy/borrow a solid type hole cutter. The cheap ones with the thin blade that clips into a multi-slot base are not really up to cutting through a good thickness of GRP. Drill very slowly from outside for a short depth, then use the same pilot hole from the inside. The blade can get very hot, so pull it back from time to time. It's better to keep it cutting to avoid too much heat being generated, so press firmly, don't hold back. If you can't get a cutter exactly to size use a file to increase. Definitely do not use those woodwork hole borers on GRP.

 

[terryw]

I have done this a couple of times with complete success. I used a hole cutter, and started from the inside, When the pilot drill had completely penetrated the hull I then drilled from the outside until the plug was removed.
There are two reasons why I started from the inside:
1) There are generally more obstructions on the inside, and position of hole and coresponding pipework are more critical.
2) With the boat in the water, by drilling the pilot hole from inside you are able to lean over the side to complete the cut from the outside without the drill wandering about.

Hope this helps.
Terry

 

[Sinbad1]

But, check first to see if your hull is cored, ie, a sandwich construction which will be of two layers of glass with a lightweight sandwich of balsa or similar inbetween. If this is the case you need to drill from the inside with an oversize hole saw until you are through the first layer of glass and into the core. You then dig the core out, glass the hole you have just made, and when fully dry, start cutting again with the sorrect size hole saw as outlined above.

 

[gtmoore]

Thanks all - excellent answers as always! I'm off to B&Q to find a decent hole cutter.

Regards

Gavin