[jimmyb]

To install a heater flue, I have to make a 5" hole through the cabin roof of my boat. The roof material is 1.75 inches of fiberglass and wood.

My intention is to use a hole saw. Unfortunately, the centre of the hole is missing. It was drilled out by the previous owner to install ventilation hardware. So I don't have a solid surface into which I can drill the pilot bit.

To make matters worse, the cabin roof is slanted, 10-15%, so I think I need the pilot bit to drill a hole that is close to vertical.

I can think of two solutions. Fasten a piece of hardwood over the existing hole so the pilot bit can bite into it. Or make the best job I can with a jigsaw.

What would you do?

Any help appreciated.

 

[mdntrdr]

You could attach a 10-15 deg. block to the underside, and drill straight up. You might run out of depth and have to clear the holesaw before continuing. :smile:

 

[MT Stringer]

Would it be possible to make a plug for the existing hole and glue it in. Then you would be starting over.

 

[mmwood_1]

I had to do exactly that in fiberglass last year. I opted to use a fiberglass repair kit and glassed a piece of 1/4" ply to cover the existing opening, Then I drilled a 5" diameter hole through with a hole saw. If your hole needs to be pretty accurate, this is a better way to go than a jig saw, and it's pretty easy.

 

[jimmyb]

I had to do exactly that in fiberglass last year. I opted to use a fiberglass repair kit and glassed a piece of 1/4" ply to cover the existing opening, Then I drilled a 5" diameter hole through with a hole saw. If your hole needs to be pretty accurate, this is a better way to go than a jig saw, and it's pretty easy.

Agree it's probably nicer than I could do with the jigsaw, but not sure I understand the reason to glass the plywood down, though it would certainly look nicer before the hole was cut.

Screwing down a 4" disc of plywood would hold just as well don't you think, at least for my purposes? Then I could easily remove the plywood if I wanted to once the hole saw had bit in an inch or so.

 

[woodnthings]

you need a plug

To drill a vertical hole, you'll need to plug the existing hole to get a pilot. If you make a plug with a hole that's already centered and use quick/fast dry epoxy that will work to get started. The other issue is you will probably run out of depth on the hole saw because of the slant. You'll need to drill from both sides, another reason for the pilot hole.

NEW DRILL BIT SET - 16 piece HOLE SAW KIT - 3/4" to 5" - Amazon.com


OR you can jig saw out a larger hole then sleeve it with a PVC or metal tube, cut on the proper slant and glass that in for a clean look and watertight fit.

Or if the existing hole is just a bit too small, you can use a drum sander attachment to enlarge it slightly. Mine have a 1/2" shaft for use in a drill press. Similarly, you can wrap a cardboard or other close size tube with very rough sandpaper and do it by hand with vertical strokes up and down.

A Sawzall would leave a "rough opening" but the longer blade will help cut all the way through. If you get good with the saw and don't use too coarse of a blade you may be able to finesse the hole out cleanly...I donno?

 

[jschaben]

To install a heater flue, I have to make a 5" hole through the cabin roof of my boat. The roof material is 1.75 inches of fiberglass and wood.

My intention is to use a hole saw. Unfortunately, the centre of the hole is missing. It was drilled out by the previous owner to install ventilation hardware. So I don't have a solid surface into which I can drill the pilot bit.

To make matters worse, the cabin roof is slanted, 10-15%, so I think I need the pilot bit to drill a hole that is close to vertical.

I can think of two solutions. Fasten a piece of hardwood over the existing hole so the pilot bit can bite into it. Or make the best job I can with a jigsaw.

What would you do?

Any help appreciated.

If you have a decent hole to start with, You can open it up with a router pretty accurately assuming you are using the same center point. You need a flush trim bit with top and bottom bearings. Swap out the tip mounted bearing for one of a smaller diameter. Make the first pass around using that bearing for a guide. The next pass, lower the router so the top bearing is guiding. That would increase the diameter of the hole by the difference of the bearing diameters.
May not work if you want the pipe vertical through the slanted roof though. :smile:

Woodnthings - I've got that hole saw kit and it's handy but the cups are only about an inch deep.. 1.75" would be way out of it's league.