Best way to make a 35mm hole in a GRP hull?

[ChasB]

Hi,

I need to route a gas pipe out of a box (where the gas bottle sits) down through the GRP double-thickness deck. The deck is 40 - 50mm thick. That's two layers of GRP with balsa between. Really hard to tell exactly how thick this is in total until a hole is made , but it seems to be too long for a copper gas skin fitting. My solution is to fit a plastic skin fitting that goes all the way through which would allow a gas pipe to pass through inside of that with some wiggle room.

Once fitted, sealant would of course have to be applied to fill in the gap around the gas pipe to stop any gas leakeage (or water!) making its way down.

The plastic fitting TruDesign Long Domed Skin Fittings (Thru Hulls) has an outside diameter of 33mm. Once measured up it would be cut to length.

So, I need to make a hole of 34mm dia. through the hull. How would I do this easily, cleanly, cheaply, and without cracks in the hull?

BTW My intention is to fit this plastic thing myself, and leave the gas to a professional.

Many thanks



p.s. Since cold things are more inclined to be be brittle, would it help to warm up the area with a heat gun first?

 

[Boater Sam]

Starrett hole saw in a drill.

 

[Hermit]

That is a big hole for a small pipe - just wonding if using one of these would be better - still can be a biggish hole through the deck, but 'pipe-sized' hole in the rubber seal and no need for loads of sealant to fill most of an, at least, 1 inch sized hole.

Scanstrut Cable Seals | Force 4 Chandlery

 

[ChasB]

That is a big hole for a small pipe - just wonding if using one of these would be better - still can be a biggish hole through the deck, but 'pipe-sized' hole in the rubber seal and no need for loads of sealant to fill most of an, at least, 1 inch sized hole.

Scanstrut Cable Seals | Force 4 Chandlery

I like your thinking.

 

[VicS]

+1 for a hole saw if you really need to make a large hole.

A bulk head fitting will be better if you can use one because it will support the gas pipe and form a gas tight seal around the exit from the gas bottle locker.

If you find you can use a bulk head fitting bore out the centre land so that the gas pipe will pass through without being cut and only use one of the compression joints.

I'd drill a pilot hole through to determine the actual thickness of the deck.

.

 

[Boathook]

Hole saw, but a decent make such as Starrett. GRP wrecks the cheaper ones very quickly I have found.
Don't forget to seal the balsa with resin to save the deck getting ruined by water tracking through.

 

[ChasB]

A bulk head fitting will be better if you can use one because it will support the gas pipe and form a gas tight seal around the exit from the gas bottle locker.

That sounds like a good idea for the gas locker. ?

 

[penfold]

As everyone else has said a decent bimetal hole saw is the answer or you could try one of these. A bulkhead fitting is the proper way to do it; either counterbore underneath if you have access to permit it to fit as bulkhead fittings aren't usually available for very thick bulkheads or bore right through and fit a small flange. Otherwise just drill a hole big enough for the pipe and seal it up afterward with bog.

 

[andsarkit]

Whichever method you use, it is important to avoid a water leak wrecking the balsa.
The West Manual has some advice on holes in cored panels and I think it would be wise to follow that. Drill the outer hole with the holesaw and remove some of the balsa. Fill with epoxy and then re-drill right through at the required size.

 

[ChasB]

Whichever method you use, it is important to avoid a water leak wrecking the balsa.

Excellent advice.

And as it happens, that's exactly the problem a previous owner left me with: a poor gas fitting seal leading to a large chunk of virtually inaccesible rotten balsa core which I then had to remove. Really difficult messy job. I dried it out (over several hot summer days and with a heater) and filled the space with resin - West of course. And it got a bit hot! So what is in there, for about 20% of the rear deck, is actually a thick block of epoxy resin!

Still, I am aware there are likely to be small gaps that could a allow another leak to travel, so that's why I thought a thoroughly sealed plastic fitting would do the job. The gas fitter then has to just poke the copper tubing through and seal up the gap around it. Any leaks in his work would not result in leakage bertween the two hull surfaces, but instead a drip that can be easily dealt with.

 

[PetiteFleur]

I would use an Index Marine deck gland, available in various sizes with several rubber bungs to suit different sizes of pipe/tubing. Similar to Scanstrut but cheaper! Is this for copper pipe or rubber tubing?

 

[Thistle]

I think I'd be inclined to find a gas fitter used to working on boats and the leave the whole job to them. They may well be able to suggest a better design for the whole system which could avoid carving big holes in your boat - with all the potential for leaks that goes with them. If holes are required a properly qualified fitter should be able to do the job in a way that protects you from water and, perhaps more importantly, gas leaks.

 

[Refueler]

Hole saw, but a decent make such as Starrett. GRP wrecks the cheaper ones very quickly I have found.
Don't forget to seal the balsa with resin to save the deck getting ruined by water tracking through.

My old 1970's boat has 50mm lower hull solid GRP ......... it wore out 3 good Hole Saws getting the speed log hole cut ... but it was worth it as the hole was true and clean to go. Allow the saw to cut without too much pressing and it will also partially seal the raw GRP of the hole ... leaving only a smear of epoxy to finish the job.