Cutting large hole in GRP berth base

[Hydrozoan]

The immersion heater thermostat (or the element) of our horizontal, under-berth calorifier seems to have failed. There isn’t clearance to withdraw it so rather than dismantle the pipework and lift the calorifier I propose to cut an access hole (large enough for the whole immersion heater head) in the (ca. 5mm GRP) berth base, finishing it off with a dinghy hatch (e.g. Plug Inspection Screw White 102 Mm BRAND Osculati 20.204.00 for sale online | eBay). Am I right that an ordinary HSS holesaw (e.g. Silverline 427630 Bi-Metal Holesaw, 114 mm: Amazon.co.uk: DIY & Tools) is the best way to cut the hole, and are there any special precautions to avoid its cracking the gelcoat?

 

[davidwf]

I use Bosch hole saws and have never had any cracking of the Gellcoat on the side I am drilling from. Mind you Bosch are twice the price of the Silverline, but I always believe when it comes to drill bits and hole-saws you definitely get what you pay for.

 

[maby]

I would expect that any decent hole saw would do the job. Do be careful about the dust - it can be very irritant if inhaled. Wear a good mask, ventilate the work area well and hoover out all dust afterwards.

 

[V1701]

Blue masking tape over where the cut will be made may help in avoiding gelcoat cracks (better than the cheap white stuff) and second the suggestion to buy good quality hole saws. If you already have a jigsaw you could use that...

 

[Fr J Hackett]

I always found Starrett to be about the best hole saws. If you use a jigsaw use a laminate cutting blade, the teeth are reversed so you will get the better cut on the top with less not no chipping and the tape trick is an old wives tale it doesn't work.

 

[pvb]

If you use a jigsaw use a laminate cutting blade, the teeth are reversed so you will get the better cut on the top with less not no chipping and the tape trick is an old wives tale it doesn't work.

Agree about the laminate blade, but if using a jigsaw on GRP it's best to cover the surface in masking tape or some other stick-on protection, as the vibration of the soleplate can mark the GRP.

 

[Hydrozoan]

Thanks to all for the very helpful responses; I don't think I'll use a jigsaw as space to turn it is probably too limited.

 

[Daverw]

I think you will find cutting hole with large hole saw easier than you think, you coukd if really worried about it put a peice of ply on the front face and cut through both, screw the ply onto the face inside the hole diameter and you will go through cleanly.

 

[maby]

Thanks to all for the very helpful responses; I don't think I'll use a jigsaw as space to turn it is probably too limited.

A scrolling jigsaw is the answer - blade separately steerable from the body. Possibly a bit expensive, but worth it if you do any significant amount of DIY

 

[duncan99210]

I have a set of hole saws I bought from a Spanish “Chinese shop”, where they sold all sorts of cheap and cheerful stuff. Been using them to cut holes in GRP for years now: I just use a fairly slow speed on the drill and keep up an even pressure on the drill once it starts to bite. Any minor chipping on the edge of the cut will be covered by the lip of the hatch when you fit it.

 

[syTaika]

A hole saw is the easiest option when you need a round hole, but for other shapes I use Fein Multimaster with a thin semi circle shaped blade. It produces a very little amount of dust compared with a jigsaw or a hole saw.

 

[Hydrozoan]

My jigsaw is a scroller but a teak trim on the GRP face might be a problem, and blade clearance from pipework inside might also be an issue. But I’ll take it to the boat and check when I measure up more carefully - the calorifier maker now fits a different heater ... which uses a different thermostat ... which is slightly complicating matters.

Yes, the lip of the hatch would cover any edge chipping so I’m probably being needlessly concerned about long cracks. (At least I’ve now stopped kicking myself for not thinking more about replacement when installing the calorifier, realising I’d have chosen the hatch solution and deferred fitting it until it became necessary.)