Cutting holes in GRP

[DanTribe]

Hi
I need to enlarge a 150mm dia hole to fit a larger portlight in my topsides.What is the best technique?
Can I use a jigsaw? Coarse or fine blades? Fast or slow?
Would it help to cut from inside to avoid splintering the gelcoat?
The hull is approx 17mm thick & new hole will have 150mm dia semicircular ends.
Thanks
Dan

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[Strathglass]

Dan,

Use a ceramic tile cutting blade in a jigsaw at not too fast a speed. Take it slowly and let the blade do the cutting and don't push it too hard. The ceramic cutting blade will not go blunt half way through the job and give you a better cut.

If the GRP panel is flat then it would be possible to sandwitch the panel between two plywood panels bolted together then use a holesaw for the ends and jigsaw in between the two holes. This can give more precice curves but I dont think it will be necessary in your case.

Iain

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[sailorman]

jig-saw with metal cutting blade.
put masking tape over area to stop scratching with "sole plate"

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[Becky]

I have just been doing it- cutting holes that is. I used a laminate blade in my jiggly saw, and it was fine. If you can cut from the back, so much the better as the blade cuts on the up-stroke and can splinter the top edge if you don't. But a battery-powered jiggly saw runs slower and can leave a smoother finished edge, if you have the choice. My youngest son was a manager in B&Q so I have access to loads of tools. Not that I am particularly interested in such things!

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[celandine]

I would use a jig saw as descibed here if you can. If the jig saw body comes up against some immovable part of the boat, stopping you from following the path that you need, you might try chain-drilling, cleaning up the resulting rough edges with a rat tail.

Mick

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[PaulR]

very timely as I have just bought via e bay two waterproof cockpit speakers and need to cut 4" round holes-was planning to hire a hole saw- any one had success with those and any tips gratefully received (had thought of using jigsaw but thought hole saw would be neater?

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[chippie]

Enlarging an existing hole with a holesaw is dependant on having something for the pilot drill to go into to start. I have screwed a piece of plywood with self tappers inside the final hole diameter . Once the holesaw has gone in part way you can then come in from the other side to prevent tearout. With care this method works well.

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[Strathglass]

Whenever using a holesaw it is essential to use a LOW speed drill and to hold the drill firmly.
As you are drilling new holes you should not have a problem with the centre drill being secure but, as sugested, a piece of ply would make it easier.
It is important to hold the cutter horizontal to the GRP surface to ensure that a complete circle is scribed in the gel coat before the cutter starts penetrating.

Iain


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[MIKE_MCKIE]

Ref all the replies before, agree that either a low speed sigsaw of a hole saw for new holes is the way to go. HOWEVER, don't forget to wear a good dust mask/respirator. The dust given off is EXTREMELY ABRASIVE (after all it contains a fairly high proportion of GLASS) & is best not ingested!!!.
Coughingly yours!
Mike

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