Trouble with a large hole saw in grp is the pilot hole getting bigger as you drill then the hole saw starts to walk, thats when they start to snag. The method I use is to rivet a small plate in the middle of the hole <anything will do> mask the area you are drilling and drill from the gel coat side inwards. Never had a problem doing it this way even with a 130mm saw. The masking tape is enough to stop the plintering of the gel coat and the small plate will stop the drill walking. Oh and go slowly with a variable speed drill.
Bob
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