Cabin ceiling
Bob- I drilled a 4" hole in the cabin ceiling (coach roof) of my O'Day 27 to install a solar vent. Location was on the CL abt. 12" fwd of the mast. Total thickness of the roof was 3/4" ( inner & outer f'glass + 3/8" balsa core) plus an approx. 3/16" thick f'glass cabin liner. Very easy to drill using a 4" dia. hole saw in a 1/2" drill. Your roof is probably a bit thicker due to the greater beam of the boat, but I doubt that the balsa core is thicker than 1/2". Before you drill, check for electrical wires behind the liner or sandwiched in the laminate itself. My boat had 2 #10 wires, which I found the hard way: saw the shiny copper on each side of the hole I made. Very hard to splice without cutting away more of the f'glass to expose enuf wire. Still don't know what the wires were used for but think it may be for bonding or a grounding circuit. Later on I played around with an electronic stud finder and it seems to respond to hidden wiring OK. When you cut thru the roof you should dig the core material back abt. 1/4" or so all around the opening, and fill with thickened epoxy, to seal the core. Otherwise, eventually, moisture will get in and cause the core to rot.
