It is still painful for me to drill holes in a perfectly good boat, but it is more painful still to drill it in the wrong spot, or have it end up too big for the hardware.
In the case of thru-hull fittings, I normally use a hole saw for the job. Picking the correct size can be deceiving, so here are a few things I do to measure twice and cut once.
One would normally start by holding the hole saw up against the fitting. The drilled hole is often slightly larger than the bit itself, so keep that in mind. The bit in the picture looked like the closest fit from a visual standpoint.
I always drill a test hole in door skin or thin wood before I think about turning it on the boat. Here the test hole works, but had a little more play than I would have liked.
I put the next size down in the chuck and gave it a test drill in the wood. Despite the fact that it had looked too small visually, it was just right for the thru-hull. If the hole is too tight, you can relieve it with sandpaper wrapped around a dowel.
Now I turn to the real thing on the boat. I measured my spot, twice, and then drilled a pilot hole with a small bit. Use this to judge your spot, because it is much easier to adjust or fill a tiny hole versus a one-inch hole. If it still looks good, drill about halfway thru on the backside.
Now put masking tape on the finished side to reduce chipping. Find the pilot hole and drill the rest of the way from the front. If you drill out from the unfinished side, it will blow out a messy exit. Spin the saw quickly, but don’t push too hard. Let it cut at a steady pace.
Now you are ready to caulk and tighten your thru-hull in place.