This all started by trying to fix the old knotmeter. After some enthusiastic inspection and troubleshooting by my pal, who decided it was broken, the old thru-hull transducer fitting sprung a leak as soon as the boat splashed. I went to the chandlery and checked out my options. There weren't very many.
One was to glass over the whole thing. There were no plugs or other devices for a temporary fix. The best option was to put a brand new transducer in the hole, so I ordered a new Raymarine ST40 Speed by priority post.
Things were looking grand as I drove to the yard with my new knotmeter, but when I tested the thru-hull fitting in the cutout, it rattled around with about 1/8" of space around the whole device. The cutout was too large by about a quarter-inch.
I decided to do an in-situ epoxy cast. My mold consisted of a flat base, pressed up against the outside of the hull, and a 2-inch outer diameter central vacuum PVC tube sitting in the old hole from inside the boat and resting on the flat base. Central vac pipes are the only PVC pipes I could find with a precise 2 inch outer diameter. I covered the flat base with a sheet of waxed paper. I cut a piece of the PVC pipe about 1.5" long and wrapped that in waxed paper also. To "glue" the edge of the waxed paper flat against the pipe I used some cross-country skiing liquid glide wax. This gets between the layers of waxed paper that I'd rolled around the pipe and makes them stick together.
I propped up the base against the hull with tape and pieces of foam on a box. From inside I inserted my home-made plug. With a syringe I put epoxy into the gap between the plug and the old cut-out. Some gentle turning of the plug ensured that the epoxy went all the way around the outside of the plug filling in the gap.
I won't know till tomorrow how this turns out. I'm hoping that if there's too much epoxy in the gap it can be sanded off, or if there are voids they can be filled in with more epoxy by hand and then sanded smooth. With luck it'll be a smooth cast with only a few burrs to clean off. The new transducer should be nice and snug in the new opening.