Re: Removing old thru-hulls
I helped a friend remove an old transducer sleeve from his hull. It had a flange outside and an inside nut which is either seized or epoxied. Finally used my Dremel with a cutting disk to go inside the opening from the exterior and cut outwards around the circumference of the tube until the inner and outer halves could be separated. There may be other ways, but that worked for us.
Alternately, stuff a broomhandle or somesuch into the opening to give something for the center bit of a hole saw to grab, then drill out the entire thing a little oversize - since you're planning to remove it and plug the hole anyway.
1998 Jeanneau SO45.2