I've remove many a thru-hull. And the best way I have found is just grind it with a rotary file, or sometimes called a carbide burr. Welders use then all the time for grinding out welds and such.
I've banged my head against the wall trying to save a thru-hulls and have finally given up that idea.
I have an over sized dramal sp? type electrical
grinder I use. An air grinder with a 1/4" collet is what they are designed for, but air is not alway available so I hord my electrical motor
Anyway, from the bottom I start grinding away the inside until I start to see threads. Then I'll go inside, if I can reach, and grind or cut the fitting off fairly flush with the inside deck
. Then with the burr grind out the inside of the thru-hull some more until I see threads again.
Then I start at it with a pick tool and hammer and start chiseling out what is left. The threads will usually peel right out. It works both on bronze and Marelon.
Bronze is a little nasty. The metal chips like to stick in the skin, so gloves and a long sleeve shirt are in order along with safety
glasses for both.
The spiral single
fine cut burrs are the best. The double cut don't cut very fast and the course tooth grab and spin around in the hole.