Just unfroze one a few months ago. It was below the waterline, so after some relatively gentle tapping didn't move it i waited until the boat came out of the water for the winter. Then I sprayed PB B'laster into the thru hull from the outside, and onto it where it sat in the hull and let it sit for a few hours. The handle then turned with more gentle tapping from a hammer, and now is as workable as all the others. Fairly certain what did it was spraying the penetrating solution up into the thru hull from the outside.