I've sheared off one or two nuts in my day. When I don't want to it's ridiculously easy.
So I'd just put a tight socket on the nut with a decent size handle and lean on it. Should shear right off.
Then grind off the stump and weld a suitable 316 bolt on. Go for something like 5/16".
You've an Offshore
38, right? the've been a few reports of skeg failure on larger versions of similar designs. My impression is that the close proximity of the propeller
to the skeg can cause electrolysis
which would not help.
I'd suggest mounting a largish anode on the skeg right behind the prop and making sure the paint
on the skeg, rudder
and surrounding are are up to scratch. Propspeed on the prop seems to help.
And just in case the stray electrical
currents get a bit out of hand I've found a big anode close to the bottom of the keel
in the middle can be a good idea. Don't ask.