C
Colin
I would cut the nut off using a hacksaw and take the two halves to a machine shop and have then make a split nut for you. They should be able to measure the nut but may have to visit your boat to get the correct thread dimensions. Such a nut would have small screws each side to hold the two halves together once assembled around the propshaft. Might cost you $100 or so, but worth it for a system that will work on the long term.