Instead of fussing with a zinc at the prop have you considered placing a streamlined zinc on the shaft just forward of the after strut? Same protection, less hassle. I tried the propeller
nuts with the built in zinc fairwater that goes over it. They always managed to fall off after a few months after the stainless steel
screw that holds it on managed to corrode away the zinc causing the whole anode to fall off. For me it was a big waste of money
. I just put two streamlined anodes at the forward end of the after strut..it works well for me.