Just saw the title to this, & had to read the thread. I'm glad that you found the prop. Mush have been quite the surprise when you went to maneuver the boat!
And, much like Jim Cate, I had the same thought about having a machinist make you a spare nut for less $. The tricky part about that, is what, if any, kind of heat treatment or semi-proprietary chemically based metallurgical alteration might the nut have?
As to keeping the prop in place, SOLIDLY, this time, chemistry could be part of the option... in addition to the already mentioned safety
wires or pins.
What I'm referring to is this. There are plenty of SERIOUS adhesives out there, for which there are also fairly common solvents which will absolutely remove them.
I learned as much when talking to ITW/Plexus about Devcon, one of their metal filled epoxy
putties. And the gent at the company that I was talking to, addressed my concerns about being able to disassemble some metal parts
glued together with their Stainless Steel Epoxy
Putty, by telling me that simply by soaking the parts
in Methelyene Chloride, I could take them apart again (that chemical's spelling may be a touch off).
So for your propeller
, I'm uncertain as to what exactly you would use, but odds are that there's an adhesive
which you could glue it on with. And then, down the road, when it's time to pull it off, you repeatedly bathe the joint in the solvent several times, for a few days. Much as one does with penetrating oil
on conventionally stuck metal parts.
That, & or apply extreme cold to the parts involved. Often enough that works as well, for conventionally stuck metal bits, as well as "glued together" metal parts. Be it; Dry Ice, Loctite Freeze & Release, or a long blast from a scuba
tank (I'm told).