I had one fall off, when the nut and key were eaten away by electrolosis, the nut had a screw in it to lock it on, that was the first thing to go! Are you in a marina with earth probs? or do you have a leak to earth through your anodes and are your anodes ok? This is all worth checking.
Yes, I am in a marina with a power hook up and did have some trip problems last week but the anode was replaced in October. Is there a way to check earth leakage? Funnily enough at the same time I,ve had the engine coolant disappear without draining into the bilges!! I just hope that is not corrosion.
I think it is likely to be the calorifier. I was told that the anode was replaced in October so had not even considered electrolysis as a threat until the propeller fell off! I'm trying to get the boat out the water ASAP.
The entire prop shaft started working its way out through the hull of my 27 ft boat two years ago when I was off start point (with 4 and 2 year old children and an unimpressed wife). The first I knew about it was that the boat suddenly lost way. Engine note didn't change or anything. I thought I had lost the prop so went over the side with a mask. The prop and shaft were sticking out far too far. Another half an hour and we would have lost the lot and started shipping water through the tube quite fast! I spent a very cold twenty minutes manually pushing the whole thing back up into the boat and rotating it ten degrees at a time so that my wife could stick a bolt through the coupling and shaft to act as a kind of sheer pin. We had to repeat his process three times because the bolts kept.....sheering. I was very upset as I was well on the way to persuading my wife that the whole thing was perfectly safe as a family past time.