Thanks for the replies. I figured I might need a new prop shaft but won't really know 'til I get it out and have a googd look. The corrosion appears confined to the very end of the shaft but it is hard to tell without removing it from the boat.
The full story about this problem is that I spent a number of years completely renovating the boat and during that time I was given a second hand prop shaft which was in much better condition than the original shaft which had serious wear at that point where it passes through the stuffing box/packing gland.
As the donated shaft was too long I got a local engineering bloke to cut it to length and turn a new taper on the end to suit my prop and to cut a thread for the prop nut. The guy was slightly out cutting the taper which resulted in the prop sliding further up the shaft about half and inch. This put the prop closer to the hull than I wanted because I had planned on fitting a zinc anode to the shaft in that space behind the prop. It also meant I had to put a large spacer in between the prop and the nut, as can be seen in the photos.
Time went on, renovations continued and the need to do something about finding a method to fit a shaft anode was put on the backburner. When the renovations were finally completed, the boat was relaunched but in the meantime I somehow forgot about fitting the zinc anode. The boat was moored in seawater at a jetty which had no
shore power but there were other boats moored nearby. Due to
health problems and
family matters the boat got little use for almost three years, but I did go down there fairly regularly to clean it and run the engine etc.
Fortunately, my corrosion problem appears to be confined to the prop and the end of the shaft. My through hulls are as good as the day I fitted them and the engine,
gearbox, and shaft coupling all appear to have not suffered any damage. That part of the prop shaft which is inside the boat shows no sign of corrosion either.
When the
weather improves I will pull the shaft out of the boat and have a closer
inspection of it. I don't know what the grade of the stainles
steel is and I understand what you are all saying about pitting and intergranular corrosion. I will take that into account when I inspect it more closely.
I doubt if I could fit a collar zinc with the setup I have at the moment but if I have to get a new prop and shaft I'm thinking I might be able to go a couple of inches longer to give some room behind the prop for a collar zinc. It would mean moving the
rudder out a bit further but I think it could be done, hopefully without too much hassle.
I will also definitely look at
electrical bonding for the boat. It has none at the moment. I will need to read up more on the subject to get my
head around it but it sounds like it's something I should have done three years ago.
I will add some more photos later when I have the shaft out.
Thanks again to everyone for the replies.
bony.