Prop Shaft Fault

[fishwife]

I have a dripless seal on my prop shaft. Above the seal in use is a spare, mounted on the shaft and held up by friction. Doing the weekly check of all the through hull fittings I noticed that the spare gaiter had slipped down and exposed a section of shaft I don't think I've ever seen. By pure chance I noticed what appeared to be a spiral hair line crack, about 4 inches long, not extending to the outside of the shaft. I had an engineer look at it and then arranged for the shaft to be pulled and x-rayed. It's definitely a crack and I'm waiting now for a new shaft. When we put the boat on the hard, the prop was in excellent condition with no evidence of ever hitting something. Equally, the bearings etc were OK and the alignment was fine.

Can anyone suggest how this could have happened?

P.

[Chief Engineer]

I have seen shafts get damaged by ahead/astern operation without allowing the shaft to stop turning.

I have seen 2 inch shafts twisted off.

Have you tried to have it magnafluxed in situ?