Cutlass bearing and Shaft replacement

[Witzend]

So it's been 10yrs on Witz since I redid the drive train (Engine mounts, Cutlass bearing, damper plate, packing). It's time to replace the cutlass bearing and packing again. She gets pulled every year for the winter and runs in fairly silty mostly fresh water, I'm guessing the silt provides enough abrasion to act on the cutlass negatively. I've got the bulk head out now and engine ready to raise to get the shaft out. While sliding the shaft aft for inspection after detaching the coupling from the tranny, I noticed that the original bronze shaft had considerable pitting only where the cutlass bearing contacted. This surprised me as being on a mooring and no AC any where near (solar charged). No where else on the shaft is there deprivation, just where the cutlass bearing runs, and it's not scoring. My zinc deprivation is ~2/3 every year.
I'm struggeling to understand why the shaft acted in this manner.

 

[dlochner]

I'd like to give a definitive answer, but I can't. One issue with SS shafts and SS in stagnant water is crevice corrosion. This occurs when SS is immersed in stagnant water, the oxygen in the water is depleted and the chemistry of the metals in the SS start to react with each other causing the corrosion.

That the pitting only occurred around the cutlass bearing leads me to think that a similar process might be happening to your shaft. The water in the cutlass bearing, especially in the small gap between the ribs and the shaft get stagnant and corrosion ensues.

While the shaft is out, be sure to check it for wear around the stuffing box and shaft log and to make sure it is still straight. Bronze is a relatively soft metal and will wear at the stuffing box and can be warped and no longer straight. It might be time to replace the shaft with a SS shaft.

 

[Witzend]

"Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not found in grooves or slots in which circulation of the corrodent is possible. "
from:

Crevice corrosion - Wikipedia

en.wikipedia.org

The gaps are way bigger within the said interface. She also is moored in a strong tidal flow ~1-2 knts so it would not appear to be stagnant water. But then again it did occur. It may not be considered "crevice corrosion". I will take some pictures when I get the shaft out.
BTW, no vibration issues.

 

[jssailem]

BTW, no vibration issues.

I would count my blessings.

 

[Witzend]

So I had a discussion about the chemistry with my 99yr old chemical engineer father at dinner last evening (will be 100 in November and still quit with it!) It's clearly not crevicing looks more like a geo topo map. (Pictures to come phone died arrg). He indicated that the vulcanization of rubber involves sulfur which does not play nicely with the copper content in the bronze. So mere prolonged contact has probably caused this.
Will be cutting the sucker out measuring it and going with Agualoy 22, with split coupling. Won't need to borrow that prop puller anymore.

 

[dlochner]

Will be cutting the sucker out measuring it and going with Agualoy 22, with split coupling. Won't need to borrow that prop puller anymore.

Good choice!