Help - SS worms!

[Gordonmc]

One thing leads to another...
Things were going well with the refit. Boat out of the water, rigging off and measured for new wire rope. Gammon iron and chainplates away for regalvanising and work started on cleaning up the hull for painting.
Then I had a look at the rudder and found the lower gudgeon is worn so the rudder will have to come off the irons to get the pin replaced. While at it... why not polish the prop...?
Off comes the castellated nut - to give my first experience of stainless steel cavitation corrosion in the propshaft.
Although the thread is sound, the shaft has become hollow to a depth of about an inch... too far to be happy about.
Clearly it is time for a new shaft... it's probably worn at the packing gland anyway. But what to do about preventing further corrosion? There is no room between the prop and the bearing to get an anode on... only about an inch and a half.
Or do I tell myself that a forty year life of a shaft isn't bad anyway?
How about a copper/carbon brush on the gearbox end of the shaft wired to a hull anode?
Any (free) advice welcome.

 

[Plum]

Glad everything else is going well for you! Never heard of "cavitation corrosion". If you have "crevice" corrosion (most often where the shaft is inside a bearing) it is probabley casued by oxygen starvation. SS needs oxygen to keep its surface layer of oxidation intact which is what stops it from corroding. Where you have free air or flowing water over stainless steel you don't usually get crevice corrosion. Zinc anodes and carbon bruches will not help crevice corrosion as these only reduce galvanic corrosion.

Does this help?

It could be that your old shaft is made from type A2 stainless steel rather than the better A4 type but it is not easy to tell the difference

 

[dickh]

Sorry to hear about your ss corrosion - better now than the prop fall off! Regarding anodes, you can get a narrow type - about 32mm wide for shafts from 1" to 1½" Ø which should clamp onto the shaft, depending on shaft diameter - see www.zincsmart.com If necessary the anode could be machined thinner to suit. Try one and see if it is eroded away after a season.
If you get a new shaft, make it a bit longer and you will have more room for an anode, alternatively fit a spacer in the engine coupling thereby moving the prop further from the bearing.

dickh
I'd rather be sailing...

 

[ianwright]

Free advice

Get the new shaft made from Monel. S/S has no place on a boat except for eating irons,,,,,,,,,

IanW

Vertue 203, Patience

 

[anmarine]

Contact MG DUFF at Chichester I'm sure they would be willing to help with advice etc.

 

[ccscott49]

Totally agree with Plum, anodes won't stop this type of corrosion they are there to protect your prop. I also agree, Monel is far superior, but not sure how much it would cost, A4 or 316 stainless would be better than A2, which should never be used under the water anyway so I doubt your shaft is A2, but as Plum says, the shaft needs oxygen to stay stainless and prevent "Crevice" corrosion, which is what you are describing.

 

[Gordonmc]

The shaft is now at a fabricators to be used as a pattern for a new one in 316. Monel bar was not available in the diameter of the old shaft, one and one-eighth inch. I will replace as was and keep an eye on things.
Thanks for the advice.