Prop Shaft Corrosion

[Appledore]

A couple of years ago I noticed slight corrosion on the prop shaft when I undid the stuffing box to check the packing. Previously I had used a hemp packing, with lots of grease on it, but obviously this deteriorated fast and allowed too many drips, which meant I had to tighten it after every trip. I also have a greaser on the prop shaft. Last year I repacked the gland with Reinforced Graphite Gland Packing (Ebay). Last week I checked the shaft again and noticed that the corrosion appeared to be worse. It is just where the packing would be on the shaft.
I read somewhere that trapped water could cause shaft corrosion, and this perhaps could have been the initial cause, but on reading Nigel Calder's book he says (on page 548) 'graphite impregnated packing material shall not be used because of the possibility of galvanic incompatibility with the shaft material', and I wonder if this is the cause of the further corrosion.
Calder also mentions Dripless Packing, which has 'a crumbly, clay-like consistency that can be rolled between your palms until it is an appropriately sized diameter to go in the stuffing box. It is sandwiched between outer and inner rings of conventional packing which keeps it form working its way out of the ends of the stuffing box'. He mentions that the original drip-free packing consisted of shredded Teflon (PTFE).
What I intend to do is clean up the corroded part but not 'flatten' it, and fill the shallow void with JB Weld or the like, then suitably sand back to a smooth finish the same diameter as the shaft. It isn't that deep, and is pock-marks rather than scoring. But what do the panel recommend for the new packing? I thought, like others, that graphite was OK, but it appears not to be the case. A new shaft will be fitted before long!

Thanks for your comments.

 

[oldharry]

Stainless's anti corrosion property is the result of the fine film of oxide which forms on the surface of the metal, preventing further attack. Anywhere on a shaft that is subject to wear, it gets rubbed off and does not get a chance to re-form while the shaft is still in motion. Cratering which is what i think you are describing may be normal corrosion, or it may be electrolytic, occurring here because the protective layer is being constantly rubbed off. It is not uncommon too to find crevice corrosion which in the early stages looks like a small crack in the metal. This can be dangerous because the weakness goes a great deal deeper into the metal than would seem reasonable from the surface evidence..

When i was maintaining workboats 30 years ago, any sign of crevice corrosion or cratering meant the shaft would have to be replaced. They would not get their Commercial Passenger Certificates otherwise as this was something that was always checked out by the surveyors. As the boats would be running up to 6 or 8 hours a day, this meant average shaft life was often less than a couple of years!

 

[Neil_Y]

I still think a dripping conventional packing gland is the best solution, it drips a little with the shaft turning thereby lubricating and cooling the shaft. If adjusted well it won't drip much, if at all, at rest.

The danger with grease, or too much grease is it can work its way to the stern tube bearing blocking waterways and allowing the bearing to over heat and wear more rapidly.

 

[vyv_cox]

You can find several examples of shaft crevice corrosion on my website. Once seen it is virtually unmistakeable as the damage exactly matches the counterface at the narrow crevice.

 

[pcatterall]

Is it so serious with a stuffing box? I had a similar issue with my lip seal ( and my location in 'stagnant water' mooring at Cagliari Sardinia) With a stuffing box doesn't the packing simply expand to fill the cavity ?

 

[vyv_cox]

Is it so serious with a stuffing box? I had a similar issue with my lip seal ( and my location in 'stagnant water' mooring at Cagliari Sardinia) With a stuffing box doesn't the packing simply expand to fill the cavity ?

I imagine it does but there is always the possibility of a crevice.

 

[Appledore]

Vyv,

thanks for your response (and others too). Looking at your pictures I can see it's the same problem. However no one as yet has suggested what I could use in the stuffing box/gland in place of graphite packing, if this is at all a contributing factor to the pitting, nor if JB Weld would help in filling in the pitting. Any suggestions please? Thanks.

 

[rogerthebodger]

Is it so serious with a stuffing box? I had a similar issue with my lip seal ( and my location in 'stagnant water' mooring at Cagliari Sardinia) With a stuffing box doesn't the packing simply expand to fill the cavity ?

That would be fine with the shaft stationary but once the shaft rotates the cavity could rip the packing or the lip of the seal causing the seal to fail.

A face seal like the PSS overcomes this problem as any cavity on the sealing face could easy be replaced by reversing the rotor or replacing the rotor.

Vyv had explained this before as Vyv reported crevice corrosion on his rotor face before.

 

[Minchsailor]

I cast doubt on repairing the propshaft - I think it is unlikely you could get an adequate surface finish. A new shaft from T J Norris is not that expensive. After suffering a similar problem with a previous owner who had used graphite packing, I now use the PTFE type. Still good after several years.

 

[vyv_cox]

I cast doubt on repairing the propshaft - I think it is unlikely you could get an adequate surface finish. A new shaft from T J Norris is not that expensive. After suffering a similar problem with a previous owner who had used graphite packing, I now use the PTFE type. Still good after several years.

I do exactly the opposite. PTFE packing gave me lots of overheating problems so I reverted to graphitised cotton. The packing had been graphitic for 40 years before my abortive trial with PTFE, no sign of any crevice nor galvanic corrosion.

Before I changed to a PSS on the Sadler it also had graphitic cotton, around 20 years, no corrosion.