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S&S Swan Maintenance - 38 Shaft Strut Corrosion
06 June 2009 - 17:27
#1
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jim
Join Date: 17 February 2007
Posts: 11

38 Shaft Strut Corrosion
I am having corrosion occur at my shaft strut.  Has anybody had this problem?

08 June 2009 - 14:56
#2
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danielefua
Join Date: 30 January 2007
Posts: 462

Jim,
finding the source of galvanic corrosion is rather difficult and most of the times the easiest way to avoid it is by the use sacrificial zincs wherever needed. Corrosion of the shaft strut is not uncommon and should be prevented with a couple of zinc discs in good electrical contact at the sides of the strut itself, fixed with a long through bolt.
The same zincs will work also as a useful protection against corrosion of the nearby propeller.
Just in case: check that the strut is not incorrectly used as ground plate. Usually, as far as I know, off the water it should be electrically insulated from both the shaft and the boat electrical system.

Regards
Daniel, 411-004

08 June 2009 - 19:24
#3
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The Professor
Join Date: 02 January 2008
Posts: 1547

Jim

The strut should not corrode, and there is some particular reason for this problem.

If there is a cable connected to the inside of the strut it is likely that the corrosion is caused by an electrical leak on board. Disconnecting the cable will stop the corrosion, but then some other underwater part may suffer instead, and it would be preferable to find the leak and eliminate it.

Are you connected to shore power? Is there a zinc on the propeller or shaft, and how long does it last?

Daniele, afraid that a zinc on the strut will not protect the propeller, because electrical contact between them is required to form the protective circuit. The shaft rubber bearing prevents this contact

Kind regards

Lars
08 June 2009 - 22:09
#4
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danielefua
Join Date: 30 January 2007
Posts: 462

Dear Lars,
I agree with you that a positive direct electrical contact must exist to produce cathodic protection but I have often seen zincs on the strut (as I wrote) that were there for "propeller protection", I was said. The only explanation I could guess was that, once immersed in salty water, an electrical connection proportional to the exposed area, although weak, could indeed be created between the shaft and the nearby strut.
Just a possibility I am not totally convinced of.

Regards
Daniel

30 July 2009 - 12:03
#5
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yacht roxi
Join Date: 30 July 2009
Posts: 2

Hello
Just a small suggestion we did have a similar probelms on our shaft strut about ten years ago before forums existed after alot of discussion with a good surveyor friend we decided it wasnt galvanic corrosion but crevice corrosion happening under the antifoul we sandlasted all the old epoxy and paint and filled the holes with JB weld we also renewed the bonding wires and its been good so far however we have now found crevice corrosion on our shaft tube so be careful, very difficult to see with out removing the PPS Seal, the coorosion is where the shaft seal touches the shaft not the seawater side
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