Mixing aluminium and zinc anodes

[stranded]

As new berth is in fresh water, I understand we should switch to aluminium anodes. No problem for standard anodes but I am struggling to find aluminium Gori prop specific anodes. We are shaft drive.

My (poor) understanding is that one should not mix aluminium and zinc anodes because the al will sacrifice some of itself to protect the zinc. As things are, the current zinc Gori anodes are seriously corroded at the end of each season while the bloomin’ gurt shaft zinc is barely touched. So my thinking is that if we get a suitably sized al for the shaft, the protection it provides for the Gori zincs might be no bad thing?

But is it more complicated than that? (!)

 

[Tranona]

No, probably simpler than that. There is little difference in potential between zinc and the aluminium alloy used for anodes - deliberately so. However proximity might be an issue as in your case with a shaft anode close to the prop anode and in the same circuit. But as your shaft anode does not seem to be doing anything it is rather a moot point. The prop anode goes first because it is in closest contact with the propeller that needs protecting. The shaft anode only comes into full play when the prop anode has gone. Your problem will be that the zinc prop anode will become ineffective in fresh water and the aluminium shaft anode will come into play earlier. Can you get magnesium anodes for the Gori?

 

[stranded]

No, probably simpler than that. There is little difference in potential between zinc and the aluminium alloy used for anodes - deliberately so. However proximity might be an issue as in your case with a shaft anode close to the prop anode and in the same circuit. But as your shaft anode does not seem to be doing anything it is rather a moot point. The prop anode goes first because it is in closest contact with the propeller that needs protecting. The shaft anode only comes into full play when the prop anode has gone. Your problem will be that the zinc prop anode will become ineffective in fresh water and the aluminium shaft anode will come into play earlier. Can you get magnesium anodes for the Gori?

Thanks for that Tranona.

The problem with magnesium in our case would be she will be stored in fresh water for around half the year but sailed in seawater for the other half. And not in two neat chunks, so not a situation where we could do a quick lift twice a year to swap (and I really hate cold water (Atlantic France) so diving to do it isn’t really an option.

Our current zinc is half way along the prop shaft so I am planning on moving the replacement much closer to the p bracket to see if that improves our excessive cutless bearing wear. With the closer proximity to the prop and the slightly greater effectiveness of al, perhaps that will go some way to addressing the issues you raise.

Thing to do I think will be to try it and keep a close eye on how it goes.

 

[[70521]]

Find some scrubbing post or do what the French do, beach their boats on a nice sand bank and swap your anode. Make sure you beach anode side up.

 

[stranded]

- you mad man!