Saildrive anodes in fresh and salt water.

[Quandary]

My boat, a Finngulf 33 is berthed in fresh water (The Crinan Canal) but all our sailing is done on the sea, about 40-50 days per year in cruises of 1 to 30 days. The only anodes are zinc and are on the Volvo saildrive and propeller. Salt water is electrically more aggressive than fresh so I am reluctant to change to magnesium anodes. Will it help if the boat goes into salt water at regular intervals, if so how often and for how long? Could I protect the drive by fitting magnesium anodes but hanging a zinc over when berthed at sea? If so where should the inboard end be grounded, (I am told that the leg is isolated.) Will there be any problems with dissimilar metals in the anodes? Is there an expert anywhere with experience of this, Volvo refer me to the local dealer who says not to worry, the zinc anodes will do ok but they do coat over in fresh water.

 

[vyv_cox]

I agree with your dealer. We kept our boat in fresh water in Holland for seven years, sailing on the sea most weekends. We always used zinc anodes. They do coat with oxide/hydroxide films but this is not a big problem, you can abrade it off at suitable opportunities. Once per year seems to be quite adequate and we suffered no underwater corrosion problems during that time.

What you should not do is to fit magnesium anodes. They will react rapidly with seawater and almost certainly will be consumed in a weekend or little longer.

Aluminium anodes are being promoted for dual-water applications, so you might like to check MG Duff's anode selection pages, which will probably direct you that way.