Saltwater/Frehwater anodes.

[graham]

Our boat is berthed and sailed quite a lot in Cardiff Bay which is fresh water.

We also lock out of the bay into the sea to go further afield.

The boat currently has one large zinc anode on the grp hull not wired to anything. I have just bought a set of shaft earth brushes to connect the anode to the shaft.
Is it wise to fit a magnesium anode on the other side of the hull and connect to the appropriate one.Maybe even install a change over switch to make it easy to switch from fresh to salt anode ?

 

[SimonD]

Easier to fit aluminium anodes. They're good for fresh and salt and anything in between.

 

[Tranona]

Why do you think you need an anode at all? If the current one is not connected, is your prop suffering from dezincification? If not then why bother at all. What sort of boat is it, as even if you do need one a hull anode may not be the best solution.

 

[Quandary]

On the presumption that you do need anodes you have to realize that zinc anodes would be fine in both fresh and salt water if they did not become ineffective in both when they aquire a passivated coating of inert material. Magnesium works well in both but can disappear after a couple of weeks exposure to salty water. I kept the boat in fresh water going to sea frequently for varying periods , the answer was aluminium anodes to protect a propellor and a saildrive leg. Prior to that in the same environment we had a Sigma with shaft drive, aluminium anodes were rare then so we simply used a zinc shaft anode, the rapid rotation of the shaft was sufficient to erode the coating, though as Tranona points out we probably would have been fine without anything.

 

[graham]

Thanks for the replies.Boat is coming ashore soon so i will have a close inspection of all the metal bits and decide what (if anything) needs to be done.