cgaa186
New Member
Just moved the boat (GRP) to the Vilaine, which is fresh water, but plan to sail in the sea for periods of up to about two months at a time. The hull and shaft anodes are currently zinc and immersion in fresh water is said to make them ineffective - certainly after a few weeks they seem to be covered in some kind of insulating layer. This isn't a problem when in fresh water as I can hang a magnesium anode (and anyway there probably isn't much corrosion then) but I would like to avoid lifting out each time before going to sea just to clean up the fixed zinc anodes.
There seem to be two possible strategies and I'd welcome any comment:
1. fit aluminium anodes instead of zinc. This seems to be common practice in the US - e.g. in the great lakes - where passivation of aluminium by fresh water doesn't seem to be considered a problem. However UK advice (e.g. MG Duff) seems to be that aluminium like zinc should not be used in fresh water. Can anyone throw light on this apparent contradiction, or have experience of using aluminium anodes in fresh water?
2. use a dangling zinc (or aluminium) anode when in salt water (so basically dispensing with fixed anodes altogether). The anode hung over the side would be about 1.3m from the propshaft and propellor, and there's a fairly low resistance (less than 0.5 ohm) between the propellor and the point of connection inside the boat. Does that seem likely to provide enough protection?
Of course these strategies could be combined ..
There seem to be two possible strategies and I'd welcome any comment:
1. fit aluminium anodes instead of zinc. This seems to be common practice in the US - e.g. in the great lakes - where passivation of aluminium by fresh water doesn't seem to be considered a problem. However UK advice (e.g. MG Duff) seems to be that aluminium like zinc should not be used in fresh water. Can anyone throw light on this apparent contradiction, or have experience of using aluminium anodes in fresh water?
2. use a dangling zinc (or aluminium) anode when in salt water (so basically dispensing with fixed anodes altogether). The anode hung over the side would be about 1.3m from the propshaft and propellor, and there's a fairly low resistance (less than 0.5 ohm) between the propellor and the point of connection inside the boat. Does that seem likely to provide enough protection?
Of course these strategies could be combined ..