CAN I MIX MY ANODES?
Q: I have a Tartan 3500 that I sail in freshwater on Lake Ontario. She is docked at a yacht club. I try not to keep her plugged in to shore power for the entire summer, unlike most boats we are surrounded by. I have a Yanmar saildrive with a three-blade VariProp. A few years back I experienced severe prop corrosion when I was using all-zinc anodes. Since then I have installed a galvanic isolator and subsequently disconnected that isolator and installed an isolation transformer. I also switched the anode on the propeller to aluminum and have been using magnesium anodes on the saildrive, both externally and for the pencil internal anode. These tend to significantly erode over the five months we are typically in the water. I had also been having trouble with blistering of the bottom paint (VC 17) in concentric rings on my lead keel (see photo at right),
recoated with barrier epoxy and repainted. But the blistering has improved with all these changes. My question is regarding the use of unlike metals for anodes. I understand that there are recommendations against using zinc with aluminum. Does the same hold with magnesium and aluminum? If so, should I switch to all—aluminum since I cannot locate a magnesium anode for the VariProp propeller? The arrangement seems to be working with the mixture except that the prop gets a coating of scale, which I am thinking has to do with the erosion of the magnesium anode. Douglas Walker, sailmail@sailmagazine.com