Answers to some technical queries at http://coxengineering.sharepoint.com
When I bought my boat new I was told that there was a known problem with the propellor eating anodes and that Volvo was aware of it. The prop was a 3 bladed Volvo folder on a 120S Saildrive. Volvo's solution was to supply me with another propellor made with a different metallic compostion. So maybe the problem is with the Volvo prop. Sorry to disappoint but the new problem actually made the problem worse. It's now in my den acting as a hi-fi headphone stand.
I have this problem too. I would like to know for sure if the anode is really necessary.
Also, I wonder if the Ambassador rope stripper affects the circuit.
The saildrive anode doesn't erode much. The prop anodes went very quickly during the first year.
We have a similar set-up on a new boat (launched April 2011). After 6 months, the prop anode was about 50% gone, and we replaced it. After 10 months, we've just replaced it again (again about 50% gone). We are keeping the remnants in case we get caught short!
The saildrive anode, after 10 months, was about 30-40% gone. But here lies a problem - it is a standard Volvo anode machined to accommodate a stripper, and we are having to mill away some perfectly good zinc from the Volvo replacement to get it to fit! Again, we'll keep the eroded one just in case.
In the meantime I found out that Volvo Penta decalares that their folding propellers are a nichel+aluminium+copper alloy.