Mixing Anodes

[Albert Ross]

Good idea or recipe for disaster?

My boat has a Zinc anode on the autoprop, a zinc anode on the prop shaft, and will also soon have an aluminium anode on the hull (connected by a copper wire and a stainless steel bolt to the engine block). All these items and anodes are in electrical continuity with each other.

If you look on-line, lots of folks say that mixing anodes "is a no no".

The boat is moored in seawater, so either Zinc or aluminium is OK.
Aluminium is currently much cheaper and lasts longer but is less widely available in chandleries etc.

My prop anode (Zn) presently only lasts a few months at most (no, I'm not in a marina, and yes, we do have a galvanic isolator when we do (rarely) plug in).

Now it seems to me that the Aluminum anode being very slightly more anodic than the zinc one will be spent very slightly faster than the zinc anodes (which themselves will be slightly protected by the aluminium anode). Thus my zinc anodes will last longer and the boat will be better protected.

Is there a chemist out there who can tell me the real truth?

Thanks.

 

[VicS]

Is there a chemist out there who can tell me the real truth?

Sounds like something I wrote a couple of days ago

I reckon you will be lucky if a aluminium hull anode reduces the rate of loss of the prop anode, or the shaft anode for that matter, by a great amount. What it will do though is maintain some protection when both prop anode and shaft anode are gone.