external anode

[geoff]

I have had on my 40 ocean a separate  external anode attached to the engine block since I bought the boat new in 2001. In that time it has shown virtually no wear at all. I am thinking of moving the internal connection to the saildrive body instead , the idea being to slow the rate of erosion of the very small sd120 ring anode . Any thoughts out there ? Geoff

[Symphony]

There is no need for an external anode to be attached to the engine block as there is nothing in the engine that needs protection - that is why you observe no wear. Connecting it to the saildrive housing may reduce wear of the ring anode. However, the effect is likely to be minimal as the ring anode wears first because of its proximity to the aluminium casing. That is exactly why it is attached directly to the casing. The best you can expect from an external anode is that it might be effective if the ring disappears completely, but even then it depends on how physically close it is to the casing.

[tiger79]

Isn't the external anode part of the earth bonding protection system?

[geoff]

I fitted the external anode myself before delivery. My earth bonding is I think a ruddy great copper conductor between the mast and a keel bolt [keel being lead on an ocean] Geoff

[Symphony]

The copper conductor is for protection against lightning - that is earthing the mast to the keel. External anodes were not fitted before around 2010 when the standards on electrical systems were revised. There is no need for an external anode on your boat which is why one was not fitted by the factory. One of the benefits of a saildrive system is that corrosion protection is incorporated in the construction - even if the actual need and execution is problematic. With shaft drive boats external anodes are often fitted and bonded to the engine to provide an electrical path to the stern gear to protect the yellow metal propeller which is in contact with a stainless steel shaft. If an engine does have dissimilar metals in the construction of the seawater system as many raw water cooled and some heat exchanger cooled ones do (but not most of the Volvos fitted to Bavarias) then there will be an anode in the seawater circuit.