Black art of anode chemistry … calling all metallurgists.

[VicS]

Thank you.

I think you have now answered my original question:

Anodes are no longer "a black art".

Thank you all.

Good. I was thinking of mixing a potion for you

...................

or getting my former colleague Getafix to make something to apply ......

 

[Rossynant]

Aluminium is far more widely used than zinc for steel structures in seawater. For example, see http://www.bacbera.dk/index.php/english/offshore/platform-anodes

Yes, so to clarify myself - they are not advised for sailboats. We built many steel boats here, having several zinc anodes on hull and Baltic is not so salty. Aluminium might be even better, but here a danger is they may be spent before next haul-out, without noticing. Can happen in 'electrified marina' for instance, this was known to happen.
That's why I think OP's arrangement is safer, as he would still have the zinc, as originally designed, in such case.

 

[NormanS]

The real problem is that Brunton's anodes are ridiculously expensive, considering the tiny amount of metal in them.
It seems that flat1 may have found a clever way of making them last longer, and provided that the propeller is adequately protected, who gives a stuff about the science?
Well done, flat1.

 

[nigelmercier]

I'd not expect more than about 0.4v... The 500 mA you measured would account for the largest part of a kg loss of zinc, or an equivalent loss of zinc and aluminium, over a 2 month period...

Thanks Vic, but where are you getting these figures?

 

[VicS]

Thanks Vic, but where are you getting these figures?

The 0.4volt from a galvanic series table. There are many on line with no doubt some small variations . I have this one bookmarked http://www.corrosionsource.com/FreeContent/1/Galvanic+Series

IIRC you reported that you could measure 500mA current flow between the engine and what you refer to as ground.
By calculation that will account for a loss of nearly 900g of zinc in 60 days .. except that I guess your original 120S anode is less than 900g so once all gone the aluminium will go at the rate of approx 4g per day.

BTW I am inclined to think that your saildrive was never isolated from the engine, nor intended to be

 

[nigelmercier]

The 0.4volt from a galvanic series table...
IIRC you reported that you could measure 500mA current flow between the engine and what you refer to as ground...
BTW I am inclined to think that your saildrive was never isolated from the engine, nor intended to be

Thanks for the info. The 500mA was with me shorting the engine to the ground (keel) with the ammeter, I hope the normal current is somewhat less.
I'm inclined to believe the same, otherwise why have to grounding relays.

 

[VicS]

Thanks for the info. The 500mA was with me shorting the engine to the ground (keel) with the ammeter, I hope the normal current is somewhat less.
I'm inclined to believe the same, otherwise why have to grounding relays.

This is drifting the thread and creating another discussing your problem. I will PM you

 

[nigelmercier]

This is drifting the thread and creating another discussing your problem. I will PM you

Yes, sorry to OP. All interesting stuff though.