Galvanic corrosion of one keel on a Moody S31

[KellysEye]

>How/ where do you fit a galvanic isolator to the 12 volt system and what exactly is its purpose ?

Exactly the same place as 240v on the earth wire. The purpose is as I said to stop stray current corrosion.

 

[VicS]

>How/ where do you fit a galvanic isolator to the 12 volt system and what exactly is its purpose ?

Exactly the same place as 240v on the earth wire. The purpose is as I said to stop stray current corrosion.

Sorry I dont understand what you mean by " exactly the same place as 240volt on the earth wire" .
A GI is fitted in the incoming earth conductor from the 240 shore supply before it connects to any part of the boat or its electrical system. There is no incoming earth conductor in the 12 volt system, its all on board, and it has no earth wire any way, so there is no "exactly the same place".

You are advising somewhere to fit one in the 12 volt system ........ Where ?

What are these stray currents you mention. A GI will only stop current originating from the very low voltage "galvanic sources". It won't stop current that originates from 12, 24 or 240 volt sources.

 

[Terpeg]

Thank you Tranona, I am in favour of removing the hull anode, if as you suggest it is unnecessary.
On inspecting the hull anode today I noted the the engine is earthed to this anode, another earthing wire runs from the diesel water separator /filter to the anode. There is a "do not earth' sign attached to the saildrive unit so I assume the saildrive although attached to the engine must in someway be isolated from the engine.
By removing the hull anode is it unnecessary to earth the engine and ancillaries ??
Pete

 

[Tranona]

Yes, the saildrive is isolated from the engine (and the rest of the system) through a membrane and washers between the gearbox and the engine. No need to have an anode attached to the engine or the fuel filter.

 

[Seajet]

Terpeg,

just another thought to add to the confusion ! I have found this relevant in my case though...

My original 1977 keel ( lifting, galvanised mild steel plate with a cast iron ballast bulb on the end ) showed signs of galvanic corrosion and eventually required replacement.

There are no electrical items whatever bonded to or even near the keel structure inside the boat, she has an outboard which is stowed in a locker on the - half tide, mud - mooring, and there are no through-hull fittings at all.

I was intrigued why this had happened, and after asking around discovered the ' Marine Metallurgy Unit ' at Portsmouth Uni' were investigating accelerated steel corrosion in Chichester Harbour.

They came and had a look at my old keel, and on chatting it seems there is a problem area from at least the Western part of Chichester Harbour all the way to Portsmouth.

Their initial theory was that the culprits were ' steel eating microbes ', things which excrete something which viciously attacks steel, the wreck of the Titanic has this apparently; I don't know if it affects her insurance...

My keel proved not to have these microbes, but they agreed something nasty was going on; one expert claimed the deep pitting in the 3/4" steel plate was caused by the speed of the boat through the water !!!

My own theory is that the nearby marina - with shore power of course - and the even nearer mains power supply to Hayling Island are relevant, everything steel nearby suffers.

There is also the fact that the area was used as a decoy to lure luftwaffe bombers away from Portsmouth, and there is a lot of ordnance in the mud, the craters are still evident from the air and when one tries to walk on the mud.

My new keel instantly showed signs of galvanic corrosion, and nobody could come up with a ' proper ' electrical explanation; thinking ' nothing to lose ' I fitted a streamlined anode on the part of the steel plate in front of the ballast bulb which stays below the hull with the keel up.

Every autumn when she comes ashore the anode is at least half gone, and the keel looks fine, no more ill effects.

So what I'm saying is, consider the locality of your mooring, it might not be something on your boat; and one can chat about theory all day on here, but if something works, grab it with both hands and ponder the causes at leisure!