Electrolysis Question Volvo 2010

[DavidTav]

I have a Volvo 2010 engine with traditional shaft. It is only 2 years old and replaced a previous engine. The previous installation had a pear shaped anode underneath the hull connected to the engine block. This has been connected to the new engine block, but it has always puzzled me if this is in fact correct because there is a do not earth symbol clearly marked on the block. I was under the impression that the drive is electrically isolated from the block on this engine, but this doesn't seem to be borne out when testing with a meter. I wondered if any more knowledgable readers could advise whether I should keep the anode or just disconnect it. It has corroded slowly. I would be tempted to fit a shaft anode instead, but there isn't enough room for that. Thanks.

 

[boatmike]

The purpose of the anode is of course to protect the prop. I am not sure why the block says do not earth. It is normal to isolate the gearbox from the engine with a saildrive cos its ally but don't see why you need to on a conventional shaft. Do you have electrical continuity between the block and the shaft? It will usually require a strap across the flexible coupling to achieve this. Is the anode as close as possible to the prop? (it should be) If it is not being depleted it probably isn't working. If there is no continuity of circuit between the prop and the anode or the anode is too far away from the prop it won't. The key question though is....."is your prop OK?" If it ain't broke don't mend it!"

 

[VicS]

I think the insulated gearbox etc applies to saildrives. See the note about Galvanic corrosion on p71 of the workshop manual.

I note there is an earthing relay fitted to these engines. It appears that when the battery isolator is open most of the electrical system is disconnected from the earth by that relay opening. I'm not an electrician so It takes me a long while to figure out some of these wiring diagrams but take a look at it for yourself on p80 of the w/s manual. I still do not understand why there should be a "do not earth warning" on the block or quite what it means.

As far as your anode is concerned the connection to the engine block sounds like the normal way of connecting to the stern gear. Hopefully any flexible couplings have a bridging wire across them. A better method is to use an "Electro eliminator brush" system immediately inboard of the stern gland see http://www.mgduff.co.uk/leisure-craft/fitting-instructions/electro.html

If you think your stern gear need cathodic protection keep the anode and leave it connected as it is. If It does not appear to be effective then you'll have to think about the brush system to make the connection directly to the shaft. If you think cathodic protection of the stern gear is unnecessary then disconnect the anode from the block but keep it connected to anything else that may require protection.

W/S manual

Repeated much of what boatmike had to say I'm afraid

 

[DavidTav]

Thanks to all for the advice