Saildrive corrosion: galvanic isolation and grounding

[Nigel]

Further to the problems that caused my 120S saildrive to become corroded: the brand new anode fitted two months ago was seriously depleted, and this time the shore power was not connected; so this at least eliminates that aspect of the fault finding. The replacement saildrive seems to be based on a 130S body, and has a massive split anode, which is nice. Obviously I need to avoid a repeat of the problem, so I've been doing some tests.

My saildrive may not be isolated from the engine, there are relays that make the engine ground connection via a 55A fuse when starting or stopping. However, the ground wire to the isolated starter motor was shorting on the casing, causing the engine to be connected to ground at all times. I think this is the cause of the problem because there is a lightning strap from the mast to the keel. Assuming the mast is grounded via the VHF antenna, it would make a full connection.

I discovered that with all the batteries disconnected, there is a voltage of 0.47V between the engine and ground, I briefly tested the maximum current between the two, it was about 500mA. This voltage is throwing my resistance readings between the two points, but I am reasonably confident there is a high resistance. I am wondering that this voltage is as expected from having dissimilar metals in salt water. Any comments on this?

I think it would be interesting to find out if I am alone in having a saildrive that appears to be electrically connected to the engine. So, is your Volvo Penta saildrive isolated from the engine?

Please post a reply confirming the following:
 Isolation status (Isolated/Not Isolated/Don't Know):
 Engine model:
 Saildrive model:
 Boat model:
 Year of manufacture:

[Odysseus]

Nigel, here are Odysseus details

Engine        Volvo 2030   5102153792

Leg             120 S-D      reduction 2.48 :1

Bavaria 38

Year 2006

My saildrive was not connected to the engine However I have now done that, also have extra anodes that hang over the side when berthed. So 12v and 240v are earthed through my "safesure " galvanic isolator through 8mm cable through engine and saildrive to the stern hanging anodes.

May I say your comment about your 240v being off therefore OK. If you are in an area with other boats they may be the rouge perpetrator of stray current and it could be as far away as 40 feet and you will get the effect, bigger the perpetrator the bigger the circle of danger.
.04v is what you would expect to see on discharge but watch out for "parasitic drain" not such a big deal of standard boats but as technology builds (like cars) it is a big issue, ask any 5 series BMW or Range Rover owner about flat batteries. (sorry that bit was extra)

Go to "safeshore" web site and read the data there, well worth the effort in gaining an understanding of the issue.