Saildrive corrosion: galvanic isolation and grounding

nigelmercier

nigelmercier

RIP
Joined
20 Jun 2007
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Live in Kent, boat in Canary Islands
www.bavariayacht.info
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 ([EDIT] may not be) isolated from the engine, there are relays that make the 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 discovered that with all the batteries disconnected, there is a voltage of 0.47V ([Edit] not 0.45V) between the engine and ground, I briefly tested the 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 if there is a connection between the keel and the ground (via the mast and the lightning strap), and this voltage is as expected from having dissimilar metals in salt water.

Any comments on this?
 
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nigelmercier said:
I discovered that with all the batteries disconnected, there is a voltage of 0.45V between the engine and ground, I briefly tested the 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 if there is a connection between the keel and the ground (via the mast and the lightning strap), and this voltage is as expected from having dissimilar metals in salt water.

Any comments on this?
Click to expand...

Hi Nigel

When you say 0.45V between engine and ground, exactly what does ground mean if the batteries are disconnected.

Second question, is this voltage with the starter still shorted to the engine or with the short removed (i.e back to the 'as designed' position)?
 
Also why on earth are there relays that connect the saildrive to the engine? The sail drive shouldn't be doing anything electrically apart from its own interaction with seawater.

Pete
 
Playtime said:
When you say 0.45V between engine and ground, exactly what does ground mean if the batteries are disconnected. Second question, is this voltage with the starter still shorted to the engine or with the short removed (i.e back to the 'as designed' position)?
Click to expand...
Ground in this case is probably the keel, as above. Voltage measured with short removed, otherwise they would be at the same potential.

prv said:
Also why on earth are there relays that connect the saildrive to the engine? The sail drive shouldn't be doing anything electrically apart from its own interaction with seawater.
Click to expand...

Sorry, didn't explain that very well: the relays are to ground the engine for starting and stopping, so glow plugs and stop solenoid can work.
 
As far as I know there is no direct connection between the keel and anything electrical on a Bavaria. There is normally a connection (copper pipe on my old 38 Ocean) between the mast and the keel but there should be nothing electrical grounding (or otherwise) the mast. It might be worth checking if there is any stray voltage between battery ground and the mast/keel connection (radar, nav lights, antenna etc.), particularly with stuff turned on/transmitting.

Another question - is it a cast iron keel or lead?
 
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The relays should only affect the engine not the drive as it is normally isolated from the engine
Are you getting continuity to the drive from the engine ?
Is the control cable making a circuit ?
 
The Saildrive should be completely isolated from the engine; the rubber bush on the drive shaft plus the insulating plastic 'gasket' and nylon inserts for the bolts should ensure this is so; if it is not isolated then it is little wonder that the anode is being used up quickly. The Saildrive anode on my Bav 38 lasts for at least 2 years, even when hooked up to the marina supply throuhgout the winter months. I'd check if there is a circuit between the Saildrive and the engine; if there is then the installation of the new Saildrive was not done correctly and I'd get the engineer back in to sort it out.

Equally, the only link of an electrical type to the keel should be a large diameter copper cable between the base of the mast and the keel; it's there to mitigate the effects of any lightening strike. Any other connections are a bit suspect. If you're getting any indications of an open circuit involving the keel and your electrics it will be down to a short circuit somewhere and you need to find the problem and cure it.
 
duncan99210 said:
The Saildrive should be completely isolated from the engine...
Click to expand...

I think this was a later modification, on mine all the bolts have steel washers, and relays to make the ground contact when needed.

As I said, there is a lightning strap from the mast to the keel. If the mast is grounded via a light fitting or the VHF antenna, it would make a full connection.
 
nigelmercier said:
I think this was a later modification, on mine all the bolts have steel washers, and relays to make the ground contact when needed.

As I said, there is a lightning strap from the mast to the keel. If the mast is grounded via a light fitting or the VHF antenna, it would make a full connection.
Click to expand...

Nigel

As said before there should be no connection between battery ground and the saildrive, even when the engine is on. The saildrive should be totally insulated from the engine. The relay is to connect just the engine to battery ground, for the glowplugs and stop solenoid when the key switch is operated.

I suggest you get a Volvo engineer to check your saildrive installation as this could be the cause of the corrosion (unless it was only modified when the new saildrive was installed, of course). Maybe you should quiz the engineer that installed the new saildrive as he should have known better!

Also I suggest checking for any connections with the mast as this could be the other half of the electrolytic cell.
 
Playtime said:
The saildrive should be totally insulated from the engine. The relay is to connect just the engine to battery ground, for the glowplugs and stop solenoid when the key switch is operated.

I suggest you get a Volvo engineer to check your saildrive installation as this could be the cause of the corrosion (unless it was only modified when the new saildrive was installed, of course). Maybe you should quiz the engineer that installed the new saildrive as he should have known better!

Also I suggest checking for any connections with the mast as this could be the other half of the electrolytic cell.
Click to expand...

I think any saildrive isolation occurs further back in the drive chain, where the flywheel bell-housing meets the gearbox (see here). This is as fitted from new with no isolation washers.

I fitted the new saildrive myself, but the gearbox was not removed.

Thinking about the mast, I have come to the conclusion that it is inevitable that it will be grounded as the VHF antenna is connected to it.
 
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I'm going to repeat myself here. The Saildrive has to be electrically isolated from the the engine and the rest of the boat. If it is not, then the immersed Saildrive leg will corrode very quickly. If you look at the exploded diagram at http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748580-44-7620.aspx parts 49 - 51 and 54 - 55 are the insulating components that you should see fitted between the engine and gearbox. I understand that you've had to replace a Saildrive lower leg; if you've got steel washers retaining the gearbox, then at somepoint someone has fitted them in error. You need to replace them with the correct components or you'll be replacing the lower Saildrive leg again.
 
nigelmercier said:
Any saildrive isolation occurs further back in the drive chain, where the flywheel bell-housing meets the gearbox (see here). This is as fitted from new with no isolation washers.

I fitted the new saildrive myself, but the gearbox was not removed.

Thinking about the mast, I have come to the conclusion that it is inevitable that it will be grounded as the VHF antenna is connected to it.
Click to expand...

I'm also going to repeat the message for the third time. You seem to be satisfied that non-isolation of the saildrive from the engine is OK as it was originally fitted this way. As others have said I do not believe that is correct.

Please check with Volvo.

Regarding the mast grounding through the VHF antenna, it depends what bracket is used. Some are plastic and provide isolation. Again, I would check it out if you can (might be difficult to be definitive, though, with the boat in the water).
 
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duncan99210 said:
I'm going to repeat myself here. The Saildrive has to be electrically isolated from the the engine and the rest of the boat. If it is not, then the immersed Saildrive leg will corrode very quickly. If you look at the exploded diagram at http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748580-44-7620.aspx parts 49 - 51 and 54 - 55 are the insulating components that you should see fitted between the engine and gearbox...
Click to expand...

I've been looking at some pictures of my gearbox casing, see below:

bellhousing_zps0164938d.png


Left to right there are three columns of screws, the ones in the grey gearbox casing, the ones on the left of the bell housing overlapping the first column, and the ones between the bell housing and the engine. Which of these should have the insulation? From the exploded diagram it looks like it is the interface between the grey and the green, but this does not tally with the info given in this post: http://www.ybw.com/forums/showthrea...Saildrive-isolation-how&p=3961352#post3961352

Thanks for the link to the diagram, but I had a 120S gearbox, so looking at a slightly different diagram here: http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748570-44-4761.aspx it seems that the following parts form an insulation kit 877369:
  • 49 Insulating bushing 3583361
  • 50 Insulating washer 3583359
  • 51 Insulating washer 3583357
  • 54 Insulating washer 3583358
  • 55 Insulating washer 3583357

However, I can see no mention of insulation on the driveshaft.
 
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The input shaft for the gearbox goes through a rubber bushed damper plate on the flywheel. The bushing provides the isolation. At the other end the standard prop also has a rubber bush so is isolated from the drive. The casing is therefore isolated and protected by the anode bolted to the housing.
 
nigelmercier said:
My saildrive is not isolated from the engine, there are relays that make the 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.
Click to expand...

Hello Nigel, perhaps your current reading is because together with the zinc anodes in the water your engine behaves like a low power battery, hence the corrosion that you experience. You have to break the circuit, leaving in the water the smallest possible mass of metal, and avoid that this mass (the saildrive + zinc anode) have any contact with the battery ground.

I think that the lack of isolation between the saildrive and engine is as the basis of the problem that you experience. Every Volvo manual warns that it is paramount that the saildrive is electrically isolated from the engine.
 
Tranona said:
The input shaft for the gearbox goes through a rubber bushed damper plate on the flywheel. The bushing provides the isolation. At the other end the standard prop also has a rubber bush so is isolated from the drive. The casing is therefore isolated and protected by the anode bolted to the housing.
Click to expand...

OK, that makes sense, thank you. I confess that I didn't spend much time testing if the saildrive was indeed isolated: I had read on this forum that only some of them were, and the screws didn't look like they had insulators on them. I thought that my system relied on the entire engine being isolated.

So in my picture above, which of the screws should have the insulation? From the exploded diagram it looks like it is the interface between the grey and the green, but this does not tally with the info given in this post: http://www.ybw.com/forums/showthrea...Saildrive-isolation-how&p=3961352#post3961352
 
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