Saildrive corrosion: galvanic isolation and grounding

[VicS]

I found that link in post #16 of this thread 16 by looking for my 120S saildrive or engine, don't remember which. From that I assumed I did have one of those gearboxes, but I have no idea if this is correct. How can I find out, it has no ID plate.

I hate these f***ing c***ing ****hole links!

The gearboxes look quite different externally ( even more different internally!)

This is the one Id expect on your engine .. the upper gearbox for a 120S https://www.marinepartseurope.com/nl/volvo-penta-explosietekening-7746710-44-2888.aspx
Pretty sure thats waht is in your picture
Look at the tops, the position of the selector etc etc

 

[Plevier]

I dont think that is the correct kit. Its for MS25S an MS25SR upper gear boxes.

Click and you will see this page http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748570-44-4761.aspx It'll contain the insulating parts on that diagram. It may not even include the insulating plate #47

The parts list on the right of that page says item 47 - insulating plate - included in kit 877369
The application list for kit 877369 at http://www.marinepartseurope.com/en/volvo-penta-877369-insulating-kit.aspx lists it for MS25 and 120S the way I interpret it (although the presentation is a bit confusing).

I see your point about the photo being more like http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-2888.aspx (that's the English version!), it really is very confusing isn't it.

 

[nigelmercier]

I see your point about the photo being more like http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-2888.aspx (that's the English version!), it really is very confusing isn't it.

The upper part of my saildrive has the rubber bush #3 like this http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748570-44-4761.aspx but I'll find a full picture tomorrow.

 

[Plevier]

The upper part of my saildrive has the rubber bush #3 like this http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748570-44-4761.aspx but I'll find a full picture tomorrow.

Unfortunately http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-4202.aspx suggests that could be an alternative to what's shown on http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-2888.aspx

Do you have the big filler cap on top or just the dipstick? That seems to be a conspicuous difference

 

[nigelmercier]

Do you have the big filler cap on top or just the dipstick? That seems to be a conspicuous difference

Big filler cap...

 

[VicS]

The parts list on the right of that page says item 47 - insulating plate - included in kit 877369
The application list for kit 877369 at http://www.marinepartseurope.com/en/volvo-penta-877369-insulating-kit.aspx lists it for MS25 and 120S the way I interpret it (although the presentation is a bit confusing).

I see your point about the photo being more like http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-2888.aspx (that's the English version!), it really is very confusing isn't it.

Missed the bit about the plate but I think if it was the kit also for a 120S it would say so on the diagram as well as MS25S

Maybe the bits will fit but i cant help wonder why its not already insulated. Perhaps the reason will become apparent if one tries to insulate it

Sorry about the Dutch website..... No idea how that crept in


The pictures now posted confirm the upper gear box to be a 120S. I think it even says so on the ID plate!

 

[nigelmercier]

The pictures now posted confirm the upper gear box to be a 120S. I think it even says so on the ID plate!

It does indeed, but I thought this only referred to the saildrive itself, not the gear box?

 

[Plevier]

The pictures now posted confirm the upper gear box to be a 120S. I think it even says so on the ID plate!

However the mountings look the same (also on http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-44-2888.aspx) and I would still bet on backward compatibility of the insulation kit but that's more on feeling than solid evidence.

It's odd that on the spares website there isn't a definitive stand alone page for the 120 under "drives and transmissions" as there is for the 130. However it is grouped in with MS25 as though the same pages apply to all. Here's the top level
http://www.marinepartseurope.com/en/volvo-penta-schematics-MarineDrivesTransmissions.aspx
and the next level
http://www.marinepartseurope.com/en/volvo-penta-cat-7748570.aspx ,
the title still includes the 120 yet none of the drawings on that page shows the one with the big top filler cap!
When you look at the listings on that page, it is rather as though the 120 designation only applies to the bottom leg not the gearbox as Nigel says.

As to why it wasn't insulated, don't you think the history shows VP moving from trying to insist unsuccessfully on isolation of the whole installation to insulating the drive only, so that you can earth the engine? They would want the solution to be backward compatible rather than have to redesign everything.

Ah, just seen an argument against me. At http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746710-21-2868B.aspx the vibration damper looks like a plate with springs in - like a clutch plate - rather than rubber so maybe no insulating effect there. Bu**er!

 

[VicS]

Ah, just seen an argument against me. At http://www.marinepartseurope.com/en/...-21-2868B.aspx the vibration damper looks like a plate with springs in - like a clutch plate - rather than rubber so maybe no insulating effect there. Bu**er!

One of the things I was going to come back and say was that one would have to be certain that the coupling was one of the bonded rubber types. It seems probably not!

The other thing would be the clearance in the bolt holes. I think on other engines, Eg 2010 etc, using a 120S the insulation is between the back part if the bellhousing and the main part. Not between the gearbox and the bell housing. So the bolt holes in the mounting flange might not be large enough for an insulating sleeve/ adequate insualation. With the box in the workshop they could probably be enlarged if necessary but would it not be so easy in situ.

 

[Plevier]

So the only definite solution is to return to total isolation of the whole power unit - relays, insulated controls, isolation from shore power earth etc etc

 

[mjf107]

As I understand it Nigel's system is already isolated electrically which protects it from stray current erosion, the point of isolation/insulation between engine and saildrive is to prevent a "Galvanic cell" developing as the engine block and saildrive are both in contact with the electrolyte (sea water)

So the only definite solution is to return to total isolation of the whole power unit - relays, insulated controls, isolation from shore power earth etc etc

 

[VicS]

As I understand it Nigel's system is already isolated electrically which protects it from stray current erosion, the point of isolation/insulation between engine and saildrive is to prevent a "Galvanic cell" developing as the engine block and saildrive are both in contact with the electrolyte (sea water)

i dont understand what you mean about the engine block and sail drive both being in contact with the elctrolyte. Surely the engine is in the dry inside the boat. Only the sail drive is in contact with the "electrolyte"

I think if you go back to an earlier thread you will find that the cause of his saildrive corrosion was a short associated with the starter motor wiring which grounded the engine block

 

[mjf107]

My thinking is the cooling water in the heatexchager and maybe the the discharged cooling water in the exhaust system as the heatexchanger in contact with the block so you have a back to back connection of different metals (engine and saildrive) both in contact with an electrolyte

i dont understand what you mean about the engine block and sail drive both being in contact with the electrolyte. Surely the engine is in the dry inside the boat. Only the sail drive is in contact with the "electrolyte"

I think if you go back to an earlier thread you will find that the cause of his saildrive corrosion was a short associated with the starter motor wiring which grounded the engine block

 

[mjf107]

Missed the second part of your reply. I did see that, but assumed Nigel's boat's corrosion was ongoing and current (pun intended) Also the voltage and current he measured was with the battery disconnected, pointing to a (galvanic cell) ?

I think if you go back to an earlier thread you will find that the cause of his saildrive corrosion was a short associated with the starter motor wiring which grounded the engine block

 

[VicS]

Missed the second part of your reply. I did see that, but assumed Nigel's boat's corrosion was ongoing and current (pun intended) Also the voltage and current he measured was with the battery disconnected, pointing to a (galvanic cell) ?

My thinking is the cooling water in the heatexchager and maybe the the discharged cooling water in the exhaust system as the heatexchanger in contact with the block so you have a back to back connection of different metals (engine and saildrive) both in contact with an electrolyte

I dont think the cooling water connection would cause a problem for much the same reasons that a hull anode offers no protection to an engine cooling water circuit.

The galvanic cell is assumed to have been via what Nigel calls "ground" and the keel. It was between the engine block and "ground" that he was able to measure a 500mA current flow.

 

[mjf107]

My understanding of Nigel's "ground terminal" is an isolated terminal used as convenient loop terminal for negative cables. The measurements being made from this terminal to the saildrive So if this is correct the measurements were in fact from the negative cabling to the saildrive not from the engine block itself. Admittedly the engine/saildrive may be connected electrical if there is no insulating plate and shaft isolation. Not sure the mA reading can be relied upon, as nothing was disconnected to insert the ammeter ie by connecting the meter a path was formed for the current to flow

The galvanic cell is assumed to have been via what Nigel calls "ground" and the keel. It was between the engine block and "ground" that he was able to measure a 500mA current flow.

 

[VicS]

My understanding of Nigel's "ground terminal" is an isolated terminal used as convenient loop terminal for negative cables. The measurements being made from this terminal to the saildrive So if this is correct the measurements were in fact from the negative cabling to the saildrive not from the engine block itself. Admittedly the engine/saildrive may be connected electrical if there is no insulating plate and shaft isolation. Not sure the mA reading can be relied upon, as nothing was disconnected to insert the ammeter ie by connecting the meter a path was formed for the current to flow

He found a short on the starter motor wiring which was effectively connecting the engine to "ground"

After removing the short he measured 0·47 volts between ground and engine and also was able to measure 500mA between the two points. That 500mA would have been flowing through the short before he removed it.

500mA is about the right current to cause the damage sustained in a 2 month period

 

[mjf107]

Thanks for the reply I now understand your thinking and agree its the most likely cause of the corrosion. It would be interesting to repeat the tests with the VHF aerial disconected to see if the volts and mA fall away

He found a short on the starter motor wiring which was effectively connecting the engine to "ground"

After removing the short he measured 0·47 volts between ground and engine and also was able to measure 500mA between the two points. That 500mA would have been flowing through the short before he removed it.

500mA is about the right current to cause the damage sustained in a 2 month period

 

[nigelmercier]

He found a short on the starter motor wiring which was effectively connecting the engine to "ground"

After removing the short he measured 0·47 volts between ground and engine and also was able to measure 500mA between the two points. That 500mA would have been flowing through the short before he removed it.

500mA is about the right current to cause the damage sustained in a 2 month period

Just to clarify, the "ground" I refer to is the battery negative, after the negative isolator. However, something seems to be bypassing the latter, so I have disconnected the batteries. At present my shunt is right next to the battery, I'm beginning to think it would be better placed after the isolator.

[Later - IGNORE I have found a more recent photo.] I've just realised something, I've got a feeling that the starter motor negative goes direct to the battery, with no isolator and not via the shunt. That explains a lot. It also opens up a whole can of worms regarding which negative things are connected to.

 

[VicS]

Just to clarify, the "ground" I refer to is the battery negative, after the negative isolator. However, something seems to be bypassing the latter, so I have disconnected the batteries. At present my shunt is right next to the battery, I'm beginning to think it would be better placed after the isolator.

[Later] I've just realised something, I've got a feeling that the starter motor negative goes direct to the battery, with no isolator and not via the shunt. That explains a lot. It also opens up a whole can of worms regarding which negative things are connected to.

According to the wiring diagram in the owners manual all positive connections and all negative connections, starter included, go to the battery via the isolators.