nigelmercier
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Saildrive corrosion: galvanic isolation and grounding
- Thread starter nigelmercier
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Plevier
Well-Known Member
All the parts seem to be available at reasonable cost http://www.marinepartseurope.com/en/volvo-penta-explodedview-7748580-44-7620.aspx
The reason for isolation is apparently to avoid exactly the problem Nigel is having.
It does not appear to be too much of a job. No dismantling of the gearbox, just sliding a plate in between and bushing one bolt and washering them all. The nasty bit would be if it became necessary to undo the diaphragm but I would have expected that to have enough flexibility.
A boatyard got the SD disconnected and out of my mate's Bav 30, into the workshop and half dismantled in less than an afternoon including removing the diaphragm.
If Nigel can pin it on the the people who fitted it without isolating it, even better!
Having said all that, I admit to not having done it and knowing unforeseen complications usually arise in a job and my wording perhaps seemed a bit flippant.
Rephrase - it would be worth investigating to see if implementing this is going to be too difficult.
PS - I have got the full official workshop manual for the SD130. It starts with the SD already removed from the engine, and nowhere in the text or drawings is there any reference to insulating components within the SD, supporting the assumption that it is all done in the mountings externally to the complete SD, no need to disassemble anything.
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VicS
Well-Known Member
Those are the parts for a 130S drive. Nigel still has the upper 120S gearbox . Although similar parts can be found.
the insulating plate would have to be cut because you could not fit it over the drive shaft with everything in place. Will there be sufficient give in the mountings and the shaft coupling to allow for its thickness?
The bolt holes would have to be enlarged for the insulating sleeves .... but you would have to look carefully at the sizes etc and how they compare with the sizes in installations where the Sdrive is isolated.
Are the holes smaller than normal to take the "normal" size bolts or are the holes the "normal" size with larger bolts. If the former then presumably the holes could be enlarged to take the normal sleeves. If the latter could the holes be enlarged and specially made sleeves be fitted?
What other complications are there?
If it really is not already isolated .... why isn't it ?
Plevier
Well-Known Member
Potentially more tricky then. I haven't seen a drawing for 120 type.
I don't think the plate would be a problem (if an appropriate one is available), it's thin, yes you would have to spring it round the shaft.
As to why the 120 wasn't isolated, I don't know. The fact is that the 130 leg is supposed to be isolated to avoid corrosion, and the bit in the water is a 130, even if the top is still a 120.
It starts to make more sense now with Nigel's original question about relays doesn't it, if the whole installation can be isolated as an alternative, but he appears to be having some problems with that route too. (Hello Nigel, sorry to keep referring to you as though you are a bystander!)
Ah, seen the drawing now from Nigel's post no 16 in this thread. The part numbers of all the insulating components are the same as for the 130 (including the plate, Nigel's list of parts in his post missed that out). I wonder if the fact that they sell a packaged insulating kit for the 120 (at under 30 euros!!) suggests it is a later mod? The same kit isn't listed for the 130, presumably because it's all fitted as standard to start with.
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Playtime
Well-Known Member
As I said earlier, I suggest that Nigel gets the installation checked over by a registered (certified?) Volvo engineer. This should be a free inspection with a view to them maybe doing some work. Depending on their conclusions you can then politely decline.
I don't think anyone on this thread has all the knowledge on this subject. However, I am still firmly in the 'it should be isolated' camp!
VicS
Well-Known Member
The thing is the 120S is isolated when fitted to other engines!
The fact that the replacement leg is a 130 or similar to a 130 should not matter. Better actually in so much that it has a larger anode that is also split so that it can be renewed without taking the prop off
There's no doubt in my mind that if his Sdrive had been isolated it would not have suffered the corrosion it did even though the culprit was probably a wire shorting to the engine block coupled with an apparent connection between what he calls "ground" and the keel.
The earthing relay is something we all know about ... Some engines did not even have an isolated starter motor so the earthing relay was in fact a heavy duty solenoid that carried the starter current as well as the other things.
Plevier
Well-Known Member
VicS
Well-Known Member
You appeared to be arguing against isolating it!
Confused a bit as well. All the arguments are there in favour of any SD being isolated but as far as I can see its a belt and braces situation isolating the SD and isolating the engine block.
As far as one can tell when fitted to the MD22 only a belt was used , no braces. Reasons not known. Unfortunately for Nigel the belt broke and his trousers fell down.
May yet be proven to be wrong of course.
nigelmercier
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Plevier
Well-Known Member
My guess would be that they found the idea of isolating the whole installation unsustainable with the increasing tendency to bond every bit of metal on the boat to the shore power earth, as well as accidental breaches by control cables, removed relays, chafing wires etc., and had to come up with another way.
VicS
Well-Known Member
Control cables ... that's an interesting thought The throttle control would have to be insulated from the gear shift control.
nigelmercier
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Unfortunately my mate reconnected the gear shift cable before I got a chance to test it.
I've now heard back from Volvo Penta, and to say I'm even more confused is an understatement! They sent the document linked below, which is not even for my engine, but how does an isolating bush on only one of the lower two bolts help?
Volvo Penta document here: http://www.dolcetto.org.uk/temp/7742977.pdf
Plevier
Well-Known Member
I can't quote reference for this but I understand all the holes have clearance, the unit is centred by the drive shaft into the coupling (have to make sure not hanging on it when bolting up), therefore if a bush ensures that one of the bolts is not contacting anything, you can be sure none of the others is either! All have insulating washers under the heads and there is the full insulating "gasket" in the join face. Have another look at the photo I posted earlier showing the SD half withdrawn from an engine.
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VicS
Well-Known Member
Interesting to see how the insulation differs on different engines/ gearboxes. Cant explain why only one bolt of several in the last example requires an insulating bush but it would probably be obvious if you were actually fitting it. I dont like Troubadours explanation that you rely on the drive shaft centering it and just one bush but he is probably right
What you need is details of the insulation for your engine and gearbox ...........but if they are not insulated there wont be any details!
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Plevier
Well-Known Member
Is it worth asking VP the specific question, is the 120S isolation kit - you've got the part number of it - retrofittable without mods to a 120S that wasn't originally isolated?
I'd bet quite a bit - well at least a pint! - that it is.
I'd bet quite a bit - well at least a pint! - that it is.
Plevier
Well-Known Member
Well you couldn't use the bolts for accurate alignment could you? They must have clearance. Ideally it would be dowelled but it clearly isn't.
mjf107
Well-Known Member
Hi Nigel
I think most posts on this subject have an element of confusion!
Get back to Volvo asking them to advise on your particular engine/saildrive combination as they have sent you information on wrong engine Tell them about this thread and the interest its creating. I'm sure definitive and concise answer to the whole engine/saildrive isolation question would be welcomed by the forum
All the best Michael
I think most posts on this subject have an element of confusion!
Get back to Volvo asking them to advise on your particular engine/saildrive combination as they have sent you information on wrong engine Tell them about this thread and the interest its creating. I'm sure definitive and concise answer to the whole engine/saildrive isolation question would be welcomed by the forum
All the best Michael
nigelmercier
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The problem is that they no longer do technical advice, they try to refer you to a dealer. The dealers are only interested in helping if they are going to make a sale.
The kit is under £30, but I need to know if I can retro-fit it. http://www.marinepartseurope.com/en/volvo-penta-877369-insulating-kit.aspx
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VicS
Well-Known Member
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
nigelmercier
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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.
PS I hate these f***ing c***ing ****hole links!
PPS I naturally wrote the above words in full, the middle one didn't get censored
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