Prop and shaft corrosion

[sailbleu]

Methinks you have little understanding of the process of electrolysis and the role of anodes.

A P bracket should not be connected to your engine. An engine does not suffer corrosion from any source outside the boat, but only from seawater that is internal to the engine, and then only if there are dissimilar metals in the seawater circuit.

For electrolysis to occur the dissimilar metals must be in direct contact and immersed in an electrolyte (such as seawater). So there may be electrolysis between the yellow metal of the prop and the stainless shaft, and an anode which is zinc and has a lower potential will be eroded before the zinc in the prop. The anode can be either on the shaft as in this case or it can be a hull anode that is bonded to the shaft, often through the gearbox housing. This is connected to the engine, but it does nothing to prevent internal corrosion in the engine - it is simply a convenient electrical path to the prop shaft.

The only way a P bracket can suffer corrosion is if it is an alloy of dissimilar metals, but rarely is this the case as Vyv explained. A P bracket is not electrically connected to the shaft, being insulated by the non metallic cutless bearing, so an anode there will have no effect on corrosion of the shaft or prop. So, rather than being a first line of defence it is a waste of time.

Afraid your description of what "stray currents" are and what causes them is also wrong, but as it is irrelevant to the OPs possible problem I shall leave you to work out why.

Methinks , you're not aware that ALL Jeanneaus are equipted with a lead/connecting from the engine to the strut.

Regards

 

[sailbleu]

Let me tell you a brief story of a fellow boater I know.
His engine was also insulated from the neg. , meaning starter and alternator were/are ( like mine) designed so no neg could/would reach the engine block.
He bought this boat second hand and was not aware the previous owner had replaced the alternator with a custom made ( and cheap) alternator that for one reason or the other alowed the neg to touch the block.
That resulted in a complete corrosion of the heat exchanger , most tubes leaking .
After analysing the problem and the consequenses they determined the cause , an electrical leak to the engine block that was not deflected to the strut

Please be my guest and ask around Jeanneau owners if their engine has a connection to the P-bracket.

Regards

 

[30boat]

Let me tell you a brief story of a fellow boater I know.
His engine was also insulated from the neg. , meaning starter and alternator were/are ( like mine) designed so no neg could/would reach the engine block.
He bought this boat second hand and was not aware the previous owner had replaced the alternator with a custom made ( and cheap) alternator that for one reason or the other alowed the neg to touch the block.
That resulted in a complete corrosion of the heat exchanger , most tubes leaking .
After analysing the problem and the consequenses they determined the cause , an electrical leak to the engine block that was not deflected to the strut

Please be my guest and ask around Jeanneau owners if their engine has a connection to the P-bracket.

Regards

All the engine installations I've seen have the negative on the engine.Not having an anode on the heat exchanger will lead to the damage you describe.

 

[vyv_cox]

All the engine installations I've seen have the negative on the engine.Not having an anode on the heat exchanger will lead to the damage you describe.

Yes, I agree. Plus, Jeanneau have a long history of poor and low cost materials selection for shafts, structural items, other parts. Several examples on my website.

 

[VicS]

Let me tell you a brief story of a fellow boater I know.
His engine was also insulated from the neg. , meaning starter and alternator were/are ( like mine) designed so no neg could/would reach the engine block.
He bought this boat second hand and was not aware the previous owner had replaced the alternator with a custom made ( and cheap) alternator that for one reason or the other alowed the neg to touch the block.
That resulted in a complete corrosion of the heat exchanger , most tubes leaking .
After analysing the problem and the consequenses they determined the cause , an electrical leak to the engine block that was not deflected to the strut

Please be my guest and ask around Jeanneau owners if their engine has a connection to the P-bracket.

Regards

Knowing what engine would be relevant and might help to understand this more fully especially as most small yacht engines AFAIK do not have a fully insulated DC system. Some do I acknowledge.

I am inclined to think that the connection between alternator and engine block is in this case a red herring.

 

[Tranona]

Methinks , you're not aware that ALL Jeanneaus are equipted with a lead/connecting from the engine to the strut.

Regards

Maybe they do, but it will not do you any good for protecting the prop unless the engine is connected to the shaft. In this case it will act like a hull anode and they have only put the anode on the P bracket for convenience, or maybe they feel because the P bracket is low grade material it also needs an anode. as already explained the normal way of protecting a shaft and prop is with either a hull mounted anode bonded to the shaft through the gearbox housing and coupling, or a shaft anode. In addition many folding a feathering propellers have their own anodes because the mechanism uses dissimilar metals, usually stainless pins in a yellow metal propeller.

Have a look around other boats with P brackets and you will find the majority do not have anodes on them for the reason Vyv gave earlier. As others have said it is nothing to do with preventing corrosion in the engine.

Important to understand the principles of anodic protection. You can get a good explanation on the MG Duff website.

 

[VicS]

Important to understand the principles of anodic protection. You can get a good explanation on the MG Duff website.

Cathodic protection but Google will find you some good reading on anodic protection if you really cannot sleep one night

 

[sailbleu]

No offence intended , but i find it strange that most of you haven't heard of completely insulated engines.
There's quite a few discussions going on other board about corrosion , pitting shafts , props etc.
Up to now mine are spotless.
This is my setup.

I agree , could of gotten away with a smaller zinc instead of this ' lump ' , and would if I could redo it .
But the holes are there to stay I guess.
Maybe a hint/suggestion to others

The pic is not that sharp , the focus was on the joint P-bracket >>>hull .
But if you take a good look you can accually notice this zinc is pulling his weight.
Meaning, there is something going on that most likely in the case of absence of a zinc would put its teeth elsewhere .

And even if my theory on a P-bracket >><< zinc was proven wrong , well ,...no harm no foul .


Regards

 

[vyv_cox]

No offence intended , but i find it strange that most of you haven't heard of completely insulated engines.
There's quite a few discussions going on other board about corrosion , pitting shafts , props etc.

Discussions about corrosion are very common on most forums. Unfortunately it is a complex subject that very many posters do not understand, so a great deal of what is written tends to be incorrect.

In your case it would seem that the shaft anode is protecting the prop and shaft, which it is designed to do. I cannot see that your P-bracket anode is doing much at all, other than protecting the steel bracket that attaches it. The fact that it is corroding doesn't mean a thing - if it was not attached to anything it would do the same.

 

[Tranona]

Cathodic protection but Google will find you some good reading on anodic protection if you really cannot sleep one night

Late night mental block!

 

[Tranona]

No offence intended , but i find it strange that most of you haven't heard of completely insulated engines.
There's quite a few discussions going on other board about corrosion , pitting shafts , props etc.
Up to now mine are spotless.
This is my setup.

I agree , could of gotten away with a smaller zinc instead of this ' lump ' , and would if I could redo it .
But the holes are there to stay I guess.
Maybe a hint/suggestion to others

The pic is not that sharp , the focus was on the joint P-bracket >>>hull .
But if you take a good look you can accually notice this zinc is pulling his weight.
Meaning, there is something going on that most likely in the case of absence of a zinc would put its teeth elsewhere .

And even if my theory on a P-bracket >><< zinc was proven wrong , well ,...no harm no foul .


Regards

You are not having problems with your prop because you have a shaft anode. Even if the anode on the P bracket is connected to the shaft it would not erode as the shaft anode would erode first as it is closer to the prop. That anode will do nothing to protect the engine, but if it is a freshwater cooled Yanmar then it may not have, or need an anode.

Just because a builder fits something does not mean that it is right. There is a lot of misunderstanding, and to an extent some disagreement, about this subject even among the "experts" and there has been a tendency in the past to just attach anodes or bond fittings without fully understanding why.