Who's eaten the prop blades?

[jeremyshaw]

Sold the boat last year after ten years. Original props still good if a little pitted - passed by the surveyor.

The new buyer got on the boat for the first time this year and found no drive on one engine (Yanmar SD saildrives). Then discovered the prop had "fallen off".
Hauled and found the blades had fallen off one prop and the other prop looks pretty sick.
But the anodes look good.

They have kept it on our berth where it has been for a few years.

I assumed this was caused by stray electrolysis. We never kept the boat plugged in but I am not sure if they did -or another boat nearby could have caused an issue?

But how come the anodes look barely worn? Or are some kinds of electrolysis not mitigated by anodes?

Might the yard have put the wrong paint on the props? Hard to see it would cause so much damage though...

As you'll have realised my electrolysis/galvanic knowledge is sketchy. I a puzzled like this and would like to give the new owner helpful advice.

Many thanks

 

[john_morris_uk]

Here's a couple of possibilities:

The boat might not have a galvanic isolator and they might have left it plugged in and there were stray currents that overcame the protection of the anodes or they might have accidentally or unwittingly disconnected the wire from the anode to the shaft/engine or broken the electrical connection somewhere? (The anodes only world if they are close to what they are there to protect and connected electrically with a low resistance connection.)

 

[lustyd]

But how come the anodes look barely worn?

It would be perfectly possible to just glue anodes to your hull and have them looking shiny and new for years to come. Were they connected to anything?

 

[vyv_cox]

There has been a similar case on this forum in the past few weeks and posted to only a couple of days ago. A scroll down may well find it. In that case the saildrive had corroded but various non-aluminium parts were OK. The conclusion was that cheap, substandard anodes had been fitted rather than big-name ones.

I cannot be certain that this was the cause as I do not know the material of your props, so more information is needed. Some props are aluminium, some are isolated from the shaft and saildrive, some have their own anode, some do not.

In the vast majority of cases 'stray currents' are a myth. Electricity follows strict guidelines and electrons do not buzz around in the water looking for something to corrode.

 

[VicS]

Some points/questions to raise.

Is the Yanamr saildrive isolated from the engine ( like many VP units) ?

Is the prop isolated from the shaft? I believe it has a rubber bush but does that effectively isolate it?

If the prop is insulated from the shaft and/or the SD insulated from the engine. hull anodes have no relevance, likewise a shorepower connection, with or without a galvanic isolator, and other vessels in the vicinity using shore power.

I take it the OP's reference to anodes is to prop anodes ????? Yes? no ??

If so another mystery. Perhaps substandard anodes as Vyv concluded was the case he refers to above. Perhaps not fitted correctly.

 

[cliff]

Sold the boat last year after ten years. Original props still good if a little pitted - passed by the surveyor.
<snip> would like to give the new owner helpful advice.

Why? Serious question. New owner bought the boat having had a survey done and the props passed. Not your problem anymore - walk away and forget about it - it is the new owner's problem and nothing to do with you.

 

[savageseadog]

I've had exactly the same thing happen on a Yanmar GM/SD20 saildrive setup some years ago. I lost two folding propellers to corrosion in the space of two or three years each. The propellers were Gori without dedicated anodes, there was light corrosion on the hub and blades but the gear teeth corroded badly enough to cause misfolding. The anodes were the original Yanmar branded zinc ones which were always replaced regularly and appeared to be doing their job. A Yanmar aluminium fixed blade prop used earlier on the same boat in the same situation corroded away very badly. The Yanmar GM/SD20 is not electrically isolated, we never had shore power, the Gori propeller hub is in two halves with a rubber bushing so most of the propeller is isolated electrically. I've had similar setups on a different boat in the same marina (not in the same berthing position) with Radice and Flexofold propellers and the later aluminium anodes without any corrosion. The Radice prop was originally isolated but wasn't when I had it due to some earlier mods. The Flexofold prop (new) is rubber bushed but when I checked it before fitting I found it was not electrically isolated. All very mysterious. I came to the conclusion it was stray currents because I'm reasonably sure that we weren't getting the problem when we stayed on a mooring elsewhere. Stray currents do exist in my opinion. If there is a potential difference in the water current will flow, if you put something between the two points of potential difference then current will flow round it or through it, or both if there are mixed materials.

 

[jeremyshaw]

You are right it is nothing to do with me, but a) I'm curious b) don't want the new owner to have a bad experience.
Thanks

 

[jeremyshaw]

Many thanks for all the replies. The anodes are Yanmar brand and fit between the leg and the prop so no probs with quality or contact.
Though like others a stray current sceptic I do wonder if this is the potential problem.

 

[KellysEye]

It sounds like stray current corrosion which has two causes. A bad earth in the marina 240v supply or when fresh water mixes with salt water it produces an electrical current. Both can be overcome with a galvanic isolator.

 

[VicS]

Many thanks for all the replies. The anodes are Yanmar brand and fit between the leg and the prop so no probs with quality or contact.
Though like others a stray current sceptic I do wonder if this is the potential problem.

The anodes are fitted to the sail drive leg and are there to protect the leg. They offer no protection to the prop which should be electrically isolated. I dont think you have said though what type of prop it is.

 

[jeremyshaw]

Thanks for explaining that about the props not being protected by the anode which I had not appreciated. So maybe they lost their electrical isolation in some way. Would that be the purpose of a loose fitting washer that goes behind the prop?
They are bronze props - like the legs.

 

[VicS]

It sounds like stray current corrosion which has two causes. A bad earth in the marina 240v supply or when fresh water mixes with salt water it produces an electrical current. Both can be overcome with a galvanic isolator.

Can you explain these stray current phenomena in greater depth. Detailed scientific explanations that will convince me and Vyv please


Just how is it caused by a "bad earth" in the marina and how does it flow to produce this type of corrosion in a prop which should be electrically isolated from the drive.

Please also explain how a current is produced when fresh water mixes with salt water..
If I take a beaker of fresh water and a beaker of salt water in the laboratory and mix them where will the current produced flow and how can I observe it, measure it or even detect it.
Again explain how this will cause corrosion of an isolated component

 

[savageseadog]

Current will flow wherever there's a potential (voltage) difference between two locations in or on a conductive material. The potential difference can be created by power sources, either AC or DC or in the case of boats by electrochemical potential created by different metals. To put one common misconception to rest, electrolytic corrosion can be caused by AC currents. The situation with mains power is that the live 240V is referenced to ground. The ground that comes with the power should be exactly the same as real ground, ie the sea but it usually isn't. AC Currents can be created by mains earth to real earth situations or leakage from the live potential to earth via faulty equipment, mains filters etc. DC potentials can be created too but I think it unlikely that these potentials would be far ranging, a few mm at most, very localised. I stand to be corrected. If there there are significant currents running through the water through DC or AC currents then there will be potential differences, if a boat is sat in a field of potential then current will flow past the boat.
I have thought about doing some experiments.

 

[VicS]

Thanks for explaining that about the props not being protected by the anode which I had not appreciated. So maybe they lost their electrical isolation in some way. Would that be the purpose of a loose fitting washer that goes behind the prop?
They are bronze props - like the legs.

I dont know why there is a loose fitting washer behind the prop. It surprises me that there would be any loose fitting component.

Loss of electrical insulation would more likely lead to corrosion of the leg or at least the leg anodes.

The props are just a generic bronze props then. I had assumed they were folding props of some particular brand.

id not realised that the saildrive legs were bronze. That does surprise me. Also surprising that the anodes would last any significant length of time.

Now totally bewildered.

 

[VicS]

Current will flow wherever there's a potential (voltage) difference between two locations in or on a conductive material. The potential difference can be created by power sources, either AC or DC or in the case of boats by electrochemical potential created by different metals. To put one common misconception to rest, electrolytic corrosion can be caused by AC currents. The situation with mains power is that the live 240V is referenced to ground. The ground that comes with the power should be exactly the same as real ground, ie the sea but it usually isn't. AC Currents can be created by mains earth to real earth situations or leakage from the live potential to earth via faulty equipment, mains filters etc. DC potentials can be created too but I think it unlikely that these potentials would be far ranging, a few mm at most, very localised. I stand to be corrected. If there there are significant currents running through the water through DC or AC currents then there will be potential differences, if a boat is sat in a field of potential then current will flow past the boat.
I have thought about doing some experiments.

So if there was a fault to earth on another boat there could be a small current flow through the earth conductor. If there is a bad connection in the earth conductor system the current flow would lead to a small potential difference between the earth conductors and the true earth and the water.

One needs to explain why the RCDs dont trip ? But I am sure that can be done.

The small PD arising between the earth conductor and the water is then the cause of current leading to corrosion of under water items connected to the shorepower earth. I guess a galvanic isolator will not block these currents because they are basically driven by the 240 volt main supply.

Current flowing through the water could conceivably create a PD between one end of a boat and the other. If there are suitable underwater fittings connected to each other current will then take the "easy path" from one end to the other through the internal wiring leading to corrosion of the fittings.

Food for thought ... much thought.

 

[vyv_cox]

I think the bronze information is incorrect. The operator's manual for one of the Yanmar SD saildrives never actually states the material but there are plenty of clues that both standard prop and the saildrive itself are aluminium, e.g. No copper antifouling, warnings about anode materials, etc. http://yanmar.com/product/marineple...perationmanualgears/SD60_OPM_0ASDM-G00200.pdf

 

[savageseadog]

I think the bronze information is incorrect. The operator's manual for one of the Yanmar SD saildrives never actually states the material but there are plenty of clues that both standard prop and the saildrive itself are aluminium, e.g. No copper antifouling, warnings about anode materials, etc. http://yanmar.com/product/marineple...perationmanualgears/SD60_OPM_0ASDM-G00200.pdf

The saildrive is aluminium and the standard prop is as well. All the folders are bronze, the Gori is definitely aluminium bronze, I don't know about the others.

 

[savageseadog]

Current flowing through the water could conceivably create a PD between one end of a boat and the other. If there are suitable underwater fittings connected to each other current will then take the "easy path" from one end to the other through the internal wiring leading to corrosion of the fittings.
.

The PD causes the current, not the other way round. I'll look at the rest after F1 and le Tour.

 

[VicS]

The PD causes the current, not the other way round. I'll look at the rest after F1 and le Tour.

A potential difference is created by a current flowing through a resistance. .

An electromotive force ( emf) drives the current.


Take a series of resistances connected to a battery. The current that flows is a function of the EMF of the battery and the total resistance of the circuit, including the internal resistance of the battery.

The pd you can measure across any one of the resistances is a function of the current and and the magnitude of the resistance.