Who's eaten the prop blades?

[savageseadog]

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.

"A" potential difference, which will be equal to emf if you measure across it, ie a voltage. The reason I used potential difference as a term is because we are talking about completed circuits and an electrolysis situation is more complex than simple batteries inevitably there will be an emf, a non linear resistance and a current in most circumstances.

Anyway, if there was a

 

[KellysEye]

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

I 'm surprised you both don't know about stray current corrosion, it's been known for a long time, including a report on TV showing how mixing fresh/salt water to light a bulb as a demonstration of the current. Also, stray current is well known in marinas with a bad earth and dissimilar metals in proximity.

Stray current corrosion in marinas: https://marine.cat.com/cda/files/949222/7/Corrosion.pdf

Current created by mixing salt and fresh water: http://www.physicscentral.com/buzz/blog/index.cfm?postid=8192106608311312838

Lagos, Portugal is known for boats left in the marina losing their props because of the current caused by the river's fresh water mixing with the incoming sea salt water, if the anodes are covered in calcium the corrosion damage happens very fast .

 

[Heckler]

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

Hohum! the man has shown you, so perhaps a less jaundiced eye!
S

 

[superheat6k]

No comment, just an observation that the prop blade material has gone somewhere ...

https://www.dropbox.com/s/3vk4af0x1v6fl6r/Corrosion - Stray Current.pdf

 

[septiclecky]

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.

May not be jeremyshaw problem but if I had owned it for 10 years with no problem I would like to know what has happened for future reference.

 

[VicS]

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

I 'm surprised you both don't know about stray current corrosion, it's been known for a long time, including a report on TV showing how mixing fresh/salt water to light a bulb as a demonstration of the current. Also, stray current is well known in marinas with a bad earth and dissimilar metals in proximity.

Stray current corrosion in marinas: https://marine.cat.com/cda/files/949222/7/Corrosion.pdf

Current created by mixing salt and fresh water: http://www.physicscentral.com/buzz/blog/index.cfm?postid=8192106608311312838

Lagos, Portugal is known for boats left in the marina losing their props because of the current caused by the river's fresh water mixing with the incoming sea salt water, if the anodes are covered in calcium the corrosion damage happens very fast .

This message is hidden because skipper_stu is on your ignore list.

No comment, just an observation that the prop blade material has gone somewhere ...

Some interesting reading in the second link about producing electricity from salty water. Whether the technique can lead to a viable method of producing electricity on a commercial scale I know not.
I am afraid I can see no similarities between this and whatever has lead to the corrosion of the props that are the subject of the thread. An interesting read but totally irrelevant as far as I can see.

The possibility of "stray" currents is a little more likely as the cause although, as Vyv says, electrons are not just wandering about looking for something to corrode like stray tom cats looking for females to mate with. There has to be a source and a cause for any current flow.

Without knowing the size of the props or the quantity of metal lost it is difficult to estimate accurately the magnitude of the current that has caused the damage described in a year.
My very rough estimate ... back of envelope calculation .... is that it will require a current of about 60mA. Perhaps the metal lost is less than the 200g I used in my calculation. If so it might explain why, if this was due to a small shorepower defect on board, the RCD did not trip.

This is considering the possibility that the damage has been caused by a small earth leakage on board the vessel affected.

If we look at the "tom cat" possibility of a mythical stray current flowing from one unknown point in the marina to another the total current required to produce a large enough local current density to cause several 10s of milliamps flow through the propeller is not likely to go unnoticed or to not trip an RCD somewhere.

For this reason i'd be looking for the possibility of a fault, or at least the source of the damaging current on the vessel concerned.


It still does not explain why the props have been corroded in isolation.
Having looked at the manual Vyv provided a link to I conclude that if the props are aluminium they are not isolated and should be protected by the ring anodes on the legs. If they are bronze props then they should be isolated.

 

[vyv_cox]

The fresh and salt water mixing paper is interesting but I have yet to see a suggestion that it could lead to corrosion in anything. There must be millions of steel piles, jetties, ground chains, etc in areas where rivers flow into the sea but I have never seen it suggested that they corrode faster than those in seawater only.

The stray current corrosion piece has been taken from a fishing boat reference and reading it shows it is aimed at steel hulled boats. The advice is frequently aired here for these boats, e.g. Not using the hull as a negative return and suchlike. Nothing at all to do with plastic boats berthed in marinas.

 

[GEB]

There is a source of stray current that should not be overlooked. In some circumstances it is possible for current from the supply network to flow into an installation via the earthing system. This current can be considerable (amperes rather than mill-amperes), and it may persist.

The conditions that can create this situation most often occurs when the supply network uses protective-multiple-earthing (PME) – this is an example of a TN-C-S system, common in Northern Europe and North America.

In simple terms such a system uses the supply neutral as the main return path for earth fault currents. A side effect of this is that the 'earth path' to the supply transformer is in parallel with the supply neutral and some neutral current may divert to earth. This can be due to the supply network not having enough effective connections between neutral and earth. A distribution main will generally have a number of earth electrodes connected along its length. Buildings along the run have extraneous-conductive-parts (metal structures, water and gas pipes, etc) bonded to earth and therefore also to the supply neutral. These can (not by design) act as electrodes on the supply neutral, and may carry diverted neutral currents.

These currents will not be detected by RCDs on the system as they look for line / neutral current imbalance – not earthing system currents.

A Supply Company is not permitted to use a PME system for parts of an installation that connect to metalwork in caravans or boats (ESCQR 2002 - Regulation 9.4). This means that if a marina has a PME supply (and many do) they must ensure that the pontoons, etc are supplied via a T.T system (a system that uses Terra firma as its main return path for earth fault currents). A T.T system can be created within an installation but it is not always straight forward.

My boat is moored in Southampton in a small marina. I inspected their installation (they commissioned my company) and during this it came to light that they where connected directly to a PME supply. This seems to have occurred when a new supply was installed. I resolved this for them by converting the supply to the pontoons to a T.T system.

I would want to eliminate this as a cause if I were investigating this case.

 

[vyv_cox]

There is a source of stray current that should not be overlooked. In some circumstances it is possible for current from the supply network to flow into an installation via the earthing system. This current can be considerable (amperes rather than mill-amperes), and it may persist.

The conditions that can create this situation most often occurs when the supply network uses protective-multiple-earthing (PME) – this is an example of a TN-C-S system, common in Northern Europe and North America.

In simple terms such a system uses the supply neutral as the main return path for earth fault currents. A side effect of this is that the 'earth path' to the supply transformer is in parallel with the supply neutral and some neutral current may divert to earth. This can be due to the supply network not having enough effective connections between neutral and earth. A distribution main will generally have a number of earth electrodes connected along its length. Buildings along the run have extraneous-conductive-parts (metal structures, water and gas pipes, etc) bonded to earth and therefore also to the supply neutral. These can (not by design) act as electrodes on the supply neutral, and may carry diverted neutral currents.

These currents will not be detected by RCDs on the system as they look for line / neutral current imbalance – not earthing system currents.

A Supply Company is not permitted to use a PME system for parts of an installation that connect to metalwork in caravans or boats (ESCQR 2002 - Regulation 9.4). This means that if a marina has a PME supply (and many do) they must ensure that the pontoons, etc are supplied via a T.T system (a system that uses Terra firma as its main return path for earth fault currents). A T.T system can be created within an installation but it is not always straight forward.

My boat is moored in Southampton in a small marina. I inspected their installation (they commissioned my company) and during this it came to light that they where connected directly to a PME supply. This seems to have occurred when a new supply was installed. I resolved this for them by converting the supply to the pontoons to a T.T system.

I would want to eliminate this as a cause if I were investigating this case.

Saildrives and propellers are isolated from everything else on the boat (or should be). Presumably your stray current explanation applies to those components that are connected to the mains/earth.

 

[GEB]

Saildrives and propellers are isolated from everything else on the boat (or should be). Presumably your stray current explanation applies to those components that are connected to the mains/earth.

Yes - but they are often connected because of links some make between the supply network earth and boat systems.

I do not bond my mains supply to anything on the boat - but that is my choice made after a risk assessment. I am not a fan of galvanic isolators, and isolating transformers would be overkill for my needs.

 

[jeremyshaw]

Prop material

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

I believe that Yanmar changed the SDs to alu in the mid 2000s. So the props would have changed too? Our legs are definitely bronze and my wife (who sanded the props one time) says they are ditto.
Thanks

 

[jeremyshaw]

Pics of the props

I now have pics attached of the props from the new owner, which show that the blades seem to have broken off in one case and the entire shebang in the other.

Also show the isolating bush.

Looks to me like maybe internal corrosion rather than "eating away" as I suggested. But I remain baffled.

Note that only 0.6 engine hours have been recorded in the last six months when the engines are turned over, so hitting an obstacle or rope damage seems highly unlikely. Esp BOTH props.

I remain baffled, and appreciative of thoughts so far. Any further suggestions welcome as I don't see why the same thing might not happen to the new props - unless this is just "old age"...

Many thanks

 

[vyv_cox]

The fractured prop you show is definitely not bronze, looks like an aluminium alloy to me. The focus is off for the fracture face but as far as I can see it doesn't look like corrosion at all, looks like a brittle type of fracture, consistent maybe with impact of a prop that was possibly weakened by some corrosion pitting.

I have to say that the leg looks like aluminium too, but I take your word that it is bronze. Assuming the colours in the photos are genuine.

Metal doesn't fail by 'old age', there is always a reason.

 

[savageseadog]

Saildrives and propellers are isolated from everything else on the boat (or should be). Presumably your stray current explanation applies to those components that are connected to the mains/earth.

To restate what I said before. Yanmar saildrives are not isolated. Some propellers have isolating bushes.

 

[Tranona]

Sorry, but they do not look anything like bronze. The props are aluminium and pretty sure the saildrive housings have always been aluminium. Never seen a Yanmar saildrive with a bronze housing.

Aluminium props do seem to corrode on their own over time - one of my Volvo props started to corrode around the boss. They are not protected by the anode as the bush clearly shown in your photo isolates them. The plastic disc you referred to earlier is to isolate the prop face from the lock nut. The anode is doing its job as the housing looks fine and the anodes are eroding evenly.

Replace the props with new standard Yanmar ones.

 

[KellysEye]

>The fresh and salt water mixing paper is interesting but I have yet to see a suggestion that it could lead to corrosion in anything.

In Lagos, Portugal I've seen two GRP yachts taken by the marina boat to the hard to have their props replaced because corrosion caused by the current caused by the mix of salt and fresh water that destroyed their props. Both boat had been left in the water so long that the anodes were badly eaten away and then covered in calcium. The same can happen with a bad earth in a marina 240v supply.

 

[rogerthebodger]

I have been monitoring this thread and did wander if the damage was not due to but damage. A friend wid a sail drive lost several props when the rubber insulating bush failed not unlike as shown in pic 2. I also agree with VyV_Cox in that the prop in pics 3 & 4 also looks like the blade broke off possibly due to hitting an underwater object and not corrosion.

 

[myquest]

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.

No components should be loose when fitted but is it possible that the washer you refer to had corroded (aluminium expands with corrosion) to the point that it and the hub became one therefore removing the isolation?

 

[savageseadog]

I have been monitoring this thread and did wander if the damage was not due to but damage. A friend wid a sail drive lost several props when the rubber insulating bush failed.....................

That is a common fault with the Radice folding props, they are bronze. There was a company that specialised in rebonding them (the rubber).

 

[Tranona]

That is a common fault with the Radice folding props, they are bronze. There was a company that specialised in rebonding them (the rubber).

You can also rebush the standard aluminium props. However, when I discussed doing that with my old Volvo prop the refurbishing said in his experience most props with any corrosion on them tend to crack around the boss when pressure is applied to insert the new bush. So any sign of corrosion and he would not risk it. Does not apply to bronze of course but suggests aluminium is prone to fracturing as in the photos.