Flexofold prop corrosion

[MagicalArmchair]

After ruthlessly jacking this thread: http://www.ybw.com/forums/showthread.php?497091-Burnishing-the-propellor/page3 I thought I'd continue here...

Background - 2 blade prop purchased in 2015 and fitted that year (see below), on lift out of this year, I was alarmed that the prob hub was showing signs of corrosion, other forumites noted a bad batch around this time.

http://www.albinballad.co.uk/how-tos/fit-a-flexofold-propeller/

I've had a very positive email response from Flexofold, I'll take some photos - I may be worrying about nothing...

 

[MagicalArmchair]

So I got down to the boat at lunch and took these shots of the prop. It is pitted very badly and looks a bit like it belongs on the Black Pearl! My old Bronze prop never had this problem for the 20 years it was in place, and nothing on the bonding side of the boat has changed. Does anybody have any thoughts at what is making my prop dissolve? :hopeless:

Here is how is looked after I fitted it in 2015

 

[MagicalArmchair]

Hmm, response from Flexofold:

"I understand that it might seem weird and being frustrating, but from you photos I´m positive that it´s coursed by electrolysis.
Not necessarily due to an electrical error on your boat – it might very well be on the shore power.
Have to say, that especially in the UK, the electrical installations often courses stray current.

The coloration on your propeller also shows clear signs that there are both older and resent damages. So even if you mounted a new propeller, that would most likely get damaged too."

What do you all think?

 

[savageseadog]

I agree with the response. It looks too bad for simple electrolysis, it looks more likely to be an active leakage current .

 

[vyv_cox]

Unfortunately this is a standard response from the manufacturers. In each of the four cases that I know of the owners were told the same. However, in one of them the hub was plastic and in another the owner had considerable knowledge and expertise in electrical systems. He was unable to find any currents of any description.

The first thing I would do is try to ascertain the composition of the metal used. Without spending a lot of money on analysis the only real alternative is colour. Try this site http://www.metalreference.com/Forms_Copper_Alloy.html although of course the settings of your monitor could influence your findings. Comparison with known alloys, e.g. plumbing fittings, can also be useful. You might be able to find a scrap metal dealer who will determine the composition with a Metascope for a reasonable fee.

 

[dom]

To the OP: I also sort agree with Flex-o-Fold's response, but there has been a sufficient number of of these 'probably-explainable' rapid degradations to raise a big ? over these props.

Perhaps there is something wrong with the metal composition/fabrication, perhaps the anode system is not adequate, perhaps the isolation is not effective, perhaps a lot of things.

I think Flex-o-Fold need to urgently comment, beyond the narrow scope of each failure. And for my tuppence worth; I will not even consider this product until they do so.

Edit: incidentally, has a Galvanic Isolator been fitted if earth is bonded? If not I'd fit ASAP as your he saildrive housing could be at risk next.

 

[MagicalArmchair]

No galvanic isolator is required as the boat is rarely connected to the mains and when she is it is only by an extension from the shore to charge the batteries or run a heater if we are sleeping on the boat in the marina. There is no 240V system on the boat as such, I posted a question about whether I needed such a beast on here some years ago .

No saildrive on Triola thankfully.

Flexofold, after I pointed out my prop that was fitted for 20+ years never suffered with electrolysis corrosion, responded with:

"Your photo is a little small to see details, however, it looks like the old fixed propeller suffered too! I see some big holes in both surface and not least edge of the blades.
Don´t know the exact type of material used for your fixed.
All Flexofold are made of NiAlBz. Stronger than “normal” bronze and same as what´s being used for navy propellers etc.
Paint might help to some extent, but can´t say for sure."

(photos can be found here: http://www.albinballad.co.uk/how-tos/fit-a-flexofold-propeller/)

So if the above is true, and its "NiAlBz" and its not the materials, how on earth do I stop this corrosion before my prop turns completely to cheese?

 

[dom]

Oops; clicked on your photo link and much clearer now

Looking at the difference in state between the anodes in the top and bottom photos, I'm wondering how long that first anode was in place? For it looks seriously degraded (BTW I'm assuming it's zinc?)

Have you considered stray current effect, e.g. a faulty bilge pump or something like that? Is this/anything-else left permanently hot when the boat is left? With the boat out of the water, this would be an ideal time to do some testing, to include the anode/shaft and prop/shaft bonding. If all good, I'd certainly follow Vyv's advice and have the prop composition verified.

Edit: BTW I still don't fully buy Flex--Fold's explanation for a problem which occurs surprisingly often in equally 'mysterious' circumstances.

 

[MagicalArmchair]

Oops; clicked on your photo link and much clearer now

Looking at the difference in state between the anodes in the top and bottom photos, I'm wondering how long that first anode was in place? For it looks seriously degraded (BTW I'm assuming it's zinc?)

Have you considered stray current effect, e.g. a faulty bilge pump or something like that? Is this/anything-else left permanently hot when the boat is left? With the boat out of the water, this would be an ideal time to do some testing, to include the anode/shaft and prop/shaft bonding. If all good, I'd certainly follow Vyv's advice and have the prop composition verified.

Edit: BTW I still don't fully buy Flex--Fold's explanation for a problem which occurs surprisingly often in equally 'mysterious' circumstances.

I've just found a much higher res photo of the old prop too, and it was fine compared to my Flexofold after two seasons. The only nicks it had was where I'll clocked it with a spanner and it was dezinced enough to be brittle and lose an edge after decades on the boat.

I never, ever, leave the electrics hot when I'm off the boat. If I am charging the batteries, I do it directly to the bank and leave the isolator off. So how would I test for stray current with the boat out of the water gang?

 

[Biggles Wader]

Surely the condition of your 20+ year old prop is a red herring if the new prop has failed so soon? There has to be a new problem with either your anodes, or lack of them, or the new prop.

 

[MagicalArmchair]

Surely the condition of your 20+ year old prop is a red herring if the new prop has failed so soon? There has to be a new problem with either your anodes, or lack of them, or the new prop.

The anode in the photos is what they generally look like at the end of a single season. This year, my shaft anode was probably 50% warn (I do replace them every year, even if they are only 50% worn), but still had very good contact with the shaft (and that's the one we are mostly talking about here in relation to the prop I suppose?).

The block anode I haven't replaced for a few years, but there is plenty of meat left on it, although that one doesn't erode at the same rate as the others. And yes! Zinc anode for salty/brackish water.

Here is how my lovely lady looks like right now (yup, new anode ready to go).

 

[MagicalArmchair]

So, I guess this comes down to:

• How do I test for the leaking current whilst out of the water?
• How do I test for the leaking current when I get IN the water?
• How do I protect against this electrolysis beyond more anodes?
• Whats best practise for bonding etc?
• As Vyv suggested, identify the metal as best I can.
• I have spoken to a couple of others who have had problems with props from this period from Flexofold, hmm, I wonder if Flexofold will ‘fess’ up?

 

[vyv_cox]

Nickel aluminium bronze (Nibral) is highly resistant to corrosion. Most ships' propellers are made from this alloy and anodes are not normally required. A zinc anode protects the galvanic reaction between the prop and the shaft, although even this is barely necessary, such is the corrosion resistance of the nibral. Your new prop photo looks rather more yellow to me than nibral should be, although the difference is subtle and the photos may be misleading.

Electrical testing is not one of my major skills but I think a check of voltage between the propeller and a well-grounded part of the boat, ideally the negative of the battery, should detect any problem.

Painting the propeller will help if there is any electrical influence, by reducing the size of the cathode. Hammerite/Velox would be my choice.

 

[greeny]

I fitted my flexofold 3 years ago and no problems as of yet. Darglow said that no anode was necessary but I went ahead and fitted a shaft anode and also added a hull anode approx 3 feet from the prop as a precautionary measure. The shaft anode needs changing each year which I change whilst afloat. The hull anode is not losing much at all over the 3 years but I guess this means that a shaft anode was needed. Maybe if I hadn't fitted one my prop would be similar to yours.

 

[savageseadog]

I had two Ni-Al Bronze Gori props dissolve away some years ago on a different boat, Yanmar saildrive. The life of each prop was about 2 years before the teeth starting locking up. My son thought it might have been the proximity of the large cast iron keel, I never managed toconfirm why it happened. I have a different boat with a similar setup with a two blade Flexofold, the only anode is the one on the saildrive. It's been in the water for a number of years without a trace of corrosion.

 

[MagicalArmchair]

Nickel aluminium bronze (Nibral) is highly resistant to corrosion. Most ships' propellers are made from this alloy and anodes are not normally required. A zinc anode protects the galvanic reaction between the prop and the shaft, although even this is barely necessary, such is the corrosion resistance of the nibral. Your new prop photo looks rather more yellow to me than nibral should be, although the difference is subtle and the photos may be misleading.

Electrical testing is not one of my major skills but I think a check of voltage between the propeller and a well-grounded part of the boat, ideally the negative of the battery, should detect any problem.

Painting the propeller will help if there is any electrical influence, by reducing the size of the cathode. Hammerite/Velox would be my choice.

Thanks all,

So I've emailed Flex-o-fold again, lets see what they come back with. I've also been doing some reading around electrolysis and can do some testing. Reading the below:

http://www.yachting-life.net/new index/frames3/electrol_frame.htm

It suggests

Automotive battery chargers are a common cause of corrosion in boats, particularly small boats without shore systems. Auto chargers often provide no isolation between the ac and dc windings and can energize the negative terminal, which also energizes the boats grounding system. Portable auto chargers should not be used on boats, and are a frequent cause of stern drive damage.

I do use a pretty decent multistage RING automotive charger for charging Triolas banks of batteries when they need it. It is on rarely, and yes, it is connected via an extension lead (the marina has RCDs) but the battery switch will remain off whilst charging. The boat has no 240V system (so no 240V ground to the hull so Galvanic isolator required.) and no installed charger - the engine block is grounded to an anode. Would a good test be to check the voltage between the negative terminal on the boat and the prop shaft whilst charging, and then whilst not charging to see if the negative poll is being energised? What should I look for when testing the voltage between the neg battery terminals and the sterngear? If there is voltage, I have a problem?

I will paint the prop with Hammerite special metals primer then boottop to offer some protection (it'll be hard getting in the teeth) and try and save whats left of my prop. Hmm, all very sub-optimal.

 

[VicS]

Thanks all,

So I've emailed Flex-o-fold again, lets see what they come back with. I've also been doing some reading around electrolysis and can do some testing. Reading the below:

http://www.yachting-life.net/new index/frames3/electrol_frame.htm

It suggests



I do use a pretty decent multistage RING automotive charger for charging Triolas banks of batteries when they need it. It is on rarely, and yes, it is connected via an extension lead (the marina has RCDs) but the battery switch will remain off whilst charging. The boat has no 240V system (so no 240V ground to the hull so Galvanic isolator required.) and no installed charger - the engine block is grounded to an anode. Would a good test be to check the voltage between the negative terminal on the boat and the prop shaft whilst charging, and then whilst not charging to see if the negative poll is being energised? What should I look for when testing the voltage between the neg battery terminals and the sterngear? If there is voltage, I have a problem?

I will paint the prop with Hammerite special metals primer then boottop to offer some protection (it'll be hard getting in the teeth) and try and save whats left of my prop. Hmm, all very sub-optimal.

You say that the engine block is grounded to an anode. Have you checked that any flexible shaft coupling is effectively bridged electrically and that you do have a good low resistance path from the anode right through to the prop?

Having verified the connection between anode and prop is good check for any voltage between prop and anode under different circumstances ie when charging, with the battery isolator on/ off. With the engine " ignition" on/ off and any others you can think of.


On a different tack a campsite mains power unit would be a good thing to have on the end of your extension lead as it would give you your own RCD, without relying on the pontoon one, and a circuit breaker

See https://www.towsure.com/caravanning...to-caravan-connector-campsite-hookup?limit=32

 

[MagicalArmchair]

You say that the engine block is grounded to an anode. Have you checked that any flexible shaft coupling is effectively bridged electrically and that you do have a good low resistance path from the anode right through to the prop?

Having verified the connection between anode and prop is good check for any voltage between prop and anode under different circumstances ie when charging, with the battery isolator on/ off. With the engine " ignition" on/ off and any others you can think of.


On a different tack a campsite mains power unit would be a good thing to have on the end of your extension lead as it would give you your own RCD, without relying on the pontoon one, and a circuit breaker

See https://www.towsure.com/caravanning...to-caravan-connector-campsite-hookup?limit=32

Thanks Vic, that’s really helpful. So, once I confirm that connection, in those different scenarios more voltage = bad and a circumstance for electrolysis to occur?

 

[MagicalArmchair]

So, in way of an update.

• I called the marina and asked them about stray current. They told me current in the marina is regularly checked and they have had no reports from our pontoon (they do run a tight ship at Chatham – they should do, they are the most expensive marina on the East Coast!). I honestly think we can discount this. If the problem is anywhere, it’ll be on my boat.
• There is a propeller specialist, Invicta Marine, at Chatham Dockyard next door to the marina, who very kindly went and had a look at the prop. He said that as it looked green on the shaft where it exited the boat, he suspected stray current from inside the boat. He said that if it was stray current from outside the boat, generally you would see green appearing on the tip of the prop first. He also said he could weld up the damage for circa £100 plus the vat (Thats a haul out at the end of season jobbie).
• Question: He also said that painting the prop was probably a bad idea, as if it chipped (think of around the teeth, very likely to chip there) it will corrode favorably there as the prop would contain some zinc. My gut still tells me to paint the bleeding thing.

I’ll go down tomorrow at lunchtime and measure:

• -Resistance between the anode the engine is grounded to and the prop.
• -Voltage between the anode and the prop.
• -Voltage between the anode and prop when the battery is charging using my current automotive smart charger.
• -Voltage between the anode and prop when the battery master switch is on.
• -Voltage between the anode and prop when the ignition is on.

I'll also scrub the prop up with a brass wire brush on my drill and then give it a coat of Hammerite special metals primer I think, dependent upon your thoughts on the Prop chaps opinion that painting the prop was bad news...

Edit - Last year I used a thick coating of prop grease on the prop, it kept fouling to a minimum, but didn't stop the corrosion

 

[Tranona]

Don't think paint chipping and exposing a bit of bare metal will make any difference. As you might recall I have the same prop as you but on a saildrive. It has been painted from new with Trilux and odd bits get worn off. It has been on the boat for 6 years and I have twice stripped the blades back to bare using brick cleaner and recoated. The hub has never been stripped. No sign of any corrosion.