Flexofold prop corrosion

[Quandary]

76 years, that is exceptional paint performance.
However I would tend to support Invicta's experts advice, if you presume everything can be protected by paint except the teeth, surely they will then be the most vulnerable part, they are already subject to wear by friction, expecting them to also bear responsibility for all the corrosion which might occur is surely not fair to the little blighters.

 

[Tranona]

76 years, that is exceptional paint performance.
However I would tend to support Invicta's experts advice, if you presume everything can be protected by paint except the teeth, surely they will then be the most vulnerable part, they are already subject to wear by friction, expecting them to also bear responsibility for all the corrosion which might occur is surely not fair to the little blighters.

Fat fingers! now corrected. Teeth are still in perfect condition. There is no anode on mine and no wear on the teeth or the stainless pins. Paint is used more to resist fouling for which it is reasonably effective.

 

[MagicalArmchair]

76 years, that is exceptional paint performance.
However I would tend to support Invicta's experts advice, if you presume everything can be protected by paint except the teeth, surely they will then be the most vulnerable part, they are already subject to wear by friction, expecting them to also bear responsibility for all the corrosion which might occur is surely not fair to the little blighters.

The teeth, thus far, have weathered quite well. I could take the blades off entirely, I have some InterProtect Epoxy left over from painting my deck that is hard as nails - it would be doing well to chip that, all seems a bit extreme though.

I guess it comes down to sorting out the electrical stuff .... hmmm, I hope I can identify something in my floundering around with my ammeter and lengths of wire tomorrow...

If anyone has hunted for stray current like this before, any hints for what I should be doing tomorrow dancing around the boat in my three piece suit would be greatly appreciated

 

[vyv_cox]

The part about the increased corrosion at paint chips is an urban myth. No different from your wrought steel gate at home - it looks bad only by comparison with the paint. Also of course there is no zinc in Nibral, therefore there is no dezincification.

 

[MagicalArmchair]

The part about the increased corrosion at paint chips is an urban myth. No different from your wrought steel gate at home - it looks bad only by comparison with the paint. Also of course there is no zinc in Nibral, therefore there is no dezincification.

Vyv is an engineering legend - very few come more expert, so as per Vyvs advice, I'm getting my special metals primer out (it is, in fact, already in the car ready for tomorrows lunch hour adventure), and I'll wirebrush and paint the prop in situ as best I can - I want to get sailing - prop made of cheese or not, and if I start taking the boat apart now I'll never get on the water .

I will only paint it once I've run my little bank of tests mind you. I'd like to get to the bottom of the underlying problem.

 

[vyv_cox]

The examples shown on the old version of the website at https://coxengineering.sharepoint.com/Pages/Electrochem.aspx include interesting case histories. In each case the corroded parts were connected directly to positive, which should be very easily measured. Some high currents were involved to cause this level of damage.

(Note that this site will disappear 31/3/18)

 

[MagicalArmchair]

The examples shown on the old version of the website at https://coxengineering.sharepoint.com/Pages/Electrochem.aspx include interesting case histories. In each case the corroded parts were connected directly to positive, which should be very easily measured. Some high currents were involved to cause this level of damage.

(Note that this site will disappear 31/3/18)

Massively useful Vyv - getting ready to head off to the boat now. You need some help moving any stuff over to your new site Vyv? Apologies I went a bit dark on that - flipping day job got in the way

 

[vyv_cox]

Massively useful Vyv - getting ready to head off to the boat now. You need some help moving any stuff over to your new site Vyv? Apologies I went a bit dark on that - flipping day job got in the way

Thanks Mark. I have been moving that page this morning. I have copied the content but really struggling with the appearance. Also, even though the appearance on the edit page is reasonable, the preview is completely different. Now I have published it, parent is Metallurgy and set as page 5, it does not appear on the site. Help pages only say what I have done. Any further help would be much appreciated.

 

[MagicalArmchair]

Thanks Vyv, I'll hop on your email sir. Right, so I wizzed down at lunchtime, scrubbed up the prop then and connected up the negative poll on the battery to a long piece of wire, and then put the other probe onto the prop as below:

• There is no continuity between the prop and the negative poll.
• There IS continuity (as you would expect) between the negative poll and the anode that is grounded to the engine block on board.
• With the battery master switch OFF, there is 0.23V flowing between the negative poll of the battery and the prop, as in the picture.
• With the battery master switch ON, there is 0.23V flowing between the negative poll of the battery and the prop, so no change.

Is that the voltage causing the below damage to my prop? If so, what on earth could be causing it with the battery switch off?

A little extra information, Triolas wiring is very simple (and a bit of a birds nest if I'm frank). Ultimately it looks a bit like the below:

 

[RichardS]

240mV on a boat which is not even in the water? There's something not right. What if you disconnect the +ve connection on all the batteries and check that the voltage has gone, then connect one battery at a time until you find where it's coming from. Surely it must be from a battery which by-passes the master switch and is shorting, perhaps a permanent bilge pump connection, or the master switch is faulty.

Richard

 

[vyv_cox]

0.23 volts is enough to create a corrosion problem , although compared with e.g. the 0.8 volts between a zinc anode and 300 series stainless steel I am surprised that the effect seems so severe. I also find it odd that the corrosion damage is so random. Electrically induced damage is usually far more localised.

So far as the source is concerned I would only be guessing. Solar panels? It needs a logical approach to find it, disconnecting things one by one. First thing to check is the switch itself, does the same apply if you disconnect the battery terminals? (although your continuity check would seem to say it is OK.) Anything bypassing the master switch such as bilge pump, VHF, Navtex? Windlass? Is your battery charger connected?

 

[dom]

To OP, as a matter of interest did you test the zero calibration of your meter? If yes, there’s a problem.

Also, is there anything else connected to the battery, or any other charging sources? It might be worth testing the cabling integrity and the battery switch for corrosion, etc.

Good news is you seem to be getting there.

 

[VicS]

. Right, so I wizzed down at lunchtime, scrubbed up the prop then and connected up the negative poll on the battery to a long piece of wire, and then put the other probe onto the prop as below:

• There is no continuity between the prop and the negative poll.
  [*] There IS continuity (as you would expect) between the negative poll and the anode that is grounded to the engine block on board.
• With the battery master switch OFF, there is 0.23V flowing between the negative poll of the battery and the prop, as in the picture.
• With the battery master switch ON, there is 0.23V flowing between the negative poll of the battery and the prop, so no change.

Is that the voltage causing the below damage to my prop? If so, what on earth could be causing it with the battery switch off?

So one deduces that there is no contiuity between the anode and the prop !

For the anode to protect the prop ( presumably its purpose) there must be a good low resistance connection between anode and prop. Unless its been fitted to protect something else ( to which it is connected) it is sitting there doing nothing !
The reason needs to be identified so that it can be remedied in due course. Flexible coupling not bridged perhaps.

However apparently 230 mV between prop and battery negative!
It might be a red herring and the consequence of using a high impedance digital voltmeter or it might idicate a leakge via something not isolated by the battery switch.

Thinking about that , esp as there is apparently no continuity between the engine block and the prop

 

[MagicalArmchair]

Thanks all - I ran out of time (and then some) for more trouble shooting.

• Nothing bypasses the battery master switch.
• I'll test for Voltage on the supposedly dead side of the battery switch when it is switched off to see if the master switch is causing this leak.
• My ammeter was calibrated, so it was a true reading.
• I'll try disconnecting the banks one at a time to find where the fault lies if its not the master switch. At least I can also troubleshoot this in the water if push comes to shove a wire to the prop shaft and then to the negative poll of the battery will give me the information I need. Clearly I'd prefer not to be in the drink with those volts flying about!!!
• (edit, just saw your post Vyv!) Its not a flexible coupling, its like Wil Baileys coupling on your page here http://coxeng.co.uk/stern-gear/coupling-types-and-removal/. Hmm, yes, that hadn't occurred to me that there should be continuity. That anode I've never replaced since I've had the boat - THAT EXPLAINS WHY . Surely the shaft anode would protect it enough though?

So, err, to sum up whats happening then (I'm trying to get my head around it), is something like the below?

 

[DipperToo]

Just as a thought;
You mention you have an ammeter - which implies that there is a shunt somewhere on the system.
I am by no means an expert, but I 'think' that sometimes these shunts go in the +ve fed and others, like mine, go on the negative from the domestic batteries.
Is there any possibility that somewhere the shunt is being bypassed by the battery +ve or -ve introducing a small voltage differential?
I would be interested to hear from any electricians how shunts should be wired to avoid issues? I use a Victron Monitor and the domestic -ve goes to the shunt AND NOWHERE ELSE with the engine battery -ve going to the output side of the shunt.
I have also heard about the copper strips that connect through the prop shaft coupling sometimes cracking - I have just gone through the process of having a large cable put onto the coupling to bridge it on a 'just in case' basis.

 

[VicS]

Thanks all - I ran out of time (and then some) for more trouble shooting.



So, err, to sum up whats happening then (I'm trying to get my head around it), is something like the below?

No because the continuity tests you have done suggest that there is no continuity between the engine and the prop.......... something to be corrected though!

Otherwise it was the sort of thing I was expecting to find.

 

[earlybird]

The OP's photo link in post 7 shows a shaft anode ahead of the p bracket. I'd have thought that this was sufficient to safeguard the prop from "normal" electrolylic corrosion, although the hull anode won't be contributing.
When I bought my present boat the Flex-o- fold prop had significant pitting, which seems to have been halted by fitting an anode. This is a hull anode, but definitely connected to the prop-shaft, including a jumper wire over the flexible coupling.

 

[alahol2]

When I bought my present boat the Flex-o- fold prop had significant pitting, which seems to have been halted by fitting an anode. This is a hull anode, but definitely connected to the prop-shaft, including a jumper wire over the flexible coupling.

I bought a second hand Flexofold (on here) that had significant corrosion on the hub (but not on the blades). My boat has a hull anode which is electrically connected to the prop, bridging the flexible coupling. The prop has been used variously polished or painted for several years now with no sign of any more corrosion.

Also MagicalA says his coupling looks like Will Bailey's on Vyv's site. That coupling IS a flexible coupling and needs bypassing.

 

[VicS]

I bought a second hand Flexofold (on here) that had significant corrosion on the hub (but not on the blades). My boat has a hull anode which is electrically connected to the prop, bridging the flexible coupling. The prop has been used variously polished or painted for several years now with no sign of any more corrosion.

Also MagicalA says his coupling looks like Will Bailey's on Vyv's site. That coupling IS a flexible coupling and needs bypassing.

If that is an R&D coupling then it should have an earthing connector consisting of a silver impregnated ribber strip, which when fitted through the axis of the coupling between the two fail safe straps gives electrical continuity although I think these may be supplied as an optional extra

 

[alahol2]

If that is an R&D coupling then it should have an earthing connector consisting of a silver impregnated ribber strip, which when fitted through the axis of the coupling between the two fail safe straps gives electrical continuity although I think these may be supplied as an optional extra

You are correct, I fitted one to my Beta installation. The earthing connector was an optional extra. Because I didn't know about the connector, I had to provide another means and, for the life of me, I can't remember how I did it; but I do know I have continuity. I check it every year.