Flexofold prop corrosion

As has been suggested several times, the most likely cause is a lack of continuity across the shaft flexible coupling.

The 0.23V can then be explained by the galvanic series potential difference between the prop material and the zinc anode in seawater.

Do a continuity check either side of the coupling.
 
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Playtime said:
As has been suggested several times, the most likely cause is a lack of continuity across the shaft flexible coupling.

The 0.23V can then be explained by the galvanic series potential difference between the prop material and the zinc anode in seawater.

Do a continuity check either side of the coupling.
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two "buts"

But 1 .... As far as the prop corrosion is concerned , the boat has a shaft anode, I now realise. . That should make a hull anode unnecessary.

But 2 ... the 0.23 V has been measured with the boat out of the water and is a lot less than the difference between zinc and Nibral on the galvanic scale
 
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OK that'll teach me to skim read and think I've cracked it.

In my defence, the circuit diagram is a bit misleading. It shows the prop in the water and doesn't show the shaft anode. It also can't be correct as drawn to get a reading of 0.23V across a short circuit (negative pole to prop).

I still think there is no continuity across the shaft coupling.

Maybe there is a leakage through the P bracket. Is there any water in the bilge around the P bracket bolts?
 
Playtime said:
OK that'll teach me to skim read and think I've cracked it.

In my defence, the circuit diagram is a bit misleading. It shows the prop in the water and doesn't show the shaft anode. It also can't be correct as drawn to get a reading of 0.23V across a short circuit (negative pole to prop).

I still think there is no continuity across the shaft coupling.

Maybe there is a leakage through the P bracket. Is there any water in the bilge around the P bracket bolts?
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I am just as guilty of "skin reading". I had missed the fact that there is a shaft anode!

You are right. I am sure about the coupling.The continuity checks in #29 more or less prove that. "There is no continuity between the prop and the negative poll."
 
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VicS said:
I am just as guilty of "skin reading". I had missed the fact that there is a shaft anode!

You are right. I am sure about the coupling.The continuity checks in #29 more or less prove that. "There is no continuity between the prop and the negative poll."
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Ah, okay, so are we saying that if I get continuity between the prop and the hull anode (and in turn the negative poll) the world will stop murdering my propeller? Probably by bridging the coupling?

Also, are we saying the voltage difference of 0.2V ish is a red herring and is just causes by connecting two, dissimilar metals (Zinc and Bronze or Zinc and Stainless steel?)

Thanks for all the help so far, I feel like we are getting to the cause.

edit: There is no water around any bolts for the P Bracket... In fact I don’t think there ARE any bolts for the P bracket, I think it is purely laminated into the hull.
 
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If the prop was manganese bronze it would be beneficial to ensure that the hull anode is electrically connected to the prop, assuming the two are in line of sight of each other. If the prop is Nibral it really will not make any difference either way.

Connecting two metals together will not generate a voltage unless they are both immersed in water. In your case the cause is not obvious.

There are no obvious bolts on my P-bracket, they and the pieces of angle are glassed over and the countersunk heads are covered by epoxy and antifouling. http://coxeng.co.uk/stern-gear/replacing-a-p-bracket/
 
vyv_cox said:
If the prop was manganese bronze it would be beneficial to ensure that the hull anode is electrically connected to the prop, assuming the two are in line of sight of each other. If the prop is Nibral it really will not make any difference either way.

Connecting two metals together will not generate a voltage unless they are both immersed in water. In your case the cause is not obvious.

There are no obvious bolts on my P-bracket, they and the pieces of angle are glassed over and the countersunk heads are covered by epoxy and antifouling. http://coxeng.co.uk/stern-gear/replacing-a-p-bracket/
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Thanks Vyv, it is supposedly Nibral. I noted some pinkening of the hub in patches , does Nibral do that when it is starting to fall apart due to electrolysis or is that a feature of bronze?

Will connecting the prop to the anode (and as a result the negative poll) negate the voltage regardless of tracking down the leak? Clearly I want to discover said leak so I’ll do some more investigation - take the positive clamps off each of the batteries in turn, check the battery switch, etc.
 
I would not expect nibral to go pink. As I said a long way up the thread, I have my doubts about the composition.

Out of the water I cannot see why a direct connection between anode, negative battery terminal and prop should make any difference at all.
 
Agree with Vyv, but you might be better off semi-sidelining potential galvanic problems for a moment and as Vyv says they cannot possibly exist outside of an electrolyte (seawater in this case).

Stray current corrosion is potentially a more serious problem over short timescales and your anode wear rate seems very high in addition to the developing prop problem. With a properly calibrated voltmeter the voltage difference between prop and negative poll should be functionally zero. If it is not, the problem must be traced and rectified. While they ultimately form useful lines of defence, your primary focus here should not be on anodes (that comes next), but on finding the leak.
 
MagicalArmchair said:
Ah, okay, so are we saying that if I get continuity between the prop and the hull anode (and in turn the negative poll) the world will stop murdering my propeller? Probably by bridging the coupling?

Also, are we saying the voltage difference of 0.2V ish is a red herring and is just causes by connecting two, dissimilar metals (Zinc and Bronze or Zinc and Stainless steel?)

Thanks for all the help so far, I feel like we are getting to the cause.

edit: There is no water around any bolts for the P Bracket... In fact I don’t think there ARE any bolts for the P bracket, I think it is purely laminated into the hull.
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What we are saying is that the hull anode will not be offering any protection to the prop unless the flexible coupling is bridged to complete the electrical connection. However the shaft anode should be providing that protection without there being hull anode, although a hull anode will provide some back up in the event of the shaft anode being fully consumed or lost. Also as Vyv says NiBrAl is very corrosion resistant.

I would bridge coupling but not as my first priority. Agreeing with Dom here. If it is an R&D coupling, as mentioned earlier, it should have a special silver impregnated rubber insert fitted in its centre. Details of these are on the R&D website.

The 0.23 volts you have measured is a puzzle. If NiBrAl and zinc were both immersed in seawater you would find about 0.8 volt between them but not when out of the water. I suspect it is a red herring. I certainly cannot explain it!

If there was leakage current from the DC system you would expect electrolysis, rather than simply galvanic corrosion due to dissimilar metals, which would very rapidly consume the shaft anode followed by attack of the prop but with the prop shaft isolated by the coupling this cannot occur anyway.

I am wondering if there is a DC leakage, combined with a very small leakage across the coupling ???????? For that reason I would want to completely eliminate the possibility of a DC leakage before bridging the coupling. We don't want to progress from some corrosion to a situation in which the prop disappears overnight

I am not so sure that we are yet " getting to the cause" I was hoping that Vyv would have had a flash of inspiration but we all seem to be equally puzzled.
 
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Very odd to have complete forum agreement over the next step! ....i.e. properly measure and, IF real, eliminate that pesky 230mV leakage.

Just in case Viv or Vic have a spare moment, take a look at the anode deterioration in MA's first post (link in post #7) and see what you think. Seems alarmingly high to me for a single season, but you'll be better judges.
 
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dom said:
Very odd to have complete forum agreement over the next step! ....i.e. properly measure and, IF real, eliminate that pesky 230mV leakage.

Just in case Viv or Vic have a spare moment, take a look at the anode deterioration in MA's first post (link in post #7) and see what you think. Seems alarmingly high to me for a single season, but you'll be better judges.
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I have four differnt ammeters (my late father was a shipwright, so I’m not short of kit!), so I’ll try that first. From when I changed my stern gland about a month ago I have a photo of the prop shaft from the flexible couple back and there are wires that travel perpendicular to the shaft, but certainly nothing that should be imparting any voltage.

I’ll check continuity across the coupling too on my next visit.

3DHwpsm.jpg
 
Also try checking the voltage across the coupling. If your circuit diagram (apart from the missing coupling) is correct then you should see 0.24V. This will confirm that the coupling is open circuit (and needs a by-pass). If that is the case then the 0.24V would appear to come from the shaft/P bracket/prop outboard of the coupling, which is hard to explain.
 
Thank you all, I have read and reread all your posts, I am very grateful for your input and I have a plan of attack (which is good, back in the soup next Wednesday...).

  • Confirm instrumentation by using another ammeter to confirm the stray current readings.
  • Measure continuity across the coupling.
  • Measure voltage across the coupling, confirm we see the 0.23Vs.
  • As per Vics suggestion, make sure there is no major DC leakage the other side of the coupling that is leaking across it.
  • Measure the voltage across the battery switch with it set to off to confirm it isn’t faulty.
  • Take the positive off the engine battery, confirm if stray voltage is still there.
  • If it is, take positive off the domestic batteries, confirm if stray voltage is still there. (At that point, no 12V will be present anywhere)

As remedial action if none of the above can shed any light on whats going on:

  • Paint the prop to offer some protection as per Vyvs suggestion. Prime it, then hard antifoul it (this is likely what protected my old prop)
  • Bridge the flexible coupling (so long as we can confirm there is no major DC leak the boat side of the coupling). Is the method here (seeing as I’m not about to take the boat apart before she goes into the water), to undo the two nuts a little in the below, and bridge them along the red line with a chunky piece of wire and some crimped on fork terminals?

qkRPRIal.png
 
So, with the three kids, wife, and nutty ships hound in tow, I went down yesterday to take a look at this problem. Not exactly the best circumstances for deep thought and contemplation (especially as I was trying to touch up the Coppercoat at the same time)

  • Confirm instrumentation by using another ammeter to confirm the stray current readings. - Using three ammeters, this is confirmed.
  • Measure continuity across the coupling. - There is no continuity across the coupling, as we thought.
  • Measure voltage across the coupling, confirm we see the 0.23Vs. - We do see it across the coupling.
  • As per Vics suggestion, make sure there is no major DC leakage the other side of the coupling that is leaking across it .- There is no DC leakage at all the other side of the coupling that I can identify (??!)
  • Measure the voltage across the battery switch with it set to off to confirm it isn’t faulty.- With the battery switch to OFF we still have north of 1V on the common side of the switch? I am thinking this is a red herring as the quantum of the leak doesn't change with the batteries set to 1, 2, both or off - weird though and I will investigate further with the manufacturer of the switch.
  • Take the positive off the engine battery, confirm if stray voltage is still there.- Took the positive terminal off and struggled a bit to take meaningful readings. The stray milliamps were still there, I had a hard time figuring out if they were less though. This needs more investigation.
  • If it is, take positive off the domestic batteries, confirm if stray voltage is still there. (At that point, no 12V will be present anywhere)- I ran out of time at this point (the crew were mutinying) and I have to undo a whole two screws to get to my new battery box and I had run out of will power by this point :encouragement:

I bodged a bridge up of the flexible coupling. If I did that, the stray volts between the negative terminal and the shaft went away entirely, as you might think. So do we suggest that's the answer....? Where are those milliamps coming from though! The shaft, aft of the coupling, is not in contact with ANYTHING that provides continuity, let along voltage? :confused:

I've also painted the prop with primer, and will put a couple of thinned coats of boot top on the prop, followed by some prop grease to keep the critters at bay. :rolleyes:
 
MagicalArmchair said:
[*]If it is, take positive off the domestic batteries, confirm if stray voltage is still there. (At that point, no 12V will be present anywhere)- I ran out of time at this point (the crew were mutinying) and I have to undo a whole two screws to get to my new battery box and I had run out of will power by this point :encouragement:
[/LIST]
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That's a bit like one of those murder mystery series on the TV where the killer is about to be revealed ....... and it suddenly says "To be continued ........" ;)

Richard
 
RichardS said:
That's a bit like one of those murder mystery series on the TV where the killer is about to be revealed ....... and it suddenly says "To be continued ........" ;)

Richard
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All we need now is one of the key suspects to be found murdered in the last minutes of the episode and the next one not until a week's time.

How will we get through the week not knowing?
 
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