Barnacles causing bronze prop electrolysis....?

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blan321

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Hi,
Boat spent April to October in Chichester marina, barnacles on prop at liftout, was polished at season start. I have now cleaned and polished the prop again. There are shallow craters and copper discoloration where they were squatting.
Is this;
Normal ?
V unusual but prob normal for Chichester?
Or do I have an electrolytic problem?
New anode 18 months ago, probably lost about 15%.
I fitted an R and D flexible coupling a year ago, will now fit an earth wire across it. I think I might try Trilux too.
Curious to see what others think, thanks.
 
I'd heard barnacles, limpets etc chemically eat into the rock they're attached to, so I guess they can attack a prop as well. Bound to be a marine biologist or two lurking on the forum who can give a definitive response.

Doesn't sound like you have an electrolysis probem from that wastage. I find Trilux (prop-o-drev) works reasonably well. Warming it first and spraying on in thin layers seems to give the best adhesion.
 
blan321 said:
Hi,
Boat spent April to October in Chichester marina, barnacles on prop at liftout, was polished at season start. I have now cleaned and polished the prop again. There are shallow craters and copper discoloration where they were squatting.
Is this;
Normal ?
V unusual but prob normal for Chichester?
Or do I have an electrolytic problem?
New anode 18 months ago, probably lost about 15%.
I fitted an R and D flexible coupling a year ago, will now fit an earth wire across it. I think I might try Trilux too.
Curious to see what others think, thanks.
Click to expand...

I am just ordering my second prop in 4 years after having the latest one condemned due to electrolysis just like the first - and I do have a link across the R&D coupling. My anode always lasts at least 3 years (and is connected correctly). Try dinging the prop with something metallic - mine goes thud with no discernible ring to the noise.

This is in Port Solent. We also had a lot of barnacles despite the rest of the hull being quite clean. Someone in the industry wondered if it is worse in lock-in marinas.

Do you have a galvanic isolator fitted? That's my next investment.
It's has only started since we moved into P. Solent - the boat is 30 years old and was on the original prop until then.

My power is normally plugged in but electrically turned off.

Cheers,
 
Piddy said:
Do you have a galvanic isolator fitted? That's my next investment.
It's has only started since we moved into P. Solent - the boat is 30 years old and was on the original prop until then.

My power is normally plugged in but electrically turned off.

Cheers,
Click to expand...

A couple of points ;

As far as galvanic isolation is concerned, whether or not the shore power is turned on or not is irrelevant. Turning the power off onboard does not isolate the ground connection. This is what can cause galvanic corrosion, depending on how the boat is wired. If the boat has an installation where the boats shore power ground is bonded to the DC negative (a requirement on modern boats) your anode is electrically connected to the marina ground and consequentally to surrounding boats anode bonding circuit.

Your anode> your DC negative> your AC ground> marina AC ground> other boats AC ground> other boats> DC negative> other boats anode.

So if the boat next door doesn't have an anode, it'll use yours. Assuming both you and he have AC ground bonded to DC negative.

Which brings me to the second point, with your boat being 30 years old it is unlikely that your AC ground is bonded to the DC negative. It would have to have been done after market as it was not normal practice 30 years ago. If that's the case, fitting a galvanic isolator it totally pointless.
 
i am hoping i have now attached a picture to give more info. Most of the prop is fine, its just where the barnacles sat!Prop.jpg
 
Fair enough! It's a zoomed in image of the hub of the prop with one blade at 10 and other at 4 o clock. It shows my nicely polished bronze prop with shallow pink craters each about 5mm dia where the barnacles were located. It's the discovered craters that are the subject of my post, common and well known result of barnacles on bronze or something else going on....?
 
I’m a marine chemist not a biologist, but I do know that barnacles and limpets are very different – for one thing, a limpet attaches to the substrate by its foot, but an acorn barnacle attaches by its head! My understanding is that both use a secreted ‘cement’, but limpets also attach by suction and grind the edges of their shells into the substrate to improve fit and adhesion.

As far as I can see, barnacle ‘cement’ is a complex proteinaceous material, and that could I think complex copper - which would tend to reduce its toxicity (so the barnacle survives on the prop) and might affect to some extent the surface, and thus surface colour, of the bronze. I do not get many barnacles on my prop, but I have not personally seen any ‘cratering’ where they have been.

It is difficult to tell from your photograph but the impression I get (but perhaps wrongly) is of a typical, thin white-ish calcareous deposit (calcium carbonate) with ‘pink’ patches where the barnacles have been. Is it possible that what you describe as ‘cratering’ is not in the bronze itself, but just the absence of the calcareous film in the places where the barnacle have been? If you remove the whitish calcareous film (physically, or with an acid prop cleaner) do the barnacle areas then look different?
 
PaulRainbow said:
A couple of points ;

As far as galvanic isolation is concerned, whether or not the shore power is turned on or not is irrelevant. Turning the power off onboard does not isolate the ground connection. This is what can cause galvanic corrosion, depending on how the boat is wired. If the boat has an installation where the boats shore power ground is bonded to the DC negative (a requirement on modern boats) your anode is electrically connected to the marina ground and consequentally to surrounding boats anode bonding circuit.

Your anode> your DC negative> your AC ground> marina AC ground> other boats AC ground> other boats> DC negative> other boats anode.

So if the boat next door doesn't have an anode, it'll use yours. Assuming both you and he have AC ground bonded to DC negative.

Which brings me to the second point, with your boat being 30 years old it is unlikely that your AC ground is bonded to the DC negative. It would have to have been done after market as it was not normal practice 30 years ago. If that's the case, fitting a galvanic isolator it totally pointless.
Click to expand...

Yes, I understand that point about the power being electrically off and the earth still being connected - that's why I mentioned it. However, you now raise a doubt in my mind about whether the AC earth and DC earth are connected. It is an aftermarket installation from before my ownership. I assumed , possibly erroneously, that because I now get this problem that they are (connected.) I will check that this weekend. If it isn't connected, why would my prop be affected before the anode - which does slowly get eroded?

The boats either side of me are quite new - HR37 and a new-ish Fairline MB. I'd be surprised if they were causing issues.

Cheers,
 
Piddy said:
Yes, I understand that point about the power being electrically off and the earth still being connected - that's why I mentioned it. However, you now raise a doubt in my mind about whether the AC earth and DC earth are connected. It is an aftermarket installation from before my ownership. I assumed , possibly erroneously, that because I now get this problem that they are (connected.) I will check that this weekend. If it isn't connected, why would my prop be affected before the anode - which does slowly get eroded?

The boats either side of me are quite new - HR37 and a new-ish Fairline MB. I'd be surprised if they were causing issues.

Cheers,
Click to expand...

If it isn't connected, fitting a GI won't help, as we know. But if it's connected and you don't have a GI it's important to fit one. It's not just the boats immediately around you, you could be providing some degree of anodic protection to the whole marina, including metal pilings etc.

However, if you are saying that your prop is dezincifying but the anode is not appreciably depleting, then i'd doubt that you have a galvanic problem. A GI is there to prevent outside forces from depleting your anodes, which isn't happening. So if the anode that you have isn't protecting the prop, that's where you need to look.

What is your current anode situation ? What sort, where, etc. Is the prop and shaft electrically connected to the engine/gearbox/hull anode ?
 
PaulRainbow said:
If it isn't connected, fitting a GI won't help, as we know. But if it's connected and you don't have a GI it's important to fit one. It's not just the boats immediately around you, you could be providing some degree of anodic protection to the whole marina, including metal pilings etc.

However, if you are saying that your prop is dezincifying but the anode is not appreciably depleting, then i'd doubt that you have a galvanic problem. A GI is there to prevent outside forces from depleting your anodes, which isn't happening. So if the anode that you have isn't protecting the prop, that's where you need to look.

What is your current anode situation ? What sort, where, etc. Is the prop and shaft electrically connected to the engine/gearbox/hull anode ?
Click to expand...

More investigation, methinks!
Cheers,
 
Slight thread drift, but as an owner of an older boat where the mains earth is not (as far as I am aware) connected to the DC negative are there any real advantages in making that connection?
 
It's easy to check with a multimeter, of course. If the mains is earthed to the DC negative then the argument is probably one of electrical safety. However, it's a remote risk and if you alleviate it by making the connection you will then need the GI.

Richard
 
neil1967 said:
Slight thread drift, but as an owner of an older boat where the mains earth is not (as far as I am aware) connected to the DC negative are there any real advantages in making that connection?
Click to expand...

The basic idea is that the earth run for shorepower through the marina to an actual earthing point is long and tortuous, so if you got a short to earth there would be still quite a voltage drop over the length of that earth wire. In other words there'd still be quite a bit of voltage at the short until the RCD tripped. The idea of bonding the DC ground to earth is that it gives a much shorter path to ground through the keel, prop etc, but bonding the shorepower earth and boat's ground can allow electrolysis.

The idea of a GI is to isolate the earth circuits within the boat from the incoming shorepower earth until the difference between them exceeds a small voltage (Vf of two diodes effectively so depends upon what material they're using but probably Silicon so 1.4V).

The trouble is that a short can blow the diodes (they're only meant to survive a few milliseconds until the RCD trips) so it's really a use once device. It's necessary if your earth is bonded.

My starting point was the same as yours. I compromised and have used the GI to connect the shorepower earth to the DC ground. My thinking is that it is there to help in the event of a short but if it blows I still have the shorepower earth I had anyway before fitting it.
 
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lpdsn said:
The basic idea is that the earth run for shorepower through the marina to an actual earthing point is long and tortuous, so if you got a short to earth there would be still quite a voltage drop over the length of that earth wire. In other words there'd still be quite a bit of voltage at the short until the RCD tripped. The idea of bonding the DC ground to earth is that it gives a much shorter path to ground through the keel, prop etc, but bonding the shorepower earth and boat's ground can allow electrolysis.

The idea of a GI is to isolate the earth circuits within the boat from the incoming shorepower earth until the difference between them exceeds a small voltage (Vf of two diodes effectively so depends upon what material they're using but probably Silicon so 1.4V).

The trouble is that a short can blow the diodes (they're only meant to survive a few milliseconds until the RCD trips) so it's really a use once device. It's necessary if your earth is bonded.

My starting point was the same as yours. I compromised and have used the GI to connect the shorepower earth to the DC ground. My thinking is that it is there to help in the event of a short but if it blows I still have the shorepower earth I had anyway before fitting it.
Click to expand...

Like fig 2 below ?

1goq9.jpg
 
Thanks Hydrozoan, I am afraid it's just my limited photography skills giving an impression of whitish deposits, in fact it's all now polished bronze, don't think there were any deposits before polishing either. Your point about barnacles cement complexing copper is very interesting, sounds like that's what is happening.
 
I think you're all barking up the wrong tree - I've found that barnacles have managed to makes small crater on all three bronze propellers I've had - and the boat has seldom been in any marina, during the last 20 years.
They do polish out, using a 3M polishing pad.
 
Calcium is a chemical element with symbol Ca and atomic number 20. An alkaline earth metal. Dissimilar metals will cause corrosion as will stray current. A galvanic isolator will stop stray current but not dissimilar metal corrosion.
 
KellysEye said:
Calcium is a chemical element with symbol Ca and atomic number 20. An alkaline earth metal. Dissimilar metals will cause corrosion as will stray current. A galvanic isolator will stop stray current but not dissimilar metal corrosion.
Click to expand...

Do you have any independent published evidence to support either of those claims?

The staining on that propeller is probably caused by the secretions from the barnacles reacting with the copper. As pointed out it will polish out with enough effort.
 
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