Beware anode advice

Javelin

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I've just watched a video released on Facebook from Yachting Monthly giving anode advice.
Pretty happy with most of it until they got to bonding - ie, linking up anodes inside with engine, gearbox etc.
Again, can live with the advice for GRP hulls but even then I worry about daisy chaining the internal links as it often leads to more trouble.

Where I really have issues is bonding anodes on classics and wooden hulls that rely on water to get the hull to "take up" as opposed to wooden hulls that are either sheathed or epoxy coated.
The issue is that wet wood is a conductor and you can end up turning your classic yacht into a big battery.
We see this very often around stern gear, rudder posts, P and A brackets and the ballast keel.
Also its a cause of blowing iron fasteners on older boats.

Better to get an anode asphysically close to the metal you want to protect rather than internal connections.
 

johnlilley

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Absolutely correct. Linking an anode to the internal stern gear or rudder tube will almost certainly cause severe electrolytic decay to the timber surrounding the tubes. Eventually culminating in a new deadwood or horn timber in severe cases. Many surveyors mistakenly in my opinion advise the fitting of anode and wiring them up internally on timber boats. The plank at the anode deteriorates as does the stern gland timber. Tell tale signs of damage is a build up of white soft crud around the anode fastenings and stern gland with furry soft timber. Damage done! It also occurs on GRP boats where the plywood backing pads on anode fastenings and beneath bonded seacocks suffer exactly the same.
 

VicS

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I've just watched a video released on Facebook from Yachting Monthly giving anode advice.
Pretty happy with most of it until they got to bonding - ie, linking up anodes inside with engine, gearbox etc.
Again, can live with the advice for GRP hulls but even then I worry about daisy chaining the internal links as it often leads to more trouble.

Where I really have issues is bonding anodes on classics and wooden hulls that rely on water to get the hull to "take up" as opposed to wooden hulls that are either sheathed or epoxy coated.
The issue is that wet wood is a conductor and you can end up turning your classic yacht into a big battery.
We see this very often around stern gear, rudder posts, P and A brackets and the ballast keel.
Also its a cause of blowing iron fasteners on older boats.

Better to get an anode asphysically close to the metal you want to protect rather than internal connections.
But if an anode is not electrically connected to the item it is intended to protect it wont do so
 

Javelin

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Yes and no. To protect a shaft you can't beat a shaft anode, likewise with a prop and a connected anode. However if you can't fit you have to work based on geography. The salt water acts as an electrolyte and electrical activity will be localised to whatever it is attacking. Therefore if you get an anode close ( a couple of feet or less) to the metal you want to protect, heel fitting, p bracket etc, the anode will tend to absorb all the local activity.
 

lw395

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Surely if an anode is close to a metal fitting but not wired to it, the circuit is completed through the wood and that's when your problems start?
You need the local anode to be connected to the hull side of the fitting, so the two are at the same potential, therefore no current flows through the wood.

Daisy-chain bonding distant fittings to an anode they won't be protected by is something of a different question. I think it can get complex and can adversely affect fastenings etc.
 

VicS

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Surely if an anode is close to a metal fitting but not wired to it, the circuit is completed through the wood and that's when your problems start?
You need the local anode to be connected to the hull side of the fitting, so the two are at the same potential, therefore no current flows through the wood.

Daisy-chain bonding distant fittings to an anode they won't be protected by is something of a different question. I think it can get complex and can adversely affect fastenings etc.
The salt water soaked wood doe not complete the circuit it only forms a parallel circuit to that through the water. A solid electrical connection is needed to complete the circuit.

The normal electrochemical reactions associated with cathodic protection that occur at the surface of the cathode tend cause an increase in pH under static conditions. with normal water movement this does not occur . but where you have a parallel circuit through the wood the pH within the wood will increase. In effect you are producing caustic soda which causes the destruction of the wood structure.
The current flow causing this will not exist if the electrical bond between the anode and cathode completing the circuit does not exist hence the "logic" behind not bonding the anode to the item it is supposed to protect. Unfortunately without the electrical bond completing the circuit the anode will be ineffective

Direct connection between the anode and the item it is to protect is the most effective way of preventing current flow through water saturated wood. One half of the circuit is then via the water, the other half via the direct connection.
 

dombuckley

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IIRC, the practice of bonding all the seacocks together originated in the US, pricipally in the Great Lakes, as a way of distributing / dissipating the effects of lightning strikes. It was never supposed to be about cathodic protection, but has (unwisely IMHO) been adopted as such.

There is also a major problem with daisy-chaining. I stand to be corrected by Vic (who has probably forgotten more on the subject than I will ever learn) but, unless all the skin fittings are exactly the same material, daisy-chaining sets up mini galvanic cells between each pair of fittings. If (for example) the anode is bonded to a bronze fitting, with a DZR brass fitting next down the line, the bronze fitting will be protected not only by the anode, but also by the less-noble skin fitting, which would not have deteriorated if it had not been bonded into the circuit. In this case, the DZR brass fitting will receive little or no protection from the anode, because of the intervening bronze fitting. The only way to protect all skin fittings, is for each to be individually bonded directly to the anode.

As for use on a wooden vessel, always use the very least number of anodes you can get away with. You may need to periodically replace a propeller, but the cost of this pales into insignificance against the cost of planking repairs.

On my own boat, I have a single small pear-shaped anode, which protects the engine (no imternal anode) and the shaft. After finding a some soft wood around the anode bolts a few years ago, I drilled out the holes slightly and glued in nylon tubes. These were packed with grease prior to reinstalling the bolts, and small PTFE backing plates fitted between the backing pad and the bolt washers. As a result, there is no direct contact between the anode bolts and the wooden structure of the vessel: fifteen years on, there has been no further softening of the timber.
 

lw395

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The salt water soaked wood doe not complete the circuit it only forms a parallel circuit to that through the water. A solid electrical connection is needed to complete the circuit.
....

Direct connection between the anode and the item it is to protect is the most effective way of preventing current flow through water saturated wood. One half of the circuit is then via the water, the other half via the direct connection.
These two statements are contradictory, from a circuit point of view.
The current flows through the wood because it completes the circuit anode > Seawater> skin fitting > wood > anode
Bonding the anode to the skin fitting, the circuit becomes anode > seawater > skin fitting > bond wire > anode.

With no bond wire, the wood is not in parallel with the seawater, because the current is going the other way. At least in circuit designer speak. We don't talk of putting a torch bulb in parallel with the battery, we talk of putting it in circuit. That may be nit-picking in layman's terms, but in my world it matters and I hope to avoid people discussing at cross-purposes.

With the bond wire, no current flows through the wood, it is 'shorted out' by the bond wire, it has no volts across it, both ends of the bond wire are at the same potential.

When the fitting is a long way from the anode, I think the situation is more complex, there may be a circuit anode> wood > fitting > bond wire > anode
So if the sea path is too long or tenuous, you start electrolysing the wood.
Catch 22.
Shaft anode, plastic fittings where possible? Local anode for each fitting?


For wood, read also osmotic GRP?
 

VicS

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You say "The current flows through the wood because it completes the circuit anode > Seawater> skin fitting > wood > anode " but this is not a circuit. It is two similar halves of a circuit in parallel with each other

The EMF , which is the result of the difference between the electrode potentials of the anode and the fitting, will be trying to drive a current in the same direction through the seawater and the seawater soaked wood. No current will flow until the anode and fitting are connected electrically to form the return path and complete the circuit.

Then the circuit becomes anode > seawater/ seawater soaked wood > fitting > bonding wire > anode

You also say, "With no bond wire, the wood is not in parallel with the seawater, because the current is going the other way", but the point is the difference in electrode potentials is trying to drive a current in the same direction in both the seawater and the seawater soaked wood.
 

lw395

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You say "The current flows through the wood because it completes the circuit anode > Seawater> skin fitting > wood > anode " but this is not a circuit. It is two similar halves of a circuit in parallel with each other

...
It's a loop, ending where it starts, that's what a circuit is.
 

lw395

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....

You also say, "With no bond wire, the wood is not in parallel with the seawater, because the current is going the other way", but the point is the difference in electrode potentials is trying to drive a current in the same direction in both the seawater and the seawater soaked wood.
The sum of the (open circuit) potential differences must be different around the two halves of the loop to make a current flow. Two different electrolytes?
Chemical reaction at one junction? voltaic cell.
 

VicS

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The sum of the (open circuit) potential differences must be different around the two halves of the loop to make a current flow. Two different electrolytes?
Chemical reaction at one junction? voltaic cell.
If you take an electrolyte, salt solution for example , stick two dissimilar metals in it , copper and zinc for example . you will be able to measure a small emf between the two equal to the difference between their electrode potentials... for copper and zinc in seawater this will be about 0.7 V.
If you now connect the two with a wire a current will flow. being carried in the salt solution by cations, Na+, migrating towards the copper and anions, Cl-, migrating towards the zinc.
The result will be a increase in pH around the copper due to the formation of sodium hydroxide and dissolution of the zinc to form zinc chloride.
 

lw395

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If you take an electrolyte, salt solution for example , stick two dissimilar metals in it , copper and zinc for example . you will be able to measure a small emf between the two equal to the difference between their electrode potentials... for copper and zinc in seawater this will be about 0.7 V.
If you now connect the two with a wire a current will flow. being carried in the salt solution by cations, Na+, migrating towards the copper and anions, Cl-, migrating towards the zinc.
The result will be a increase in pH around the copper due to the formation of sodium hydroxide and dissolution of the zinc to form zinc chloride.
Not arguing withthat.
 

lw395

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Such as resistor with its two ends joined ?
A physical resistor with the two wires joined would indeed be a circuit. The resistor would generate a thermal noise voltage or current. The two wires would have inductance and resistance as well as capacitance due to its physical size.

Perfect resistors only exist in schematics. Real ones are pretty complicated at radio frequencies.
If you try to analyse an ideal resistor shorted with an ideal wire, some circuit simulators crash!
 

Javelin

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Can we at least agree that you have to be very careful on the positioning of anodes on wet wooden hulls and that any bonding needs to be monitored very closely.

We have 7 classics that live on our river that we haul every year and at least 4 have had issues with bonding in the past.
They have all been de-bonded for at least 3 seasons now with no issues.

In fact the only anode issue we've had in the past couple of years was, ironically enough, with matthewriches
His big plastic gin palace (sorry Matt, but it is..) managed to eat its way through the anodes and then one prop on Starboard and the 1 1/2"shaft on port, all in 8 months.
We're blaming it on a steel yacht with lots of panels that he was moored next to for 6 months.
 

VicS

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Can we at least agree that you have to be very careful on the positioning of anodes on wet wooden hulls and that any bonding needs to be monitored very closely.

We have 7 classics that live on our river that we haul every year and at least 4 have had issues with bonding in the past.
They have all been de-bonded for at least 3 seasons now with no issues.

In fact the only anode issue we've had in the past couple of years was, ironically enough, with matthewriches
His big plastic gin palace (sorry Matt, but it is..) managed to eat its way through the anodes and then one prop on Starboard and the 1 1/2"shaft on port, all in 8 months.
We're blaming it on a steel yacht with lots of panels that he was moored next to for 6 months.
If you remove the anode bonding you will avoid the electrochemical decay of wood associated with cathodic protection but as I pointed out in #3 the anodes will then no longer be providing any protection to the fittings structures they were fitted to protect. You might just as well not have the anodes there at all. Provided all these structures are made of corrosion resistant materials, eg bronze , you will have no issues. Any structures that are not corrosion resistant are best protected by anodes directly mounted on them , where this is possible.

As far as Matthews problems are concerned the fact that props and shaft were lost indicates that there was more at play than just galvanic corrosion due to dissimilar metals. It was most likely due to electrolysis being driven by a fault on a 12 or 24 volt DC electrical system. It could have been on any near by vessel connected to the shorepower supply or even on Mathew's own boat. A steel hull alone will not cause corrosion of the more noble metals used for props and shafts
 

johnlilley

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Fitting an anode with insulated washers and bolt tubes isolation might help reduce plank damage but if connected to stern tube and rudder tube inside will not prevent electrolytic damage occuring here. This damage if extreme and unnoticed , can cause , in some cases, in a large vessel tens of thousands of pounds. Even a small ,less than30ft it can be as much as £10000 if the aft horn timber and deadwood require renewal. Look out for the classic build up of a white soft crud, around any wired & protected metal internally. Damage more severe internally because the area is not constantly flushed away by external seawater. It becomes concentrated
 

Keith 66

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In 45 years working on boats many of them wooden i have seen way more problems on boats where the skin fittings & stern gear are bonded than not. The phenomenon is called delignification, as the sodium hydroxide builds up in the timber around the metal part it destroys the lignin in the cells of the wood itself. While the wood stays wet or damp there may be little apparent decay beyond a bit of furriness but as it drys out the wood will disintegrate.
 

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