Galvanic current

[davierobb]

A friends motor boat nearly sunk when the Annode disappeared and the mild steel retaining bolt was dissolved.
Boat is now back in the water with a new set of annodes, Rudder, shaft, Hull and metal frame. The starter motor, alternator, bow thruster motor, charger and heater have all been replaced.
I've used a silver chloride reference cell to check the annodes are providing protection and all reading are consistent and considered good, at around -0.90V. We've checked that the readings are consistent regardless of what elements of the system are on off, shore power, batteries, all dc circuits active etc. I'm guessing that we probably fixed the issue when we replaced one of the damaged items.
The question for the forum is, we now have a current of 30ma flowing into the boat bonding circuit from the annode, does this seem correct? This is stable regardless of electrical configuration.
My own boat, eclipse motor sailor has an annode current of around 11ma.
Any informed opinions appreciated.

 

[Tranona]

This sounds very odd. What sort of boat is it? If it is GRP then the need for anodes and any form of "bonding circuit" is very limited. Normally all that needs protecting from galvanic action is the propeller(s) against the stainless shaft. A bow thruster has its own anode and unrelated to any other fitting on the boat. As it is a motor boat the rudder(s) may be bronze with a stainless stock which may require their own anode. Shorepower and DC electrical installations have nothing to do with galvanic action. Through hulls and seacocks do not need any form of anode or bonding.

I know that does not answer your question, But the test of whether the anode is bonded correctly to the item it is protecting is to measure the resistance between them. It should be negligible - usually figure quoted is under 0.2 ohms.

 

[VicS]

A friends motor boat nearly sunk when the Annode disappeared and the mild steel retaining bolt was dissolved.
Boat is now back in the water with a new set of annodes, Rudder, shaft, Hull and metal frame. The starter motor, alternator, bow thruster motor, charger and heater have all been replaced.
I've used a silver chloride reference cell to check the annodes are providing protection and all reading are consistent and considered good, at around -0.90V. We've checked that the readings are consistent regardless of what elements of the system are on off, shore power, batteries, all dc circuits active etc. I'm guessing that we probably fixed the issue when we replaced one of the damaged items.
The question for the forum is, we now have a current of 30ma flowing into the boat bonding circuit from the annode, does this seem correct? This is stable regardless of electrical configuration.
My own boat, eclipse motor sailor has an annode current of around 11ma.
Any informed opinions appreciated.

The loss of anode and retaining bolt would lead me to suspect an electrical fault causing electrolysis, especially if this occurred in a short time scale. However you readings with an Ag/AgCl electrode and anode current being normal and unaffected by connection/disconnection of shore power and DC supply suggest all is now OK.
The anode current will enable you to calculate a rate of loss of zinc from the anode. (I make it approx 0.3 kg per year, but please check my arithmetic.)
If there is an automatic bilge pump check that this does not affect the anode current.

 

[PeterBoater]

Could be a fault in the shore power or on an adjacent boat (ie it is using your friend's anodes).

 

[davierobb]

Apologies, I missed some very pertinent info, the boat is never left with shorepower attached and batteries are always isolated before leaving. An automated bilge pump and heater are the only live circuits when boat not in use. Bilge pump was blocked by unused tywrap bracket which was probably there since new.
it is a Merry Fisher with inboard diesel.
Thanks for previous replies. My best guess is that the issue was caused by one of the replaced items.
Our only concern was whether the 30ma current draw from the anode was within an expected range.

 

[Farmer Piles]

If it's in a marina then I think that Peter's suggestion above is not unlikely.

 

[PaulRainbow]

If it's in a marina then I think that Peter's suggestion above is not unlikely.

If it's in a marina then I think that Peter's suggestion above is not unlikely.

But post #5 says "the boat is never left with shorepower attached"

 

[davierobb]

If it's in a marina then I think that Peter's suggestion above is not unlikely.

It is in a marina but Shore Power is never left connected when boat left unattended.

 

[William_H]

My reaction to 30ma anode current is that it is far too high. (with no experience on my part) I would check it with heater and bilge pump disconnected. ie batteries disconnected. ol'will

 

[davierobb]

The loss of anode and retaining bolt would lead me to suspect an electrical fault causing electrolysis, especially if this occurred in a short time scale. However you readings with an Ag/AgCl electrode and anode current being normal and unaffected by connection/disconnection of shore power and DC supply suggest all is now OK.
The anode current will enable you to calculate a rate of loss of zinc from the anode. (I make it approx 0.3 kg per year, but please check my arithmetic.)
If there is an automatic bilge pump check that this does not affect the anode current.

VicS, your comment about 30ma draw resulting in the loss of 0.3kg of anode was something I hadn't thought of. I found some other information on the subject and you arithmetic was spot on, thanks. The anode is 1kg so I'm thinking a theoretical 3 year lifespan suggests that the 30ma draw is more than likely to be OK.

 

[davierobb]

My reaction to 30ma anode current is that it is far too high. (with no experience on my part) I would check it with heater and bilge pump disconnected. ie batteries disconnected. ol'will

My own boat is 10ma.
We also disconnected both batteries on friends boat and there was no change, anode current remained consistently around 30ma. Connecting disconnecting the main also had no affect.
With no knowledge on the subject 30ma did indeed seem to be too high but after considering some of the responses then I'm thinking it's probably OK. The 0.3Kg loss of anode mass at 30ma drain gives me the confidence to think that the anode is not going to dissappear in less than six months, the last anode and retaining bolt disappeared within six months.

 

[Tranona]

My own boat is 10ma.
We also disconnected both batteries on friends boat and there was no change, anode current remained consistently around 30ma. Connecting disconnecting the main also had no affect.
With no knowledge on the subject 30ma did indeed seem to be too high but after considering some of the responses then I'm thinking it's probably OK. The 0.3Kg loss of anode mass at 30ma drain gives me the confidence to think that the anode is not going to dissappear in less than six months, the last anode and retaining bolt disappeared within six months.

As I suggested in post#2, galvanic circuits in this context are nothing to do with the DC or AC circuits as the flow is between the cathode (the bit of metal being protected) and the anode. Never thought of it in terms of measuring the flow as this (maybe?) only occurs when both are immersed in the electrolyte (seawater). The test of whether an adequate circuit exists is measuring the resistance of the wire between the 2, or in the case where the anode is attached directly (shaft anode, prop anode etc) between the 2 items. The challenge with many installations is bonding a hull anode to the shaft/prop assembly where the wire is often attached to the gearbox and then across the coupling to the shaft. A lot of connections. A better alternative is the MG Duff Electroeliminator that has brushes running direct on the shaft. This is what is fitted to my boat as in the pic. Not always feasible as many boats don't have enough room, but very common on big MOBOs.

Getting back to your friends boat, I have seen Merry Fishers with anodes on the prop nut and no other anodes, but guess they don't last long. What other anodes does the boat have?

 

[PaulRainbow]

As I suggested in post#2, galvanic circuits in this context are nothing to do with the DC or AC circuits as the flow is between the cathode (the bit of metal being protected) and the anode. Never thought of it in terms of measuring the flow as this (maybe?) only occurs when both are immersed in the electrolyte (seawater). The test of whether an adequate circuit exists is measuring the resistance of the wire between the 2, or in the case where the anode is attached directly (shaft anode, prop anode etc) between the 2 items. The challenge with many installations is bonding a hull anode to the shaft/prop assembly where the wire is often attached to the gearbox and then across the coupling to the shaft. A lot of connections. A better alternative is the MG Duff Electroeliminator that has brushes running direct on the shaft. This is what is fitted to my boat as in the pic. Not always feasible as many boats don't have enough room, but very common on big MOBOs.

Never seen a MG Duff Electro eliminator on a big MOBO. They usually have a bridging wire across the flexible coupling.

 

[Alex_Blackwood]

My own boat is 10ma.
We also disconnected both batteries on friends boat and there was no change, anode current remained consistently around 30ma. Connecting disconnecting the main also had no affect.
With no knowledge on the subject 30ma did indeed seem to be too high but after considering some of the responses then I'm thinking it's probably OK. The 0.3Kg loss of anode mass at 30ma drain gives me the confidence to think that the anode is not going to dissappear in less than six months, the last anode and retaining bolt disappeared within six months.

30Ma does seem high but the current will depend on the area being protected. i.e. It will take more to protect 2 square mtr. than 1. I know that may be teaching Grandmothers about eggs. However have you considered if the system is protecting something nearby? A pile or another boat?
I think, off the top of my head that 2Ma / square foot rings a bell

 

[davierobb]

As I suggested in post#2, galvanic circuits in this context are nothing to do with the DC or AC circuits as the flow is between the cathode (the bit of metal being protected) and the anode. Never thought of it in terms of measuring the flow as this (maybe?) only occurs when both are immersed in the electrolyte (seawater). The test of whether an adequate circuit exists is measuring the resistance of the wire between the 2, or in the case where the anode is attached directly (shaft anode, prop anode etc) between the 2 items. The challenge with many installations is bonding a hull anode to the shaft/prop assembly where the wire is often attached to the gearbox and then across the coupling to the shaft. A lot of connections. A better alternative is the MG Duff Electroeliminator that has brushes running direct on the shaft. This is what is fitted to my boat as in the pic. Not always feasible as many boats don't have enough room, but very common on big MOBOs.

Getting back to your friends boat, I have seen Merry Fishers with anodes on the prop nut and no other anodes, but guess they don't last long. What other anodes does the boat have?

There's a shaft, 1kg Hull and 1kg fitted to a mild steel frame that protects the prop and shaft from grounding. Only the 1kg Hull anode is bonded inside the boat.
The silver chloride ref cell is possibly a more consistent method of deterring the level of protection. If you get different voltage readings around tge system then there is an earth problem. I believe this is the method used when performing a galvanic survey on commercial ships. Thanks for the input.

 

[PaulRainbow]

There's a shaft, 1kg Hull and 1kg fitted to a mild steel frame that protects the prop and shaft from grounding. Only the 1kg Hull anode is bonded inside the boat.
The silver chloride ref cell is possibly a more consistent method of deterring the level of protection. If you get different voltage readings around tge system then there is an earth problem. I believe this is the method used when performing a galvanic survey on commercial ships. Thanks for the input.

Which model of Merry Fisher, sounds like it could be an 805.

The anode on the prop protector is a total waste of time if that's the case. It's just some steel welded together and bolted to the skeg/hull. Just one type of metal, only fixed to GRP, no need for an anode. I had an 805, had a prop anode, shaft anode, hull anode and about 4 round ones on the prop protector. The piddly prop anode didn't last that long and the shaft anode just about made 12 months. I got rid of all of them, fitted a bridge wire on the shaft coupling and a much bigger hull anode. Never had a problem.

 

[Alex_Blackwood]

davierobb said:
There's a shaft, 1kg Hull and 1kg fitted to a mild steel frame that protects the prop and shaft from grounding. Only the 1kg Hull anode is bonded inside the boat.
The silver chloride ref cell is possibly a more consistent method of deterring the level of protection. If you get different voltage readings around tge system then there is an earth problem. I believe this is the method used when performing a galvanic survey on commercial ships. Thanks for the input.

Which model of Merry Fisher, sounds like it could be an 805.

The anode on the prop protector is a total waste of time if that's the case. It's just some steel welded together and bolted to the skeg/hull. Just one type of metal, only fixed to GRP, no need for an anode. I had an 805, had a prop anode, shaft anode, hull anode and about 4 round ones on the prop protector. The piddly prop anode didn't last that long and the shaft anode just about made 12 months. I got rid of all of them, fitted a bridge wire on the shaft coupling and a much bigger hull anode. Never had a problem.

From all of the above, have you considered that you may be over protecting? I don't know but may be worth considering in the equation.
Also have you considered moving to a different location i.e. anchor and take readings there to see if they change. May point to something in the present mooring area.