Stray Current Isolation Testing?

[Dave Lochner]

Originally Posted by RaymondR

According to a paper on cathodic protection the current producing potential for zinc is 780 Amp hours/kg of zinc consumed. 780Amphours /.03Amps = 26,000 hours or about 3 years if your shaft anode weighed a kilogram.

A standard zinc weighs about half a kilogram, so in theory it would have about 13,000 hours of life with a .03 Amp draw. That would be equal to about 18 months of life assuming the zinc is completely consumed, or about 9 months if consumed to 50% which is not too far out of line with the OPs original experience of zincs lasting 6 months. At 6 months the zinc would be at 66% of original weight.

Going back to the original question, what changed on the boat between the 2 years? Might it be something as simple as a better prep job on the shaft before installing the new zinc, yielding a better more electrically efficient connection?

[Schooner Chandlery]

Originally Posted by boatbod

Yes you need a silver-silver half cell and the corrosion reference work book. Basically measure from the half cell h immersed over the side) to various bonded metals in the boat. 750-950mV is good. Anything less is suspect.

The range you propose is perhaps good for a fiberglass boat but would be disaster for a wood boat. Wood boats also do better if NOT bonded. You can ruin the bronze fasteners as well as the wood by bonding a wood boat and letting all that current run through.

Our boat is wood so we have to be very careful to assure that the we have just the right size of anode and maintain it. We use a zinc on the prop shaft but on the hull/keel/rudder interface, we use mild steel as, on our boat, zinc anodes are too aggressive and the wood adjacent to the anode could be damaged. So, we maintain between 500 and 630 millivolts. If we go over 800 millivolts or under 400 millivolts we risk damage to the wood and bronze/brasses respectively. We replace the zinc on the prop shaft every 8-12 months or so, the mild steel anodes are replaced every 4-8 months or so to maintain the proper voltage potential. Fiberglass boat owners don't have the same concern on hull material degradation and typically can go with a larger anode and longer between service.

We use a silver-silver chloride half cell to measure voltages and keep track of whether or not we're in an electrically active marina/near a boat leaking current into the water, etc.

On the shore power, we have a toroidal isolation transformer rather than a galvanic isolatorr. Works great.

[boatbod]

Good info Schooner. It's easy to overlook wooden boats and they behave quite differently in so many ways.

[zstine]

Originally Posted by RaymondR

...
According to a paper on cathodic protection the current producing potential for zinc is 780 Amp hours/kg of zinc consumed. 780Amphours /.03Amps = 26,000 hours or about 3 years if your shaft anode weighed a kilogram.

Lot of variables.

If you want to ensure the submerged metal parts don't corrode and try to reduce the consumption of your shaft anode I'd go the external anode connected to the bonding route. Go buy a big anode and hang it over the side on a wire which is connected to the bonding system.

RaymondR.. numbers, it's like music to my engineering ears! for clarity, i was running 2 shaft zincs and they were gone in 3mo... lead keel. I currently have one zinc on the shaft and a large fish zinc hanging off the shroud too.

solving for the leaking current for my lost zinc (assuming 1kg for the pair I had) based on your calc...
leaking Amps = 780Ahr/(3mo*30day/mo*24hr/day) = 0.36Amps or 360mA
Now i have an idea of what I'm looking for... it's less than I thought it'd be.

I ordered the reference cell yesterday and boatzincs shipped today! should be here soon and I'll report my findings. As for the question of what changed, I put in a battery monitor and replaced the bilge pump but cannot find any issue with them specifically.

Thanks!

[RaymondR]

Originally Posted by Dave Lochner

A standard zinc weighs about half a kilogram, so in theory it would have about 13,000 hours of life with a .03 Amp draw. That would be equal to about 18 months of life assuming the zinc is completely consumed, or about 9 months if consumed to 50% which is not too far out of line with the OPs original experience of zincs lasting 6 months. At 6 months the zinc would be at 66% of original weight.

Going back to the original question, what changed on the boat between the 2 years? Might it be something as simple as a better prep job on the shaft before installing the new zinc, yielding a better more electrically efficient connection?

Yes it is possible that a better connection between the zinc and the shaft allowed more current to pass and that caused faster zinc depletion.

There are numerous factors which include the overall resistances within the galvanic system. Be mindful that it is a closed circuit which includes the bonding system, the bonded items(the anodes and cathodes) in contact with the water the vessel is immersed in (the electrolyte) and that there are also exposed surface parameters.

One of the things I noticed when I launched my steel boat was that the anode life shortened as the immersion period extended. This was back in the days when we were allowed to use TBT antifouls and the boat was in the water for nearly five years before it's first slipping. The first set of anodes lasted about 2.5 - 3 years and after that I needed to renew them every twelve months or so.

What I think had occurred was that when the epoxy coatings were new they proved excellent insulators and consequently there was very little surface area exposed to the sea water. However, epoxy's will absorb around 2% - 3% of their weight of water and as this occurred the water absorbed provided a lower resistance, current path for a greater area of steel and consequently more zinc was consumed. (This phenomena is also what causes blistering of coatings where oxides are created on the metal surface underneath the coating if one over zincs)

So, it could have been better connections improving the bonding circuit, a spell of exceptionally dry weather leading to a salinity increase in the water, the water finding it's way through the coatings and thereby lowering their resistance to current flow whilst exposing more metal to the electrolyte. And/or, there's probably some my old mind has forgotten or that I'm unaware of.

[boatbod]

I once worked on a C&C who's lead keel was so reactive that both the keel and saildrive were eating each other away despite all attempts to get coatings to stick etc. In the end we disconnected the bonding wire between the two components and now both coexist quite happily. The keel floats at it's own potential and the engine & saildrive (a Yanmar) are bonded to the vessel grounding system and galvanic isolator.

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[RaymondR]

Originally Posted by boatbod

I once worked on a C&C who's lead keel was so reactive that both the keel and saildrive were eating each other away despite all attempts to get coatings to stick etc. In the end we disconnected the bonding wire between the two components and now both coexist quite happily. The keel floats at it's own potential and the engine & saildrive (a Yanmar) are bonded to the vessel grounding system and galvanic isolator.

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The galvanic potentials of lead is about -0.19V to -0.25V and aluminium alloys about -0.76V to -1.0V so they will generate about 0.5V difference when immersed in an electrolyte. This was an instance when isolation appears to have worked. Measuring the voltage or current flow between the lead keel and the bonding system would have identified the problem.

[boatbod]

Originally Posted by RaymondR

The galvanic potentials of lead is about -0.19V to -0.25V and aluminium alloys about -0.76V to -1.0V so they will generate about 0.5V difference when immersed in an electrolyte. This was an instance when isolation appears to have worked. Measuring the voltage or current flow between the lead keel and the bonding system would have identified the problem.

Yes, that's exactly how I found it.

[zstine]

I got my boatzinc reference probe and did some testing. Testing was done at anchor with the probe hung over the side near the prop. Here’s the results….
Test conditions. “Fish” zinc on shroud and 1 shaft zinc

______________Bat sw OFF _____ lots ON _____ Eng@1100rpm
Shaft__________ -957mV__________ -965mV_______ -935mV
Engine_________ -850mV_________ -885mV________ -903mV
Bond Wire @ bat -855mV__________ -878mV________ -900mV
Keel bolt_______ -867mV__________ -858mV________ -850mV
Bilge Water____ -579 to -649mV___ -574 to -640mV*_ -581 to -776mV
Bnz Thru (unbond) -146mV________ -146mV_________ -146mV

* Bilge pump running/not running had no effect on voltage in bilge water
Bilge water voltage would start out high (more neg like -660) then reduce over a few min to -580mV… maybe because I’m using el cheepo DMM?

Partial Repeat with fish zinc out of circuit, only 1 shaft zinc

_________________Bat sw OFF________ lots ON
Shaft______________ -890mV_____________ -875mV
Engine_____________ -657mV_____________ -663mV
Bond Wire @ bat_____ -663mV_____________ -670mV
Keel bolt____________ -658mV_____________ -675mV
Bilge Water_________ -385 to -420mV______ -402 to -470mV
Bnz Thru (unbond)____ -145mV____________ -144mV


I would love people to weigh in on some conclusions. I’ve drawn my own, and have some questions, but I don’t want to influence you, so….
The bonding wire was connected for all tests. Runs from the negative side of battery, to engine, engine to keel bolt, keel to mast. I have lost quite a bit of lead in my keel.. a few pounds probably over the last 2 seasons.

Thanks for your comments/suggestions.
Zach

[boatbod]

Most obviously the engine and shaft are not well connected, so if you don't have hull zinc, your underwater metals other than the shaft (with it's own zinc) are largely unprotected.

I think you should also investigate the difference in potential between dc off and on readings. The change may simply be due to current flow in the dc negative wires, but it's non ideal since you don't really want current flow in any part of the bonding system.

[Doodlebug]

I would think the Zinc fish should have been connected to the grounding bus bar not the shroud. Incase the shroud is not bounded or has a bad connection.

[Lepke]

Some marine electronics, if wired backwards, will allow current to flow even with the item off. And yet operate normally. Also corrosion buildup on electrical switches, etc., can bleed over. I would put a mv instrument between the - Bat and it's cable and disconnect each device and dc line to see if there is a change.

[Dulcesuenos]

-900mv seems correct according to the zinc experts.
Some good info here
http://boatzincs.com/corrosion-quiz.html


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[zstine]

Originally Posted by boatbod

Most obviously the engine and shaft are not well connected, so if you don't have hull zinc, your underwater metals other than the shaft (with it's own zinc) are largely unprotected.

I think you should also investigate the difference in potential between dc off and on readings. The change may simply be due to current flow in the dc negative wires, but it's non ideal since you don't really want current flow in any part of the bonding system.

Totally agree with the observation that the shaft and engine are not well connected electrically. That could explain the high amount of lead consumed off my keel, but I would expect lower shaft zinc consumption, not higher, b/c of that...hmm

The difference between DC ON to OFF does get up to the 30mV range, so that's not great. But based on the amount of zinc I'm going through, the calculation above suggests a few hundred mV is the magnitude of concern. I will certainly keep my minds eye on the DC equipment, but it's not a smoking gun.

What about those low -145mV readings on the bronze unbonded through hull. Does that provide any info/concern?

[zstine]

Originally Posted by Doodlebug

I would think the Zinc fish should have been connected to the grounding bus bar not the shroud. Incase the shroud is not bounded or has a bad connection.

thanks, The shroud is connected to the bonding system via mast & 'lightning wire' to keel. You can see the approx -200mV shift in the bonding system potential between the 2 data sets, with and without the fish zinc. I think the fish is helping me keep the lead on my keel!