Tracking down 12 volt electrolysis issues

Rich Stidger · Mar 2, 2015

I would recommend getting a book on electrolysis and a silver/silver chloride half cell. The measurements need to be done in the water using the silver/silver chloride half cell as a reference. You should be able to figure out the source of the leakage that is causing your zincs to vaporize.

centerline · Mar 2, 2015

have you tried disconnecting the battery for a month and see what happens to the zinc?... the problem can also be DC related, and unhooking the batts will tell without any tools, although with the proper tools you can find out within a few minutes.

there is a LOT of information online about it.. google "severe zinc erosion on prop shaft", and you will get all the reading you want..

sea_casper · Mar 3, 2015

centerline said:
have you tried disconnecting the battery for a month and see what happens to the zinc?...

I would hate to see a boat sink from a relatively small leak, with the battery disconnected.

With the battery selector set to "off", the bilge pump should still be active, most everything else will be disconnected from the battery, except for engine alternator.

njlarry · Mar 3, 2015

Read Don Caseys book on boat on electrical systems or Calder's book on matainace. They both are good resources.
They both will recommend disocnnecting your through hulls from the bonding system as marina stray currents will only accelerate damage to bonded thru hulls. I have been studying electrical grounding all winter and this spring all my thru hulls will isolated. The bonding sytem will go to the keel NOT the engine.
Finding the source will take some time but a VOM meter will do it.
You can save yourself some money with a zinc hung over the side connected to a VOM and the other lead to your shaft or any other grounded metal. The silver/silver anode is big bucks for a very limited use. Search this topic on this site for more details.
But you do need to find the problem before expensive damage is done.

Bill Roosa · Mar 3, 2015

The boat is fully bonded. That is probably the problem. You have electrically connected all the metal parts and the water completes the circuit. If you have ANY dissimilar metals bonded together then you will get electrolysis. Bonding only works when all the metals connected are identical.
Do you have different metals in your below the water line thru hulls, prop, shaft etc?

mitiempo · Mar 3, 2015

FBSabre36 said:
There is no way to easily completely disconnect the batteries or shut down the Link monitor.

Yes there is. Take the positives off the battery posts.

centerline · Mar 3, 2015

sea_casper said:
I would hate to see a boat sink from a relatively small leak, with the battery disconnected.

With the battery selector set to "off", the bilge pump should still be active, most everything else will be disconnected from the battery, except for engine alternator.

If the boat fills with enough water in a month or even two months that would even cause one to wonder if it may sink, then it isnt a small leak...

Maine Sail · Mar 4, 2015

FBSabre36 said:
My Sabre 36 burns through shaft zincs pretty quickly (one a month and then some). The boat is kept on different moorings and is not near any shore power sources so I am assuming this is not AC related. There is no corrosion on the prop and the heat exchanger consumes one pencil zinc every three months.

The boat is fully bonded with non-tinned copper wire. I assume there is resistance in some of the connections causing the issue? What is the most effective way to test this when then boat is out of the water?

Are there any other places to look for causes?

The alternator output goes directly to the house bank positive with no shutoff. When the engine is off can this cause current leaks back through the engine (if this makes any sense)?

Thanks,
Peter

First questions:

What is your prop-shaft material?
What is your prop material?
What type of packing do you use?

Quick Test Procedure:

Turn off all DC items that are normally switched off for mooring storage.

Remove the main negative terminal from the house bank

Set your DVM to amperage & 10A scale

Insert the DVM between the main negative conductor and the house bank negative terminal.

What is your reading?

If 0A change the scale to the next lowest which is often 2A

What is your reading?

Now disconnect the Ah counter.

What is your reading? Did it change?

Now disconnect the alt + etc. etc.

Next test the start battery using the same method.

Continue on this path until you find all items that lead you to a current reading.

One of these circuits will be your culprit.

Most often it is the bilge pump circuit but any wire that runs through the bilge can be it.

Bonding Circuit:

Find where your bonding circuit attaches to your ships ground. This should be ONE point, with Sabre it is usually the engine and BTW that crazy SS busbar for neg connections on the engine block needs to go. SS is a horrible conductor...

Do the same with your DVM, insert it between the engine ground and the bonding system green wire.

Now go and turn on DC circuits, one at a time, being careful not to use circuits that draw more than 10A. To test higher amp circuits you will need a DC clamp meter. Your bonding system reading should remain at 0.00A always. If you get a reading doing this then something is using the bonding system as a DC negative return to the battery bank.

Start with those simple tests then we can move onto the next level....

These tests should best done in the water because a DC leak is usually using bilge water as its path and they won't show up on land. Only your parasitic loads will show up on land but even one of these circuits could have a break in a wire...

If you suspect a DC leak I would not advise unbonding until you have it sorted out. The bonding system will spread the load and slow the corrosion. I have seen boats get unbonded and sink a month later because the DC leak now went straight for the closest & easiest path, a single seacock where as before the path was all the underwater metals thus letting more metal dissipate the leak. You may not even have a DC leak or only a leak when something is active, such as a bilge pump.

As you now know it is a myth that corrosion only happens in marinas. A huge portion of the DC corrosion folks blame on marinas is often originating on-board their own boats in the form of DC leaks. DC leaks are far more rapid in erosion speed and far faster than any AC leak. I do nearly as much DC corrosion repair work on mooring boats as I do on docked boats...

I believe on this forum only Charles Irwin and myself are professionally trained, educated and practice in marine corrosion issues. There is a lot of myth and lore out there so be very careful with some advice you may get.

In a perfectly wired boat unbonding can be a good practice but in a poorly wired boat this can be a very bad idea. Unfortunately most boats fall into the "poorly wired" category... If you can't solve this on your own you can go to the ABYC site and find someone in your area who is an electrical systems specialist. What I have give you above is a quick and dirty test there is a lot more but starting there, while in the water, will lead you down the path you are looking for in terms of DC leaks.

njlarry · Mar 4, 2015

Maine Sail said:
First questions:

If you suspect a DC leak I would not advise unbonding until you have it sorted out. The bonding system will spread the load and slow the corrosion. I have seen boats get unbonded and sink a month later because the DC leak now went straight for the closest & easiest path, a single seacock where as before the path was all the underwater metals thus letting more metal dissipate the leak. You may not even have a DC leak or only a leak when something is active, such as a bilge pump.

Can a single seacock still corrode if it is totally isolated, ie: unbounded and only connected to a rubber hose and fiberglass hull?

Maine Sail · Mar 4, 2015

njlarry said:
Can a single seacock still corrode if it is totally isolated, ie: unbounded and only connected to a rubber hose and fiberglass hull?

Yes! If that seacock sits in bilge water or even has a salt trace an on-board DC leak can find its way through that seaock and corrode it. If the seacocks are all at the same "potential" they share this "load" and the corrosion will be considerably slower. The problem still needs to be solved, but unbonding a boat, on a whim, without understanding why you are doing it, can create a more dangerous situation than it prevents. The OP is NOT in a marina so if he is having rapid erosion of anodes the issues could be a potential on-board DC leak.

Folks really need to understand the difference between galvanic corrosion or stray current in a marina, originating off your vessel, and DC current corrosion originating on board your own vessel, and that point is so often entirely missed.

Stan Honey's article is the one most often misunderstood:[FONT=&quot]

"It's worth understanding the reason. In an increasing number of marinas, there are substantial DC electric currents running through the water. If your bits of immersed metal are bonded, the electric current will take the lower resistance path offered by your boat in preference to the water near your boat, and the current will flow into one of your bits of metal, through your bonding wires, and then out another bit of metal. The anodic bit of metal or through-hull that has the misfortune to be on the "out current" side of the current running through your bonding system will also become "out metal" and will disappear, sometimes rapidly."[/FONT]

[FONT=&quot]This is the part everyone misses:[/FONT]

[FONT=&quot]"In the old days, the technique of bonding everything together worked okay. In its defense, the "bond everything together" approach makes your boat less sensitive to electrolytic corrosion that can result from faulty wiring on your own boat. The problem is, the "bond everything" approach leaves your boat totally defenseless to wiring errors in nearby boats and nearby industry, that cause stray DC currents to run through the water. [/FONT]

[FONT=&quot]Today the technique of bonding everything together would still work fine if your boat spent all of its time on the high seas, in remote anchorages, or in marinas that were wired perfectly and in which all of the nearby yachts were wired perfectly."[/FONT]

[FONT=&quot]I recently helped Stan design the lithium battery system for his own boat and we had a good conversation about that article. He agreed that it is often misunderstood.
[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]We specifically discussed the boat where the owner was having issues so he unbonded. He was on a mooring and the DC source was his own boat.......:cussing: His rapid anode erosion alerted him to the problem but he chose not to fix it. Instead he unbonded and one seacock would up with greatly accelerated erosion, enough to start a leak. This seacock was very close to a bad crimp on a bilge pump that often got submerged in bilge water.
[/FONT]

njlarry · Mar 4, 2015

Thank you, very much!
I somehow stumbled upon Stan Honey's article and learn something with every read.

sea_casper · Mar 4, 2015

centerline said:
If the boat fills with enough water in a month or even two months that would even cause one to wonder if it may sink, then it isnt a small leak...

You assume that because you have no known leaks today, you will have no leaks the next day/week/month/season.

You can believe that if you will, but I personally would not leave my boat unattended without a functioning bilge pump.

It doesn't take a lot of uninvited water to spoil your day/week/month/season.