Zinc Wasting

[Frankly]

The 1 volt with the silver electrode is about correct (memory a little stale on that). To measure current you have to be in series with the load (unless you have a DC clip on ammeter and most not accurate at the current levels you are expecting).

Just trying to get a handle on leakage/ protection current levels. No way to measure how much of the zinc is being consumed by that great big AP but if the bonding is up to snuff then you should be able to measure the current flow protecting everything connected by the bonding wires. There is an approximate relationship between current flow and zinc consumption, but don't remember it off the top of my head.

[senormechanico]

I think 950 millivolts is more than you need.

I built my own circuit and am very happy with long zinc life and no corrosion.

See the thread:

http://www.cruisersforum.com/forums/...ll-192161.html

[a64pilot]

Originally Posted by senormechanico

I think 950 millivolts is more than you need.

I built my own circuit and am very happy with long zinc life and no corrosion.

See the thread:

http://www.cruisersforum.com/forums/...ll-192161.html

That is a little beyond my capabilities, But thanks

[a64pilot]

Originally Posted by Frankly

The 1 volt with the silver electrode is about correct (memory a little stale on that). To measure current you have to be in series with the load (unless you have a DC clip on ammeter and most not accurate at the current levels you are expecting).



Just trying to get a handle on leakage/ protection current levels. No way to measure how much of the zinc is being consumed by that great big AP but if the bonding is up to snuff then you should be able to measure the current flow protecting everything connected by the bonding wires. There is an approximate relationship between current flow and zinc consumption, but don't remember it off the top of my head.

I understand it’s amperage that is the actual current here, and yes even a cheap multimeter will measure low amperage if connected in series and all I have ever had have a fuse in them to protect them. I don’t think a clamp will measure small amperage very well as of course it’s measuring a magnetic field, and small current = tiny field.

I don’t think current flow is from the zinc through the half Cell though, it’s from the zinc to the water, and your not in that circuit, so you can’t measure current?

If I understand this half Cell thing, it’s measuring the difference in voltage to try to come up with current flow. You create a cell with it being half the Cell, the zinc is the other half. Any impressed current if I use that word correctly will change the voltage of the Cell.
If I understand just dropping in a half Cell won’t necessarily tell you a whole lot, have to get a voltage measurement with all electrical devices disconnected, to include the shorepower cable, not just turned off, but disconnected.
Then reconnect power and see if there is a voltage change, then start turning on devices one at a time looking for a jump, if you get a jump then of course you have a leakage in that circuit

[CharlieJ]

a64pilot:
1. You are correct. The anode must be electrically connected to the metal it is to protect and both the anode and the protected metal must be in the same body of water.
2. -950mVDC against a AgAgCl reference cell is a nearly ideal hull potential in sea water for a fiberglass boat with no underwater aluminum components.
3. When you disconnected the shore power and the hull potential did not change from -950mVDC against the reference cell, that shows that your galvanic isolator is performing and that no cathodic protection current is leaving your boat. BTW, if you tested the "dock potential" (dock safety ground against the reference cell) you would find that it is generally in the -500 to -650mVDC range.
4. Monitoring hull potential while cycling all electrical equipment is how own boat stray current is found. When a leaking piece of DC equipment is turned on, the hull potential will jump positive very significantly. The -950mVDC would jump more positive to; e.g., -875mVDC. Since you did not observe that, your boat is not creating stray current.
5. Galvanic corrosion is a slow (months, years) process. Stray current corrosion is a very rapid (hours, days) process. Since you have not observed this kind of wastage, stray current is ruled out.

Is the boat bonded? If so you may have inadvertently increased the cathodic protection current requirement on the hub anode.
>> Is the bonding system in good condition?
>> Have you changed any of the underwater metal components?
>> Have you added an underwater metal component?
Have any of the underwater metal components changed? Have you added any additional underwater components?

I recommend that you change the anode to a Mil-Spec Al anode. This will provide about 50mVDC more potential and will last about 50% longer.

Finally, reconnect the N-G bond at your inverter to comply with ABYC Standard, to increase your personal safety and because it has nothing to do with the problem at hand.

[a64pilot]

Boat is bonded, condition unknown as it the bare wire wrapped around the seacock and secured with a hose clamp type of bond, plus I have added over the last couple of years two thru hulls that are not bonded.
I will replace all the bonding, don’t see any other way than bare wire and hose clamp though.
I have added a large Dyna Plate, but problem was previous to the addition.

I guess best thing is to rebond all thru hulls and go back to hanging my guppie zinc, cause I can’t add any more zinc without going to a hull zinc or similar

[jamhass]

Besides all of the great suggestions above, in my experience, I find that:

1. zincs waste away much quicker in marinas than at anchor.
2. even with a galvanic isolator, plugging into the marina increase zinc waste rate
3. often, a neighbor boat is sufficiently "active" that zincs waste more quickly, and moving slips can help.
4. Consider adding a hull zinc.
5. When in a marina, I drop a bonded zinc overboard. This both adds an extra zinc and serves as a canary -- letting me know easily if my zincs are eroding unusually fast.

[CharlieJ]

a64pilot
Since we are dealing with about 1 volt; resistance becomes a big issue. The wire and hose clamp is probably > 1 ohm, the ABYC requirement for electrical connections in the CP system. So, to be effective, all connections in the CP system must be < 1 ohm.

That said, if all of the thru hulls are adequately bonded then the hub anode is being challenged with more uw metal than it was designed for. My recommendation is to:
1. Change to Mil-Spec Al anodes.
2. Add a "diver's friend" plate anode (Al) at the next haul out. Bond your thru hulls to this plate.
3. The hub anode will protect the shaft/propeller system.

[a64pilot]

I actually hung my guppie zinc today, I usually do in a marina but haven’t trying to resolve this issue. I did connect a 10ga wire directly to the engine block that has a ring terminal on it with a silicone bronze bolt that is now in the cockpit ice box so that I have a direct connection for the guppie zinc to the engine block.
Previously I connected it to a chain plate and as SS is such a poor electrical conductor I’m not so sure how well that worked, now I have Ti chainplates, and Ti is an even worse conductor than SS, so I ran the wire.

[boatbod]

950mV is good hull potential because you have zinc. The question is how much dc current is flowing in the bonding system, which is what ultimately determines zinc life.

What kind of keel? Lead or Iron, Encapsulated or External?

I've seen rapid zinc wasting (and keel corrosion) on external lead keels with bonded shafts. There are things that can be done to fix it, but it's a PITA. Need to understand the whole picture first.

[a64pilot]

She is an IP, so it’s a full keel, fully encapsulated. Prop, shaft and thru hulls are the bonded metals, there is a steel channel on the bottom of the rudder that connects to the keel though, and a large Dynaplate.
I assume if there is any current, then that .950VDC number would be different? Either way higher, or way lower?
I’m basing my belief that I don’t have any stray current solely on that voltage reading.

[senormechanico]

Go ahead and take my advice for what it's worth, and what I'm charging, nothing.

We went for YEARS with a single rectangular zinc on our Lord Nelson 35.
It was insulated from the bottom paint by a thin vinyl sheet and equipped with a pushbutton analog voltmeter calibrated with a spot on the face of -0.550 volts.
We had a 90% silver dime permanently mounted on the hull, similar to our current boat. The dime had a natural negative voltage (as our current boat does) of 0.130 volts.
Therefore, the -0.550 volts was actually -0.680 volts.

We had a 3 blade Maxprop (with its own zinc as it was insulated from the bonding system with a shaft flex coupling), and a couple of large copper ground plates/ one for lightning, and another for HAM radio as well as all bonded thruhulls.
Our 3"x5" zinc lasted for around 3 years and we never had any underwater metal damage.
The only difference between that boat and our current one is the automatic zinc saver circuit I have posted.

IMHO, 900 millivolts is doing nothing but burning bottom paint and costing money.

[boatbod]

-0.68V would be low to unacceptable, symptomatic of insufficient zinc or dc leakage (if value changes as loads are turned on/off). -0.9 is a good target. Don't go above -1.0.

The absolute value tells you very little about what it's going on, other than whether you presently have cathodic protection or not. You have to either measure the milliamps flowing in the bonding system, or look for voltage fluctuations to make that determination.

[fallingeggs]

Originally Posted by boatbod

950mV is good hull potential because you have zinc. The question is how much dc current is flowing in the bonding system, which is what ultimately determines zinc life.

What kind of keel? Lead or Iron, Encapsulated or External?

I've seen rapid zinc wasting (and keel corrosion) on external lead keels with bonded shafts. There are things that can be done to fix it, but it's a PITA. Need to understand the whole picture first.

Boatbod, curious if you could expand on this or send me a PM. I have been battling a zinc issue and, unlike the OP, have a lead keel with a bonded shaft and Max Prop.

[a64pilot]

Problem is, the zinc saver is likely beyond my level of abilities, to say noting of gathering the supplies, now that Radio Shack is gone, where do you get plain simple things like resistors, diodes etc?
The other concern of mine, is if I get it wrong, something supplying electrical current to my underwater metals could be bad.