Zinc Wasting

[a64pilot]

My only zinc is a prop “B” nut. I was going through an excessive amount of zincs. In two months the zinc was wasted away, but at one month only seemed about 25% consumed.
I have been trying different strategies to try to eliminate the problem, the last thing I tried was disconnecting the neutral to ground connection on my Magnum MS2812 inverter / Charger. It has a relay that will connect the two when it is used, and a plug that you can disconnect this feature. I was trying one thing at a time trying to isolate the problem as I had no half cell ($125 that I just haven’t spent yet)
Anyway a dock neighbor has a half cell corrosion reference electrode that he let me borrow.
With the Cell in the water and my Fluke connected directly to the engine block I measured approx 950 or so millivolts with shore power disconnected and battery switch set to off and power removed from bilge pumps. All form of power removed and shorepower cord disconnected.
Long story short as I connected shore power and my bank etc I might get a small jump in voltage of a few millivolts but it would very soon return to approx 950 millivolts.
I tried both battery chargers on, shorepower off and inverter on, every single AC and DC appliance I have on, one at a time, and voltage stayed always close to 950 millivolts. Cranked the engine with shorepower charges off so alternator would charge, cranked the generator and used its AC power. Nothing made much of any difference.
I even connected my “guppy” zinc at one time and got a small increase in voltage differential, maybe 10 or so millivolts, but that was it.


Is this normal? Can I assume hopefully that I don’t have a stray current or other problem that will consume my zinc now? I’ll still monitor of course, but was hoping I could find a circuit that I could pinpoint as the problem, but didn’t find one, unless of course it was that neutral to ground connection through the inverter.

[Seymore]

The way to use a half cell reference is, take readings everywhere and note the differences. The differences are what counts because they tell you where the current is flowing---the bigger the difference, the higher the current flow.

I don't have a feel for 950 mv any more.

Look for stray current due to the grounding wire in the shore power, among other things. That grounding wire (and the return) is connected to every boat in the marina. Boat electrics are notorious for wiring jumbles, connecting hot wires to the ground, and connecting electrically passive or active items to ground.

[sailorchic34]

950mv is pretty close to the difference between a silver reference cell and a zinc. Really depends on the zinc but to me it looks close enough. So it shows that the zinc is doing what it's meant to do.

If you were to remove all zinc's the voltage mv would drop.

[a64pilot]

I guess what I am asking is if there were any stray currents. Wouldn’t the voltage be a whole lot higher, and wouldn’t it jump when power was applied? Or said another way, with just slightly less than a volt, the zinc won’t waste away very fast, will it?
Reason for only connecting the half Cell and the engine block, is the zinc is connected electrically to the block

[Kenomac]

Do you have a galvanic isolator? If not... get one.

[hellosailor]

I'm wondering if "just a prop nut zinc" is the elephant in the room. I would expect that on a boat that size (38') you'd have one or two more robust zincs on the shaft itself (I know, no exposed shaft, no room on your boat) and still need to change those every year if not sooner. Perhaps the one zinc is being consumed so "quickly" is because it simply isn't up to the task, and was never supposed to (or inadequately designed to) carry the entire boat by itself?

Any IP38 owners you can reach out to, to compare notes?

[Frankly]

I am going to assume that your set up is similar to Cbreeze as to Zinc saver (galvanic isolator) . In our era the thinking was to locate the GI between the DC ground and the AC green ground. Current enlightened thinking ??? of ABYC is to now locate the GI in series with the AC ground and use a direct connection between the AC and DC ground. Assuming you have a VOM measure the current between the AC and DC grounds with the GI in series with the meter. If you get any significant current (> 20 ma AC or DC) then remove the GI and repeat the measurements. Make these measurements with shore power on and off. Be careful, would hate to think someone else would get to enjoy those titanium chainplates.

If the only zinc is the AP zinc would not expect it to last too long. I have the AP zinc, a shaft zinc, and an large teardrop zinc mounted on the side of the keel. Get about 6 months between changes (but Cbreeze has a lot of SS in the centerboard system).

Also if you are using those silly nylon screws that come with the AP zinc, deep 6 them and get some metric SS ones.

[a64pilot]

Yes I have a galvanic isolator, it is connected in the AC ground circuit, but the AC and DC grounds are connected together after the isolator, that was done at manufacture.
I get no significant voltage change, shore power on or off, cord connected or disconnected. In fact nothing I do electrically changes voltage much at all, maybe 10 mv, but then it goes back to 950 mv.
I have an adapter that was made by Islers diving out of bronze that replaced the factory AP zinc and allows a standard “B” prop nut to be fitted. I believe a “B” nut weighs a half pound. However that is all the zinc there is. I have considered fitting a hull mounted large zinc like power boats do, but am under the impression that you don’t want to “over zinc” a boat. Too much isn’t good either?

Now like I said, I changed something before I measured the voltage with a half Cell, I disconnected the relay that connected the inverter neutral and ground together whenever it was being used. So maybe I won’t be eating zincs like I used to. Be one more month before I will know, then I get to Dive the boat in January. Looking forward to that, let me tell you.

[Island Time O25]

My zinc use varies from season to season but since I went to at least 2 on the shaft the wasting slowed down considerably. May be it was just a coincidence as my mooring neighbors keep changing but that's what I've noticed. Last 2 seasons the zincs were barely wasted. At first my reaction was "I must've bolted them on with gaps" but subsequent check of the prop shaft, etc. confirmed they were doing their jobs. Was told by several pros that sometimes it is what it is and will vary considerably from season to season or may be even due to the quality of the zincs themsleves. I tend to buy them in advance, whenever I see them on sale. But two at the same time will definitely last 2 seasons with more zinc left on them then if one per season was used.

[Frankly]

You need to get out of that voltage mindset. You are dealing with more of a constant voltage/ variable current situation (think solar panel). The rate at which the zinc disappears is a function of current flow. Disconnect the AC/ DC ground connection and measure the current (AC and DC) shore power connected, shore power dis connected between the AC and DC ground. If you can locate the connection between the engine and the bonding common terminal strip, disconnect the engine connection from the terminal strip and measure the current flowing in that circuit. DC only with shore power connected and disconnected.

[IdoraKeeper]

Given your location, you seem to be within limits. Get more spares and replace as needed. Over zincing will damage things you may not easily see.

[a64pilot]

Originally Posted by Frankly

You need to get out of that voltage mindset. You are dealing with more of a constant voltage/ variable current situation (think solar panel).

I understand what your saying, however amperage would be just as easy to measure as voltage, yet it’s not?

I guess what I was after was a corrosion analyst to pop up and tell me that less than 1 volt, your fine, don’t worry about it, you must have fixed your problem. I assume current in this case is tied to voltage, or else we would measure amperage as well as voltage?

[geoleo]

Originally Posted by a64pilot

My only zinc is a prop “B” nut. I was going through an excessive amount of zincs. In two months the zinc was wasted away, but at one month only seemed about 25% consumed.
I have been trying different strategies to try to eliminate the problem, the last thing I tried was disconnecting the neutral to ground connection on my Magnum MS2812 inverter / Charger. It has a relay that will connect the two when it is used, and a plug that you can disconnect this feature. I was trying one thing at a time trying to isolate the problem as I had no half cell ($125 that I just haven’t spent yet)
Anyway a dock neighbor has a half cell corrosion reference electrode that he let me borrow.
With the Cell in the water and my Fluke connected directly to the engine block I measured approx 950 or so millivolts with shore power disconnected and battery switch set to off and power removed from bilge pumps. All form of power removed and shorepower cord disconnected.
Long story short as I connected shore power and my bank etc I might get a small jump in voltage of a few millivolts but it would very soon return to approx 950 millivolts.
I tried both battery chargers on, shorepower off and inverter on, every single AC and DC appliance I have on, one at a time, and voltage stayed always close to 950 millivolts. Cranked the engine with shorepower charges off so alternator would charge, cranked the generator and used its AC power. Nothing made much of any difference.
I even connected my “guppy” zinc at one time and got a small increase in voltage differential, maybe 10 or so millivolts, but that was it.


Is this normal? Can I assume hopefully that I don’t have a stray current or other problem that will consume my zinc now? I’ll still monitor of course, but was hoping I could find a circuit that I could pinpoint as the problem, but didn’t find one, unless of course it was that neutral to ground connection through the inverter.

I had pretty much the same experience--then discovered that the clock in the microwave via the inverter caused the 950 millivolt drain--when the micros plug was pulled the 950 millivolts went away. Cant explain why---then 2 different Surveyors checked entire elect system and said everything was fine...Said the original 950 volt reading was not a problem--as for zincs going fast- a year earlier has same problem--then started 'cleaning up electrical everywhere and zinc problem disappeared----------don't know why...damn boats--now if I could just understand why a cold light bulb gets warm when lit ....................

[geoleo]

Originally Posted by a64pilot

My only zinc is a prop “B” nut. I was going through an excessive amount of zincs. In two months the zinc was wasted away, but at one month only seemed about 25% consumed.
I have been trying different strategies to try to eliminate the problem, the last thing I tried was disconnecting the neutral to ground connection on my Magnum MS2812 inverter / Charger. It has a relay that will connect the two when it is used, and a plug that you can disconnect this feature. I was trying one thing at a time trying to isolate the problem as I had no half cell ($125 that I just haven’t spent yet)
Anyway a dock neighbor has a half cell corrosion reference electrode that he let me borrow.
With the Cell in the water and my Fluke connected directly to the engine block I measured approx 950 or so millivolts with shore power disconnected and battery switch set to off and power removed from bilge pumps. All form of power removed and shorepower cord disconnected.
Long story short as I connected shore power and my bank etc I might get a small jump in voltage of a few millivolts but it would very soon return to approx 950 millivolts.
I tried both battery chargers on, shorepower off and inverter on, every single AC and DC appliance I have on, one at a time, and voltage stayed always close to 950 millivolts. Cranked the engine with shorepower charges off so alternator would charge, cranked the generator and used its AC power. Nothing made much of any difference.
I even connected my “guppy” zinc at one time and got a small increase in voltage differential, maybe 10 or so millivolts, but that was it.


Is this normal? Can I assume hopefully that I don’t have a stray current or other problem that will consume my zinc now? I’ll still monitor of course, but was hoping I could find a circuit that I could pinpoint as the problem, but didn’t find one, unless of course it was that neutral to ground connection through the inverter.

PS put a shaft zinc on inside _ I know you do not have room outside...........

[a64pilot]

I believe to be any good a zinc has to be in direct contact with the water on the outside of the Boat, plus my shaft is about four feet above the bilge.
I have another month before I can inspect and see the zinc.
I disconnected and removed ALL power on the boat, and still had the 950mv, which is I believe what you want, you don’t want zero voltage.
Apparently the silver half Cell is one side of a battery Cell and the other side is the zinc, put them into electrolyte (salt water) and they generate current. If you have stray current, then of course you should measure way more than the static voltage of the Boat with all power removed
Or at least that is they way I think the half Cell works anyway, I may be wrong