Electrical discharge

[PEACETIME]

A fellow in our marina has had a problem with his zincs going away in 6 weeks.
We disconnected his AC power and then measured from the chain plate to the water and found .6 volts DC. We then measured from the engine to the prop shaft stuffing box and got nearly the same .5-.6 VDC. I am not sure if this is enough to eat the zincs in that amount of time or not. He discovered this problem while out on a mooring so I'm pretty sure it is not related to AC discharge. Though he does use the inverter for TV, computer ect for a couple of hours every nite. For a comparison I measured my boat the same way and got .002 VDC and I have never had any excessive zinc wear. Any suggestions would be helpful, as he is new to boating.

[Andina Marie]

If it happens on a mooring and it is agressive electrolysis there is the possibility that a wiring problem is allowing the 12 volt battery voltage to get to underwater metal.

A bad connection in the negative wiring could be passing part of the current through the water on the way back to the battery. All connections, particularly in everything connected to 12 volts negative circuit should be checked.

A fairly easy check is to measure the voltage between the battery negative terminal and every piece of underwater metal while turning assiciated DC items on and off. Those measurements should be less than 10 millivilts. If you find one considerably higher, check the ground connections to that device, or if no device you could try bonding it to the battery negative.

Any items that connect both to the 12 volts and have underwater metal suche as a depth sounder should be checked. You would disconnect the underwater metal and re-connect the ground wire through an amp meter. If you measure more than a few thousandths of an amp the device could be contributing to the problem. If disconnecting and measuring the current is not feasible, you can measure the current flow on the posititive supply with the device OFF (which often will not be zero). Then disconnect the cable to the underwater part and see if the measurement changes. If it decreases that would indicate how much current is going into the water.

If you measured those voltages with shore power connected a galvanic isolator would be highly advised. For $100 it is a first line of defense.