Stray current in marina

[mitiempo]

Originally Posted by jjjacq

Try DC and AC as you don't know what sort of stray current you may be dealing with.

AC current does not cause corrosion - DC current does.

There would not be any corrosion on the unconnected middle boat. Any current in the water would dissipate a very short distance after the source. The boat would have to be connected to shore power to realize any effect from the others or the marina's wiring.

[Neptune's Gear]

Originally Posted by boatpoker

Think I'll give up on this thread. My years of training, education and certifications in Marine Corrosion and Marine Electrical systems don't seem to buy me much credibility among the dock experts.

Don’t give up. Please. Rational and educated responses are what’s needed here. I totally agree with all your posts in this thread. I’m not a corrosion specialist, as you are, but a Marine electrician.

[verkerkbr]

Originally Posted by Neptune's Gear

Don’t give up. Please. Rational and educated responses are what’s needed here. I totally agree with all your posts in this thread. I’m not a corrosion specialist, as you are, but a Marine electrician.

I'am not a corrosion specialist, but a surveyor. Here is what I have seen:

Look for other sources.

Once I have seen a problem with an alu saildrive. The alu housing was damaged due to severe corrosion in one season. The zinc on the saildrive however looked as new.

Searching for the cause of this problem, it appeared to be a leak in the the isolation of the top navigation light. The cable was a too tight fit on the foot of the toplight.

This light was standing on the alu Mast. The mast was supported by a stainless support in the cabin and was connected to the keel-bolts.

Via the top light, there was 2 volt on the metal keel !



Bram

[CharlieJ]

but it remains possible that some of the current will pass through the 3rd boat.

We are in violent agreement on that point!

[Painless]

I have a question regarding the result of mixing anodes...We have a Volvo outdrive with a horseshoe zinc surrounding part of the lower unit, West marine was out of zinc anodes for the trim tabs and magnesium anodes were placed. We noticed small galvanic bubbles forming on our aluminum prop and it was noted by our hull cleaning service. We changed the anodes so they all "matched" materials and it seems to have stopped. Is this probably what caused the issue? (we ARE hooked to shore power) We never had this before and had no electrical changes to any systems. Thanks for any input

[CharlieJ]

Painless #65
If you are in salt or brackish water, the magnesium anodes provided cathodic protection current such that the potential (against a AgCl reference cell) was too high. The recommendation is to not allow underwater aluminum components' potential to be driven more negative than -1200mVDC by a cathodic protection system, either passive anodes or impressed current.

[CharlieJ]

Post #61

There would not be any corrosion on the unconnected middle boat. Any current in the water would dissipate a very short distance after the source. The boat would have to be connected to shore power to realize any effect from the others or the marina's wiring.

If the victim boat is not plugged in to shore power and if the victim boat has some or all of its underwater metal components bonded, and if the victim boat is in the electric field set up in the water column by Boat #1, Boat #1 is plugged into shore power, then the stray current will enter the victim boat's bonding system via the bonded underwater fitting closest to the leaking boat, use the victim boat's bonding system as the preferential conductive path (copper is much, much more conductive than seawater) and then leave the victim boat at an underwater bonded fitting close to the third boat that is connected to shore power. Stray current damage will occur on the victim boat's bonded underwater metal components.

If the victim boat is unbonded, then the same scenario is played out, only involving the victim boat's prop, shaft and shaft anode.

Any current in the water would dissipate a very short distance after the source.

This is not correct. Remember that we can shift the potential (against a reference cell) of a bonded metal thru hull -200mVDC, or more, when the thru hull (e.g., anchor wash down source) is fifty feet from a plate zinc anode mounted on the transom. And the zinc anode only provides about -1000mVDC of potential. If a boat is leaking 12VDC into the water column, its effect will be noticeable some distance from the injection point.

[boatpoker]

Originally Posted by Painless

I have a question regarding the result of mixing anodes...We have a Volvo outdrive with a horseshoe zinc surrounding part of the lower unit, West marine was out of zinc anodes for the trim tabs and magnesium anodes were placed. We noticed small galvanic bubbles forming on our aluminum prop and it was noted by our hull cleaning service. We changed the anodes so they all "matched" materials and it seems to have stopped. Is this probably what caused the issue? (we ARE hooked to shore power) We never had this before and had no electrical changes to any systems. Thanks for any input

Aluminum or zinc. Magnesium simply won't work.

[hellosailor]

As CharlieJ said. This is one of the reasons that some folks say NOT to bond all fittings together, because it creates a physical circuit run.

This is also the same reason that lightning safety practices are to squat on the ground with both feet tightly together (or better, just one foot on the ground) so that there is no difference in potential that can create a "circuit" up one leg, through the body, and down the other. (Or hand to leg, arm to arm, etc. if you're lying down instead of squatting.)

Marelon, the real thing not just lack plastic, simply eliminates the problem. Mainly having just the disadvantage that you can't stand on it, or drop heavy things on it.

[alaskasail]

Something happened to me like this once because there was a bare power leg of the dock power in the water.

Change slips ad see what happens.