Stray current in marina

[senormechanico]

If you have a Volvo engine and transmission, check your owner's manual before connecting the prop shaft to the engine.
Mine (MD2020) is very specific about NOT doing that.
The transmission has a raw water cooling passage before the engine and the case is aluminum.
The transmission is completely insulated from the engine to prevent galvanic corrosion.

[groman]

Originally Posted by senormechanico

If you have a Volvo engine and transmission, check your owner's manual before connecting the prop shaft to the engine.
Mine (MD2020) is very specific about NOT doing that.
The transmission has a raw water cooling passage before the engine and the case is aluminum.
The transmission is completely insulated from the engine to prevent galvanic corrosion.

Yes you are quite correct. It behoves every boat owner to carefully read all his equipment manuals before making any changes.

[Doek]

I also have a 1972 Pearson 33 that I am refitting. As usual, some advice here is OK, some horrible. You have a galvanic corrosion issue caused by the water, dissimilar metals, and being hooked up to your battery system probably adds to it. Dissimilar metals are creating a strong enough current to eat your anodes quickly. I expect that your anode is on your prop shaft. I have that also, but i installed a large faired "zinc" to the hull through bolted through to a heavy black wire connected directly to the ground point on the engine. It probably 3" x 9" x 1/2". I installed it on a standoff so that water can access/eat all sides. Camp Pacemaker Zinc CAM-A-90. Problem solved. By the way, if you are using magnesium "zincs" they get eaten up quickest of the three choices (zinc, aluminum, magnesiun). In your situation, a true "zinc" is probably best.

[CharlieJ]

In your situation, a true "zinc" is probably best.

Not really correct.

In salt or brackish water, zinc anodes are ok, aluminum anodes are better for the following three reasons:
1. Al alloy anodes contains more Ahr/pound than do zinc anodes. Approximately 50% more.
2. Al alloy anodes provide approximately 70mVDC more cathodic protection potential.
3. Al alloy anodes do not contain Cd, a heavy metal that settles to the bottom of the water column during the cathodic protection process, is ingested at the bottom of the food chain, and works its way up the food chain until consumed by humans.

[wildcat35]

A bad alternator or one that is wired wrong can cause this.

[River Cruiser]

Originally Posted by boatpoker

We must learn to stop calling everything a "zinc". They are "anodes" of which zinc is one type.



Zinc anode for salt water only.



Magnesium anode for fresh water only.



Aluminum alloy (indium) anode for all waters.

I boat in freshwater the upper Mississippi and doubt I’ll ever go south any farther than Tennessee. I use magnesium a friend with a aluminum houseboat uses aluminum anodes. The aluminum are much cheaper. Is it worth saving a few bucks to use the aluminum for either of us? I replace every 2 years.

[jjjacq]

Originally Posted by hellosailor

gl-
If you have no AC connected, and no battery connected, then you cannot have a current flowing and you cannot having normal galvanic corrosion eating the zinc.

That's not quite correct, I can imagine a scenario with 3 boats, one, incorrectly wired injecting a DC current into the marina, the 2nd not connected to anyting, and the 3rd connected to shore earth. The stray current injected into the water by the 1st boat may find its way to the 3rd via the middle boat's anodes, it will enter at the node nearest itself and leave at the anode nearest the 3rd boat (I'm assuming all the anodes are correctly interconnected internally on the 3rd boat).

So stray currents can exist even when the boat is not connected to anything, not even its own battery.

[boatpoker]

Originally Posted by River Cruiser

I boat in freshwater the upper Mississippi and doubt I’ll ever go south any farther than Tennessee. I use magnesium a friend with a aluminum houseboat uses aluminum anodes. The aluminum are much cheaper. Is it worth saving a few bucks to use the aluminum for either of us? I replace every 2 years.

Either will work fine in your situation (assuming you are an FRP boat). With an aluminum boat I'd stick with the aluminum anodes.

[boatpoker]

Originally Posted by jjjacq

That's not quite correct, I can imagine a scenario with 3 boats, one, incorrectly wired injecting a DC current into the marina, the 2nd not connected to anyting, and the 3rd connected to shore earth. The stray current injected into the water by the 1st boat may find its way to the 3rd via the middle boat's anodes, it will enter at the node nearest itself and leave at the anode nearest the 3rd boat (I'm assuming all the anodes are correctly interconnected internally on the 3rd boat).

So stray currents can exist even when the boat is not connected to anything, not even its own battery.

Absolutely not.

[Paul Elliott]

Originally Posted by boatpoker

Absolutely not.

I think jjjacq's scenario is plausible, and in fact I had considered proposing something similar.

But, given the conductivity of salt water and the distances between boats (even in a marina) I think the resulting current would be too small to make much of a difference.

[Doodlebug]

In order for boat #1 to inject DC current into boat #2 and #3 boat #1 must be connected to the shore power to complete the circuit.

[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.

[jjjacq]

Originally Posted by Doodlebug

In order for boat #1 to inject DC current into boat #2 and #3 boat #1 must be connected to the shore power to complete the circuit.

I agree, boat 1 and boat 3 need to be connected to shore power but boat 2 can be isolated, just sitting in the the path of the circuit. The current will always take the path of least resistance.

[CharlieJ]

The current will always take the path of least resistance.

Not really and not to put too fine a point on it but...current will take all paths back to its source. The amount of current flowing in each path is inversely proportional to each path's resistance.

[jjjacq]

Originally Posted by CharlieJ

Not really and not to put too fine a point on it but...current will take all paths back to its source. The amount of current flowing in each path is inversely proportional to each path's resistance.

I agree, but it remains possible that some of the current will pass through the 3rd boat. It can be easy to measure, just disconnect the wire from one anode and place a meter between it and the other anodes and see if you have a reading. Try DC and AC as you don't know what sort of stray current you may be dealing with. (I'm assuming the hull is plastic, if it's aluminium or steel you won't be able to do that since in this case the hull itself is "the wire" bonding all the anodes together). The current will be small, maybe even very small. I have a meter capable of detecting nanoamperes, not milli, not micro, nano, and yet those insidious nanoamps were capable of causing serious grief with explosives in the mining industry.