Corrosion and shore power

R

Rwnorton

Well-known member
Joined
Aug 13, 2015
Messages
118
Fluid Motion Model
C-24 C
Non-Fluid Motion Model
Former R-27 owner
When I measure my shore power cable, connected at both ends with power either on or off, with a clamp on multimeter I see 0.7 amps. This is suppose to be zero. Power is disappearing into the water. When I measure the voltage at the power pedestial between the green and white I get about 0.02 volts. This occurs at a dock with no other boats within 100 yards and nothing else connected to the shore power system.

What's happening? Is this an onboard problem or a dock problem? How do I know?
 
Is the current being measured in the green wire only? To measure this, see if you can get to the green wire coming from your shore power cord connector on the boat to the ELCI breaker. There should not be any current in the green wire. If there is, as you state, it could be a fault on board or an issue with the power system at the dock (not uncommon even if there are no other boats around). Do you have a galvanic isolator on the boat? If not, you should probably get one. A galvanic isolator will block DC voltages up to 1.0 to 1.2 volts or so on the grounding (green) wire. I believe AC current will still pass through code compliant isolator so you should also make sure you are measuring D.C. Current. AC current is less of a problem for corrosion.

If the current is not in the green wire then you have something else going on.

Here is a great reference for boat owners regarding corrosion.

https://www.amazon.com/Boatowners-Guide-Corrosion-Reference-Professionals/dp/0071475443

Curt
 
Thanks
Yes the current is only in the green wire. Today I installed a galvanic isolator and the readings are as before. I think the GI only stops lesser amounts of current.
 
Hmm... the GI should block current flow when the voltage across it is less than 1 volt. The voltage you measured is not enough to allow DC current to pass through the GI. There is something else going on here. Are you sure you are measuring D.C. Current? Most inexpensive clamp meters will not measure DC current.

If you are still reading .7 amps DC, what is the current reading when you disconnect the shore cable (disconnect cable, not turn power off or on)? Also, what is the D.C. Voltage reading across your GI with the shore power cable connected and disconnected?

Curt
 
My 0.7amp reading is AC. Why would I look for a DC Reading on an AC power cord?
If I set the multimeter to Dc Amps, the reading is zero with the power either on or off

Measuring across the GI I get 0.42 v dc with or without the shore power cable and with or without power.
Thanks
Ron
 
Hi Ron,

Most Galvanic Isolators only block D.C. current (at least the code compliant ones only do). There is a capacitor across the GI diodes that passes the AC current even at low voltage. For safety reasons the code does not allow even small AC currents to be blocked as even small AC current can be a safety issue. This is OK from a corrosion standpoint as AC currents do not cause the same level of corrosion as DC currents. The DC Voltage on the grounding wire can come from other boats or equipment on the same shore power system and that is the voltage you are concerned about for corrosion issues.

In your case since the DC current reading is zero either the the GI is doing its job or there is no D.C. Voltage on the grounding system at your dock to worry about. The .42 volts DC across the GI is likely the galvanic potential generated by your anodes. Since it is below 1 volt that is why there is no DC current flowing.

The .7 amps of AC current measured is likely due to some issue in the shore power system. The shore power grounding wire is only tied to earth back at the connection to the utility. With long dock systems with various connection points and different systems connected I can imagine it is difficult to keep that grounding wire at zero volts AC. If the .7 amps were due to something on your boat the current would have to come from either the hot or neutral on the boat and even a very small difference between the AC hot and neutral would trip your ELCI main breaker. To be certain it is something in the shore power system you could have someone else at the dock do the same test with their boat and see if they get similar results. If so, you should probably let someone at the marina know there may be an issue.

You should be good as far as electrolytic corrosion due to the shore power system with your GI in place. If you are still having corrosion problems either there could be a 12 volt system problem like a loose wire in the bilge (this would likely result in extreme corrosion issues) or more likely you just need to add more anode zinc to the boat. I added a transom zinc to our boat and put a zinc fish in the water when not at the boat to reduce how often I need to replace the anodes.

Are you experiencing extreme corrosion issues?

Curt
 
Yes, see
viewtopic.php?f=5&t=8716#p60692
And several other post in the days since then.
I'm new but have gone through or lost 3 prop zincs and 2 sets of trim tab zincs in 6 months and all my question are about finding out why.
I appreciate your help and will investigate the short power side of things.
 
Where do you clip your zinc fish in your boat?
 
Attaching fish;
Andrew Custis said earlier: I would connect the grouper zinc to the bonding ground block under the lift up port side hatch. It is the one that has all of the green wires coming out of it. The galvanic isolator I have used in the past is a Pro Mariner FS30. You can also install that in the same hatch as the bonding block and tie into the shore power ground at that point.

I found the grounding block to be virtually inaccessible because of all the things already there and the proximity of the generator. So in the engine compartment there are two thru hulls for the generator and AC bonded together with a ground wire running to Andrew's ground block. I attached the fish to the grounding wire at the thru hulls.
 
Curt,
How,where, and what size zinc did you add to the transom?
 
Hi Ron,

I added a Mini Divers Dream to the starboard stern under the swim grid. It is a bit of a tight fit on the R 27 to get accessibility from the inside for drilling and bonding. I was down in the space below the aft cockpit seat in the area that on the port side that has the stern thruster.

A bit scary drilling holes in the boat under the water line! :shock: (even while on the trailer!) I used long stainless bolts (I can't remember the actual length, maybe 2 1/2" to 3") such that I could get the bonding wire and a fender washer on the inside and a fender washer, two nuts, then the zinc, and then the zinc and additional washer and two nuts on the waterside to lock it secure. I filled the holes before inserting the bolts with enough 5200 to ensure that it squeezed out around the fender washers on both sides of the hull. I included the bonding wire and fender washer under the bolt head prior to pushing it through the hole filled with 5200. In retrospect it would be better to use threaded rods (nuts on the inside and outside of the hull) so the bonding wire could be removed from the inside without removing the threaded rods. On our boat it is very tight to fit the two bolts between the swim grid mount and yet stay within the accessible area under the cockpit seat. Here is a picture of the mounted mini divers dream.

 
Additionally, if you are losing prop zincs here is what I have done. I believe that the prop zinc often spins off or breaks up long before the useful life of the zinc is used up. This is because the zinc is held to the prop shaft by the zinc material itself which is reducing in size over time. While it is still important to tighten the zinc really tight I have found the factory lock washer and fender washer solutions to be insufficient by themselves. The problem is the prop zinc is reduced in length as well as width as it is sacrificed. This results in it either spinning loosely and eventually breaking up and falling off even with minimal length reduction. My solution, besides adding the additional zinc to the boat, is to paint both ends of the prop zinc before installing such that the length is not reduced so readily (the zinc will not readily sacrifice through paint or other sealer). This allows the prop zinc to sacrifice radially without reduction in its length for a longer time. While I have not had enough experience with the solution to prove 6 months of life yet (4 months no problem) I am confident that this season it will last 6 months as I have added a little more paint to each end (actually a permanent marker!). Note that this result is with the added transom zinc and fish as well.

Here is a photo of installed and painted prop zinc as well as one after 4 months in saltwater in the NW.



Curt
 
Regarding the connection of the fish. Call me crazy but I connect mine directly to the battery negative. It is just more convenient from that location. The only reason I can think of why they don't recommend this is the danger of a spark igniting battery gasses in the battery compartment. If anyone else knows of another reason let me know. First, on our R27 we have AGM batteries which are sealed and thus have a much reduced chance venting gas. Second, the voltage produced from putting the fish in the water is extremely unlikely to cause a spark when connecting. Third, I make sure to ground the fish connection against the engine block first before moving it to the battery (if I ever move it) to reduce the chance of any static charge (highly unlikely). Fourth, I leave it connected permanently to the battery ground and just pull the fish out of the water when not at the dock. No chance of spark near the batteries. I have been doing this for several years now and I am still here. If I blow the boat or myself up I guess you will just have to read about it in the papers and know to change the process! :ugeek:
 
Red Raven, that is the way most of us do it.
 
It was recommended to me to grind down a penny and slide that against one the flats on the prop shaft along side the prop zinc. This way the zinc says tight on the shaft. So far so good. I pull the boat tomorrow to head for home on the trailer. I'll see how the prop zinc liked a couple months in the Florida saltwater. In think Mike Rizzo offered up the info on using the penny.
 
Oh yeah, I forgot. I did the wedge thing as well but with a stainless washer instead of a copper penny. I did it more to make sure I still had electrical contact to the zinc (given that I had painted the ends and thus losing electrical contact there) than to keep the zinc from spinning off. While the penny or washer will provide additional tightness of fit I was still concerned that it would eventually become loose due to loss of material lengthwise. Thus I painted the ends of the zinc as well. We'll see this season if I can get to my target of lasting 6 months with the prop zinc mods and all the extra zinc on the boat. Last year I lost the first prop zinc after two months (without paint or washer but with the transom zinc and fish on the boat). It was completely missing just as the previous season. After adding the painted zinc ends (installed by diver without the added penny or washer) the zinc lasted 4 months with a fair amount left.
 
Curt and others, thanks for the advice
 
Add a galvanic isolater. We lost all zinks in 5 weeks before. After isolater all except trim tabs good after 14 months. Trim lasted a year, believe tab stainless set up battery with zincs.
This is one of the few RT flaws, guess it is trailer boat mentality.
Another is small auto diesel full. Luckily Volkswagen makes an adapter for cars at truck stops.
Bill
 
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