Corrosion/ stray current location?

[sailcruiser]

Hello, we are seeing some through hull and rudder paint corrosion and I like to first rule out/correct our boat as the problem. (I believe it is at least part of the issue.). The situation is this: 30 year old boat with shore power, in a marina with 'questionable' electrical situation (still trying to get details on what the issue is), galvanic isolation/isolation not present (YET), no genset or inverter, no wind or solar, AC wired with GFI in galley and head, bonding situation unknown, no grounding plate either. The through hulls are on the same side of the boat but 10' part and both are drains. This boat is in fresh water and has an aluminum sail drive that I don't want to turn into an expensive anode. We use a single magnesium collar anode on the drive which was almost gone at haul out. (survived June to October)

We are on the hard for the winter and I plan to get this resolved before launch. I have read a lot on this topic on the what/why but I haven't found a good checklist of how to find the cause. I'm sure the lack of a galvanic isolator or better is at least part of the issue. I am seriously considering an isolation transformer. The PO mentioned the saildrive being pitted so he sanded and repainted it but never tried to correct the cause so this issue has been going on before our purchase and relocation of the boat.

I have a good meter and willing to buy necessary probes. The survey was clean. I own Nigel Calder and Don Casey's books and so far haven't found a methodology to help work through identifying the problem. Right now I'm focused on stray current and not dissimilar metals although I'm sure this is also a factor to some degree.

The wiring in the bilge is run through PVC. The bilge is pretty dry with the saildrive. In reading it looks like current is exiting through the corroded areas but there doesn't appear to be an obvious reason why.

So is there a good trick to locate any current out of the water? The bilge pumps are at the top of the list to check. A new battery charger is being installed this weekend as well replacing the old battery boiler.

All suggestions welcome. If I can't find he issue I will bring in a marine electrician but I want to try first.

SC

 

[Maine Sail]

1- One of the biggest sources of corrosion is often your own vessels DC system. The biggest mistake I see time and time again is DIY's mistaking lightning or bonding "Earth" ground wires as DC negative returns and "tapping" into them. Green grounding wires for lightning or bonding systems should NEVER, EVER carry any current yet people tap into them all the time to add DC devices such as bilge pumps, water pumps, shower sumps, electronics, fuel pumps etc. etc..

ALL DC circuits should run back to the battery negative or buss bar and not take a shortcut to the engine or Earth potential grounding wires.

2- DC negative returns should return to the same spot, usually a DC negative buss, then that DC negative/ground should have ONE wire heading off to ship ground, usually the engine. Multiple DC neg ground points lead to corrosion. Bonding & lightning "Earth potential" wires should terminate at this same location but if the engine neg wire carries your alt and starter current the bonding and lightning should really not tie into the DC neg buss BEFORE the ships grounding point. Care must be taken to ensure that the green "Earth" wires for bonding or lightning can never carry any current.


3- Battery chargers, especially cheap ones, or non-marine units can often be a source of "leakage". Good battery chargers will isolate AC from DC inside the charger via a small isolation transformer. The same can be said for some non-marine inverters.

4- A galvanic isolator or isolation transformer are necessary devices if plugging into shore power. An ELCI or RCD AC main breaker is also a good idea.

5- AC leakage corrosion is far slower than DC and generally requires higher current than you seen in normal small AC leakage issues. It is very often a DC problem and very often caused by your own vessel though stray DC can still be carried in by the AC green wire of you don't isolate your vessel. An isolation transformer or galvanic isolator should be used to minimize any AC issues but can also help protect against external DC issues on the grounding wires too..

6- A reference cell and multimeter can be used to determine your corrosion level potential and they run about $130.00-$170.00.

7- You'll want a marine corrosion specialist not just any old marine electrician who has not had training in corrosion. Call the ABYC and find out who in your area has taken these courses or who has an ABYC corrosion certification. At least then you'll know the person you are paying has taken the time to have a better understanding than your average technician.


8- There are varying positions on bonding and corrosion even amongst the "industry guru's"... I have taken ABYC courses with both Rifkin and and Collier's teachings and read lots by Honey and to say at the least there is often disagreement even amongst the "gurus" of corrosion.

Anything by David Rifkin or Everett Collier are excellent when it comes to corrosion issues. Everett Collier's book "The Boat Owner's Guide to Corrosion" is currently used as the text for the ABYC Corrosion Certification Course.


9- Master your DC system and make sure it is not the root cause. It is often is a big contributor...

 

[sailcruiser]

Thanks for the tips MS. Sounds like this will be a long process of elimination. I agree that we are likely to find some issues in the DC system. Just trying to find a point to start testing is a little overwhelming.

I'll be interested to see if we have any ABYC corrosion specialists in the area. (lots of experts mostly schooled in hard knocks and I don't want to be a lesson.)

SC

 

[heart of gold]

Maine Sail is a usual, spot on. One of the simplest things you can do is install the isolator in the shore power ground. This should localize problems to the boat itself. Then you can start eliminating those issues. I did a survey at our marina which included intalling a milliammmeter in the shore power ground and it was pretty amazing how many boats had DC current running on the ground wire, up to 18 ma.