common ground for DC and bonding circuits?

Engines, Drive trains, Propellers, Steering, Ground Tackle and other mechanical system

common ground for DC and bonding circuits?

Postby SVBAGATELLE » Sat Oct 22, 2016 5:24 pm

There is some disagreement between the experts on this subject apparently. My problem is that I have recently put a new barrier coat on my lead keel because of seepage behind the existing barrier coat. The keel was first ground down to the bare metal. After just one season, the new new barrier coat had many blisters. It seems that this this may be a symptom of what Everett Collier in his Boatowner's Guide to Corrosion calls "cathodic disbondment." Though the lead keel does not corrode, it is electrically connected to other underwater metals on the boat since the are all grounded to the keel (in particular, the propeller and shaft, and my refrigerator's keel cooler). With the salt water, the zincs on the prop shaft and the keel cooler are forming a circuit with the keel. While this protects them from corrosion, the flow of current destroys the barrier coat on the keel. Probably the culprit is the keel cooler (for my Frigoboat fridge), since it is close to the keel and has two zincs. The fact that the keel cooler is always covered with a pasty substance and the zincs are always very worn away after a season tends to support this view.
I am thinking of removing the cable that connects the DC ground to the keel (so that the engine alone serves as DC ground), but retaining the bonding circuit's connection to the keel. This would maintain lightning protection of the mast and other external metallic objects, but isolate them and the keel from the engine, fridge, and the DC circuits generally. Any reactions or experience with doing this?
While the experts recommend a common ground, they also admit that it may cause problems in some circumstances. Don Casey and Nigel Calder differ on this. In Sailboat Electrics Simplified Don Casey says the preferred solution is to use the lightning grounding plate for all ground connections and isolate the the prop from the engine, “but stray current corrosion is a risk. The solution in this case is to keep the lightning ground electrically separated from the engine ground (for the DC and AC circuits).” (p. 167). Nigel Calder does not admit that this setup could be best: “A grounding point or bus should be established as the boat’s common ground point. All the non-current carrying grounding circuits are fastened to this bus... [including DC negative, the AC grounding wire, and lightning/bonding circuits]“ (Boatowner’s Mechanical and Electrical Manual, p. 232).

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