If the green grounding cable on your boats AC inlet is connected to the DC ground as the American Boat and Yacht Council recommends, you may be asking for trouble.
As soon as you plug into shorepower, youre connecting the underwater metal on your boatstainless steel propeller shaft, bronze prop and through-hulls, zinc anodes, aluminum saildriveto the underwater metal on all the other boats in the marina via the green grounding wire. In effect, youre creating a battery in which a direct current (DC) flowing between two dissimilar metals immersed in an electrolytein this case, saltwatercauses the metals that are most susceptible to galvanic corrosion, like zinc and aluminum, to gradually waste away.
Metals are ranked on the galvanic scale according to their electropositive (anodic/less noble) and electronegative (cathodic/more noble) properties. Anodic metals that corrode easily include zinc, aluminum and galvanized steel, while stainless steel, copper, brass and bronze are at the other extreme. Thus you can imagine a nightmare scenario where your boats aluminum saildrive unit, its zinc anode having already sacrificed itself, is nibbled away by the stainless steel prop shafts and bronze propellers of the surrounding boats or even the metal parts of the dock itself.
Of course, you can always unplug from shorepower (thus defeating the purpose of having it in the first place) or succumb to the temptation to cut the green grounding wire. The latter move will certainly interrupt the flow of DC current along the grounding wire but it will also mean that your boat wont be grounded; if a fault develops somewhere in the AC system, say a short circuit in an inverter or charger, the whole boat could be energized via the DC ground and bonding system. Not only might someone suffer a shock inside the boat, the stray AC current could paralyze anyone swimming near it. Dont do it!
So you need to maintain the grounding circuits integrity to keep yourself safe, but must at the same time interrupt it to ensure the safety of your boats underwater metals. The best way to do this is to install a galvanic isolator, also known as a zinc saver. These simple 30-amp or 50-amp devices, costing from $100 to $300, incorporate a pair of diodes to block low-voltage DC currents (under 1.2 volts) from flowing alongside AC current along the green wire and connecting your boat to others in the marina.
A capacitor is also installed so that if theres a short circuit or other electrical fault on board, the galvanic isolator will let the full AC current pass through to ground. Thus you get the best of both worlds; neighboring boats cant eat away at your anodes and possibly your expensive saildrive or propeller, and there is no risk of electrocution.
Although installing a galvanic isolator is an easy project, you should never forget that AC current can kill; engage a professional if you have any doubts about your ability to work on AC circuits.
Heres how we went about installing a Guest 30 amp galvanic isolator on a boat with an existing shorepower setup.
The Guest isolator is a simple and compact device. The fins shed heat and the gold-plated terminals ensure a solid connection
The labeling couldnt be clearer and leaves you in no doubt as to how to connect the wires
The isolator needs to be mounted as close as practicable to the shore power inlet