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ELECTRICS
Stray currents and corrosion
Q Some boats in the marina allow their shore power cables to go into the water between berth and vessel. I’ve heard it said this causes or helps galvanic corrosion, and that the earth wire contributes to this in some way.
For shore power, the boat is grounded to the shore via the earth wire, but boats typically use shore power to run the battery charger and possibly some outlets, which are not connected to the boat’s ‘earth’, at least for wood/GRP hulls, typically a bolt on the inboard side of the zinc anode.
So, the shore power earth only refers to the 240V devices, not the 12V related devices, engine/generator and metalwork in contact with the seawater (seacocks, propshaft etc) which, if anything, are earthed to the anode.
So, the anode protects the bronze prop and seacocks in the water, but what is the cathode? Is it just other more reactive ‘noble’ metals in the water, like stainless steel?
Does the shore power cable in the water have any effect by inducing a current that encourages electrolysis between the various metals in the water in some way?
Duncan Kent replies: Boat owners often confuse the terms earthing and bonding. Earthing is an AC term and is when the neutral conductor is connected to ground at the point of generation, providing the green/yellow ‘earth wire’ in a terrestrial AC circuit. With marina shore power, often the earth and neutral wires are connected again locally, where the power first arrives on site.
Bonding, on the other hand, is where you connect all the metal objects together in order to prevent galvanic corrosion. Put simply, this is where two dissimilar metals react with each other in an electrolyte (in this case seawater), with the least ‘noble’ (in the galvanic series) metal being gradually eaten away. Usually, the engine, gearbox, propshaft, propeller and metallic skin fittings will be bonded together with the battery negatives and a sacrificial anode. The anode (least noble metal, usually zinc or aluminium) is there to protect those submerged metallic objects from galvanic erosion, by eroding itself.
When you connect to shore power you’ll be bringing the earth on board too, which should be connected to the same circuit as for bonding, but at a different point to where all the others are connected. This
