Groundless Fears

Bill Schultz of Bay City, Michigan, asks:

“I am installing a 24-volt battery bank in the bow of my 40-foot sailboat to power a bow thruster. Do I need to run a grounding wire to my engine block? Or can I use an alternative grounding site? What size wire do I need?”

Nigel Calder replies:

Technically, if the bow thruster has a case ground (separate from the negative terminal) that is used for “bonding” purposes (lightning and corrosion protection), this wire needs to be the same size as, or one size smaller than the negative cable to the bow thruster assembly. It also needs to run back to the boat’s common ground point. This results in a large, heavy and expensive cable running pretty much the length of the boat. Consequently, I have never actually seen one installed and don’t have one on my own boat. I would just make sure the positive and negative cables are properly sized to minimize voltage drop (no more than 10 percent across the total cable run) and leave it at that.

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