Question: Recently I was snooping around in the bilge area of my new boat and I ran into the green and yellow wire attached to a bronze stud in my bilge. The stud appears to go through the hull bottom. I have no clue about what this wire does or what purpose it serves. Any ideas on what this wire is doing?
Answer: I can say for sure that the cable in the photo you sent in is serving as a grounding conductor, or as the Europeans refer to it, “earthing." I can also say that although you didn’t tell us, your boat was built in Europe unless it was made here in the US by Island Packet. The reason for recognizing this is that wire color code standards allow for the use of either green, or green with a yellow stripe, as appropriate for grounding conductors.
Here in the US the only builder I’m aware of that uses the green with the yellow stripe option is Island Packet. They actually use both the plain green and the green with yellow stripe. They use both options as a means to differentiate between grounding and bonding conductors throughout their boats. The differences there are a topic for another day. The Europeans have always favored the green with a yellow stripe option for their grounding conductors.
What I can’t tell you by looking at your photo is which piece of equipment is being grounded by the wire. Trace it back to the equipment to learn that. Whatever the component is, it appears that the designers wanted an isolated grounding system for their device. That’s what you have.