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Almost forgot...don’t get hung up on trying to isolate the downriggers from the boat or “leaking” voltage from a short in the boat. Nothing is isolated because the boat is in the water and the downriggers are in the same water. Nothing is isolated. Even if your boat isn’t leaking voltage, the current will come from galvanic corrosion. I’ll spare you the science, but in short, aluminum, stainless steel, and zinc anode protection on boat motors/pumps create current between themselves through a process known as galvanic corrosion. That is most likely where the voltage is going to come from.