I think this is related to anodes. In following this article
https://www.protroll.com/bb-all-chapters
"Even if you may never intend to purchase a Black Box, you should carefully follow these steps. You can save yourself a lot of grief from corroded boat parts and may considerably enhance your fishing results.
Whenever a boat is in water, the different underwater metal parts interact with each other to form a weak battery. Electrical currents flow from one metal part to another depending on the type and placement of the metals involved as well as the mineral content of the water. Typical metals used on boats include aluminum, copper, steel, brass, stainless steel and zinc as sacrificial anodes. If a boat is set up properly all the corrosion is channeled so it dissipates harmlessly in the zinc sacrificial anodes. As it does so, it creates a positive field around the vessel which can be helpful in attracting fish. There are three seperate electical sections:
1. The zinc anodes are located on the outboard motor, outdrive or propeller shaft. Because of the low nobility of zinc, these will be charged positive.
2. The hull itself and all the metal parts in contact with the water will have a negative charge. These parts include the hull if it is metal, motor shafts, outdrives, electric trolling motors and thru hull fittings. All these metal parts must be interconnected with a bonding wire.
3. Other metal objects in contact with the water like stainless downrigger cable or wire fishing line that are not connected to the boat will be charged positive. By using a voltmeter with a scale from zero to one volt, a series of measurements can be taken which indicate whether or not the boat is being protected from harmful electrical corrosion and if the downrigger cable has the proper positive field to attract fish. Following is the step-by-step procedure."
I found the jet drives are not grounded, they're isolated. The anodes seem to be working for the most part, but there is some corrosion showing up on the jets. Will grounding help?
