Do I need a grounding plate???

[originalsin]

Earlier today I was installing a Christmas present--new Furuno 7000 combo GPS/Sonar--and I found the remains of what seems to have been a grounding plate mounted about five feet forward of my transom. I bought the boat used earlier this year (Mac Solo in my signature) and this was the first time I've had a reason to be on the ground under the back of the boat. The plate appears to have been copper but I'm only guessing. It was corroded so badly that only after inspection of the bilge and discovering wires (semi hidden, previously unseen) hooked to it did I realize what it was. I checked the wires and of the three, only one was still connected to anything. It is the ground that comes from the sending unit on the main fuel tank.

Interestingly enough, the sending unit on the auxillary tank is hooked to the common ground on the fuse panel under my dash while the main tank is/was hooked to this newly discovered plate--the gauge for the aux tank works fine while the gauge for the main tank doesn't function (both gauges are new). I guess for a test I could hook the main tank sending unit ground to the common ground to determine what is going on. If the guage magically begins to function, I guess I will have learned a few things.

Anyway, a few questions:

1. I can't imagine it was still functional seeing how badly it was corroded. Should I replace it or just seal the holes.

2. Why was it there in the first place? I have two other boats, one with a sealed in fuel tank and one without, and neither of them have a grounding plate.

3. Could I hook the ground of the sending unit to the neg terminal on the battery and achieve the same effect? This is how me auxillary tank sending unit is wired and it seems to work fine.

4. If I do replace it should it be hooked to the neg terminal on the battery or only the grounding posts on the equipment that has such?


Note: By common ground above I was referring to a terminal block that is wired directly to the negative post of one of my batteries. All of the grounds for my electrical equipment are in turn wired to the terminal block.

[TTaxi]

O/S- The grounding plate was probably a Dynaplate made of sintered bronze I believe, such as these: http://www.yachtbits.com/dynaplate/s...ing_plates.php .

There were routinely installed as the required hull ground plane for LORANs and SSB's . Sometimes they are used as ground plates for bonding systems and related lightning protection plans.

If you take a wire brush to the outside does it disintegrate? If so , obviously replace or discard and prepare/fill holes with marine epoxy if you determine it is no longer needed for bonding hull ground. It may just clean up( don't paint over.)

There is a good chance your main tank gauge will function properly if the sender is grounded to the neg. bus. That said, the tanks should also be included in the vessel's bonding system.

[originalsin]

Thanks. Your link shows a picture that makes sense. It appears to have been the same dimension, or shape anyway, before the salt went to work on it.

Now tell me what you mean by "bonding system." I'm don't believe I've ever heard the term.