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Random Quote: All the world's a bowl and us just boaters in it. We have our entrances and our exits...
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.
__________________ Mac Solo
1985 25' Parker Sou'Wester---2005 F250 Yamaha
A grounding plate serves only two functions in a boat of this size--as a ground for a SSB radio (which you don't have) and if you have a lightning protection system--which it does not sound as if you have either. I would seal the holes, put the negative from the sender to the battery or negative bus bar.
A 'true grounding plate' servese several functions as mentioned above. In fact, most power boats are using the negative cable on the battery. In practicality (most cases) the negitative cable goes to the motor, the motor into the water; therefore grounded. Books have been written on this...maybe I should get one! As far as the GPS/Sonar; most do not use the ground lug/stud at all because of no grounding system/plate. In your case, (if you will not use the grounding plate, that is bad) I would be sure toreplace or remove and fill holes before it falls off.
__________________
Boston Whaler, "MUMBLER", 24 Outrage, twin 175 HP Evinrude Ocean Pros
originalsin - 12/31/2005 3:09 PM This brings up another curiosity. Should my metal tanks be grounded???
YES SIR
I would have guessed so. Will the ground wire on the sender suffice or should I add some other form of ground? And just for clarity, this can be done using the neg terminal of the battery, correct?
__________________ Mac Solo
1985 25' Parker Sou'Wester---2005 F250 Yamaha
If the plate was on the '85 Parker it was very likely installed as the required hull ground plate for a loran set. If it is the commonly installed Dynaplate it is sintered bronze ( rough appearance) and should not have been bottom painted if still used. Sometimes they look much worse than they are & can be wire brushed clean. If it is falling apart & unused , best to get it out of there and fill the 2 hull holes with marine epoxy.
Do I need a grounding plate??? 
