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My trolling motor for my boat has a black and a red connection. The red goes to the positive on the 12 volt battery. The black goes to the negative on the 12 volt battery. Could I connect the black to the giant metal boat I am driving, and then connect the negative on the battery to a different part of the metal boat? Since I use 2 12 volts (one for the trolling motor and one for the main engine starter and aerator) could I use only a single connection from one battery to ground the entire boat?

Or am I doing this all wrong?

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    Probably could, but the connection from the battery to the metal hull to wherever you would connect the black lead of the motor is of undetermined impedance. You could have a substantial voltage drop resulting in a loss of efficiency and lower motor speed.
    – John D
    Commented Jul 11, 2016 at 21:32
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    Is the boat connected to the negative of the battery? Otherwise there just won't be a closed circuit..
    – Eugene Sh.
    Commented Jul 11, 2016 at 21:34
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    You may also induce corrosion - particularly in salt-water conditions.
    – Transistor
    Commented Jul 11, 2016 at 21:40

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You could do this, but you would not want to. By running current through the surface of the boat, you will induce a voltage potential across it. This voltage potential will cause current to flow from one part of the boat, through the water, to another part of the boat. This will cause electrolytic corrosion, which you definitely don't want.

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  • What if my boat was metal but had a rubber coating on the hull?
    – USER_8675309
    Commented Jul 11, 2016 at 21:47
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    @USER_8675309 Then this would not be an issue if the rubber coating were perfect, but would be a much more serious issue if the rubber coating had even a few unobservable pinholes.
    – David Schwartz
    Commented Jul 11, 2016 at 22:09
  • This is the reason boat places sell big chinks of zinc to bolt to your hull, because even small electric currents (in this case, galvanic reaction) can have a big effect, especially on a big metal thing floating in electrolyte. Copper wire is cheaper than a new hull.
    – John U
    Commented Jul 12, 2016 at 9:31
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You could do what you are suggesting, but to what end? It seems to me you'd have a much greater chance for something to go wrong. Here are a few things to think about:

If you are running your trolling motor off of both, there is the distinct possibility you'll drain both batteries and then you'd be stranded. Keeping them independent will help ensure this won't happen. Your primary battery should be like what you'd find in a car, which provides a quick discharge with high amperage. The one you are using for your trolling motor should be a deep cycle battery, one which will discharge over a much longer period of time. While they could be tied together, you really don't want to do this.

Another thought is you probably should be running your aerator off of the trolling motor battery. For the same reason as stated above, you can drain your primary battery, again, leaving you stranded. Wouldn't be pretty either way.

You need to charge the one which does the trolling, which means disconnecting it on a regular basis to get it done. Keeping it separate will help in that endeavor, I'd think. The other one will probably be charged by the main boat motor, so you don't have to worry about it as much.

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  • I find it interesting that you think the aeroator should be running on the trolling motor. The main engine has an alternator, and I very rarely am required to pull this battery. It also runs the rest of the boats electronics, including the bilge, lights, and sonar. I can leave the boat on automatic pump (for the aerator) for nearly a week before needing to switch that battery out. However, I frequently burn through 2 12v deep cycle batteries a day with my trolling motor, depending on how much fishing I do.
    – USER_8675309
    Commented Jul 12, 2016 at 11:48
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    If it will last that long than I guess no issues. I just don't like the idea of getting stuck up the creek without a paddle, if you will. My main thing in the statement was run the stuff on deep cycle you don't absolutely need, and leave the main battery alone to start the motor when you need it to go home.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 12, 2016 at 20:30
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    Pretty good idea there. In my situation -- I could paddle the boat home in 30 minutes at most with a single paddle, and it wouldn't take much longer to pull the boat on shore and walk back to the cabin. I frequently start the main engine before leaving the dock and let it run for 10-20 minutes (assuming I'll be using the trolling motor to traverse the lake) to ensure that it is charged and ready to go. If I take my boat to a different lake where I am less familiar, I always take a backup charged 12volt battery that I can switch out for either one in case of emergency. Thanks!
    – USER_8675309
    Commented Jul 12, 2016 at 21:22
  • And that's why you are a well prepared person ... covering you bases! Excellent choice :o)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 12, 2016 at 21:23
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If there is no connection to a boat shell at all, then better stay away from grounding. It might be designed that way because you will feed a fish with 12v.. Not sure, but I would concern about it.

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    I don't think there is any danger to the wildlife as the potential is across the boat, so the fish don't actually complete any circuit and are safe.
    – André Borie
    Commented Jul 12, 2016 at 0:33
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Apart from the corrosion issues:

If you do this, one day you will be moored to the dock doing some maintenance work on your boat's electrics, when some klutz accidentally drops a live mains power cord extension into the water.

Salt water is a pretty good conductor, so your boat electrics will just have been powered up by a 120V supply when you least expected it. Not a good situation to be in!

And if you are ever out at sea during a thunderstorm, a lightning strike into the water might not do your electrical system any good, either.

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  • Unlikely, even if it is dropped really close to your boat, the vast majority of current will be via the least resistent path. For your boat to even be on the least resistent path, the dropped cord end would basically have to be touching your hull. And in that case, the least resistant path is through the shortsection of your hull closest to the plug (rather than around through your boats whole electric system). While with senstivive equiptment you might be able to measure the potential difference (ie voltage), negligable current (Amps) is going to flow, so ~ no power (Watts) is going run.
    – Frames Catherine White
    Commented Jul 12, 2016 at 15:05
  • Additionally, the boat is only moored at my dock, where there are no extension cords. Also, I make a habit to not be on my lake during thunderstorms :). But I certainly understand your point
    – USER_8675309
    Commented Jul 12, 2016 at 15:12
  • Accidents involving boats and unintentional electrical connections to bodies of water definitely kill people. It's hard to get good statistics because this doesn't have its own "official" classification as a cause of death, but in the USA it's the same order of magnitude as deaths from lightning strikes. It doesn't have to be "close" to your boat, unless you count 100 yards away as "close". If you ever used high voltage equipment (e.g. something powered from your boat's battery by an inverter) you could be putting other people innocently using the waterway at risk.
    – alephzero
    Commented Jul 12, 2016 at 17:38

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