Need to Ground 12V DC system?

Discussion in 'Electrical Systems' started by fuzzy, Apr 10, 2008.

  1. fuzzy
    Joined: Apr 2008
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    fuzzy New Member

    I'm helping a friend wire a small sailboat for lights, and we're planning on 5 lights being run from a 12V DC battery. The hull is fiberglass, and there is a small gas-driven outboard motor which is hand started and has no electrical connection to the system we're installing. We're not planning on installing a radio in the forseeable future. Charging will take place either by removal of the battery and charging on land or by temporarily attaching a charger.

    I have an electrically-related engineering degree, but no practical experience on sailing systems, so I'm working from Casey's "Sailboat Electronics Simplified." On pg. 38, Casey states "The negative post of all batteries should be connected to 'ground' to hold it at 0 potential -- relative to earth."

    I can't figure out why the electrical system would need to be connected to ground for this type of installation. Is it really required? If so, can anyone exaplin why?

    Pardon me if I've missed a previous thread addressing this, I only saw tangentially related posts while searching.
     
  2. Ike
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    Ike Senior Member

    Ground on a boat is important because of problems with galvanic corrosion and shock hazards. But on an extremely simple system like yours the negative terminal is the ground. The whole idea is to establish a point of zero potential. Just use a two wire system and it should work fine.

    Normally on most boats with 12V DC systems that are more complex than yours, the engine block or block of the outboard is the ground. But when there is no engine then a ground buss usually becomes the ground and all negative conductors are connected to the ground buss.

    By the way, Casey's is a pretty good reference. But many of these books are witten for systems with a lot more 12v equipment than just lights.
     
  3. Frosty

    Frosty Previous Member

    The engine will be protected by its own system as it was designed by the manufacturer.

    It is assumed that a small hand start engine would be on a dinghy or small boat that would not have complicated electrics. A 2 wire system is fine.
     
  4. rjdybus1
    Joined: Jun 2008
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    rjdybus1 New Member

    Wireing design for 1972 mercruser 250 L6 (165HP)

    Need help, looking for the engine wireing design for the motor for a 72 Mercruser 250cid at 165 HP. If you can help let me know at rjdybus@yahoo,com .....................thanks
    Rich
     
  5. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    I concur with Ike & Frosty here; plain old 2-wire should work for this simple setup. Ground problems can really f*** up a radio and can cause galvanic corrosion problems if you have a more complicated setup. But what's been described here already is exactly what has worked for seven seasons on my own boat with not a single electrical problem, similar equipment list to yours. My electric-start outboard charges the whole works up and, like virtually all of its kind, has the requisite regulation built-in.
     
  6. Patrick Greer
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    Patrick Greer New Member

    Similar issue, but a bit more complex.

    Good day, I'm brand new to the form...

    I just found this post, and am having a similar issue, however I have a radio on board, and a charger on my outboard. The radio is running directly to the battery, and the pos. / neg. is running back to the charger on the outboard.

    The only shore power I have is a 30amp (i think) shorepower cord with a 110 volt adapter on it.

    I bought a portable charger, but the directions state specifically to remove the battery from the boat for charging.

    I think I need a grounding plate, but am not really sure. Does the outboard fulfill this need via the negative ground to that from the battery?

    I pull my outboard up and out of the water when not in use, and even if it did ground the boat when in the water, I don't particularly want to leave it there all the time.

    Any help is much appreciated.

    Thanks,
    Patrick
     
  7. Ike
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    Ike Senior Member

    The outboard is your ground. And it is best to charge your batteries ashore unless you have a marine charger that is ignition protected and a marine shore tie with plug, marine cable, and circuit breaker panel. Just running a cord from shore onto your boat is dangerous and presents a serious shock hazard. However, if your boat is trailered, you can then charge your battery without removing it, while the boat is sitting in your driveway. But do not do this while the boat is in the water.
     
  8. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Just to expand a bit on what Ike said (and he's right):

    If you're going to bring 120 VAC (since you're in the USA) power aboard the boat, the boat needs to be properly set up with a fixed 120 VAC system that meets applicable marine standards. This involves marine-rated shore power cables, circuit breaker panel, proper bonding of the ground (green) wire (unlike a house, the neutral/white is NOT grounded aboard the boat as doing so can kill swimmers near the vessel), add a galvanic isolator or transformer if you don't want your engine to dissolve away atom by atom, etc... it becomes a very involved installation.

    Why can't you just bring an extension cord on board? Let's say your charger has a frayed wire somewhere. Its casing is now live. You are now reliant on every link in the ground line to carry that leaking current back to shore so that the fuse can overload and blow. If that ground link is anything less than perfect at every step of the way, the fuse won't blow and the casing stays live. When you touch it, you become the ground link back to the water (the ocean/lake is at ground potential and conducts rather nicely). Not good.

    Bottom line- if you are bringing shorepower onto the boat, there are three options:
    (1) A proper, fixed shorepower installation with appropriate safety gear
    (2) Charge it only when it's parked in your driveway on the trailer
    (3) Take the battery out of the boat
    Simply dragging an extension cord aboard ship, or trying to duplicate a house-style installation aboard ship, is likely to kill someone.
     

  9. Patrick Greer
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    Patrick Greer New Member

    Thank you very much for the replies. I appreciate it!
     
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