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Thread: Continuity through boat hull

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    Default Continuity through boat hull

    On my new to me boat, I bolted the black box to a compartment. When I ran the case ground to the battery, that made the hull a ground. Is this a bad thing or a non issue?

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    The hull is already grounded as soon as the motor is installed. Just don't use the hull for a ground. Always run a ground wire to the equipment.
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    Are the electric circuits on the motor bonded to the case of the motor? The motor being bolted to the boat doesn’t give it continuity

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    The negative post of the cranking battery is grounded to the outboard engine block.....the outboard is isolated from the hull thru rubber bushings in the pivot point. Never ground anything to the hull.....ground to negative battery post or ground buss bar on your fuse panel. Grounding to Hull can set up a electrolisis and over a period of years will cause pinholes all in your hull. Electrolitic corrosion.
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    I wasn’t going to use the hull as a ground, I just wondered if the continuity was an issue. I wasn’t sure if that would cause electrolysis or not. With the motor not running, and the LS case ground unhooked from the battery there is no continuity to the boat hull via the negative battery post.
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    Perhaps you could barrier isolate the black box in the compartment so it won't ground to the hull.
    Thin rubber mat material, nylon fasteners, etc.
    I fully agree with always running grounds from devices to the battery, always the best choice. Even with the trailer. I know some people don't, but I feel better and my led trailer lights appear brighter grounding thru the wiring to the battery/charging system.
    Come to think of it, if you don't and your boat is metal and you lean over out of the boat to wash your hands - it kind of makes you an anode, doesn't it?
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    I did put a rubber washer between the black box and the aluminum. I didn't do it for an electrical reason, I did it to give a little air gap. I thought about changing to nylon screws but I don't think they will be strong enough to hold the box up. I've got it mounted upside down under the top of the deck and I don't know if they would support the full weight of the box over time.

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    If the engine has electric start, the hull is grounded. Ground straps at various locations on the outboard insure that the entire platform is in the negative ground circuit. Drop it in the water and the circuit will improve. I don't recommend using the hull as a grounding lug for reasons including electrolysis and electrical noise interference between electrical equipment. The most important take away is to know that if positive cables ran to the starter battery are allowed to contact the bare hull by way of poor installation or scuffed insulation, you will blow fuses or smell smoke pdq.
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    Sliderman, I donít want to sound argumentative because Iím not trying to be. I just donít understand that. Nobody that knows me has ever called me an electrician. I can fix anything but just get by on my electrical knowledge. If the motor was grounded to the hull then why do I lose continuity when I take the LS case ground wire off the battery? With the ohm meter set on X1000 I get nothing as long as that wire is off. I can put the LS case ground wire back on the battery and with the meter set on X1 it pegs it out.

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    Between what two points are you taking your continuity measurements? If you have an aluminum hull and an electric start motor, you should have some level of continuity between the hull and the negative post of your starting battery. If you are connecting your accessory to a battery other than the one that the outboard is connected to, then the accessory is the point of ground to the hull for that battery. Also keep in mind that if the factory ground straps on the outboard have a high resistance connection for whatever reason, when the boat is placed in the water, continuity will exist between the bare areas of the grounded outboard and unpainted surfaces of the boat that are below the water surface. A stainless steel screw in the transom is an example of a common point of the circuit bridge. Keep in mind that the paint is an insulator. You will also find that the steering cable and helm will be grounded and complete the circuit back to the starting battery.
    I'd rather be fishing.

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