How much 12 volt in your hull?

[ALUMINATOR]

I am finishing my boat makeover and have been watching my volt meter as I am doing some new wiring. Today I hooked up a wire and with my meter red on + battery and the - to the hull I got 6 volts. I then touched the red to the hull and nothing sparked at all. Next I started looking around and even when I touched a bundle of wires I could get the meter to move around a little. It is easy to get 12 volts to the hull just by touching the ground buss and the hull with the other hand. As I hooked up a few more wires I would get one 1/10 of a volt sometimes for each wire added to the buss. i am guessing that this is just small amounts of 12 volt without any amps. Should I worry yet?

[JETTYWOLF]

Very interesting. But I would think you should get nothing. But then again I know zero about the subject.

I hope I get no voltage anywhere!

[welder]

Is your grounding block isolated from the Hull [ Do not use the Hull for any connection ] and be sure to run a 2 wire system to everything .
If you can take your Volt meter and touch the Neg. to the hull and the Pos. to a hot wire at any fixture and read 12V that means your grounding out some where to your hull .

[IronwoodIsland]

A good way to check on the quality of the insulation in your boat (and whether you have some unwanted grounds somewhere) is to disconnect both the positve and negative leads on the battery and switch your multimeter to ohms and measure the resistance between the hull and the negative and positive wires with all breakers off or fuses out and everything switched off.

If your wiring is truly isolated and of good quality, you should get either open circuit (infinite resistance) or, if you have a good quality multi meter ... something up in the mega-ohms.

In practice you will get something less than mega-ohms due to salt on all the wiring terminals etc. Likely something in the hundreds of kilo-ohms might be acceptable (ie leakages of .02 mA at 500kohm or 2mA at 50Kohm).

Probably the reason you are getting funny readings is that any leakage current causes voltage drops that your meter may be sensitive enough to read and the leakages from both sides of the circuit relative to ground may be of similar magnitude, dividing the 12V battery voltage into two leakages each causing 6V of drop to the hull (if the wiring is a floating ground).

Anyways, worth poking around with your meter - remember with your meter on "volts" you can't do any damage.

I've got an unintended path to ground on my alloy Ironwood at the 12V powerpoint on the dash - these things only mount by attaching the negative side to ground - My boat came with a big "Mecca" ground, - all black wires leading to a big hull ground nut which I am thinking of removing. I'll find out how the hull is wrt to electrolysis when I haul for the first detailled inspection next week. Only had a few minutes to look over the bottom when I bought the boat last year.

[ALUMINATOR]

Thanks guys! If you only knew how much extra work that I have done to be sure that this didn't happen. I have a separate battery for starting and even have a solenoid on the neg cable to disconnect it when not in use for cranking. All wires are pairs and if any load uses the base for ground I have it mounted in such a way to isolate it from the hull. I am guessing that I should of got a cheaper meter and maybe I would not see the little volts :lol: .

I like the ides for checking the oms and this after noon will tell the story.

I am not sure what you meant by saying "meter on volts you can't do any damage"???? I am starting to think that if I can pass 12 volts from ground to the hull just by touching it with my finger I might be chasing a ghost.


PS. Wouldn't a continuity test say with my meter or a light bulb tell if there was a problem or does it take to many volts to lite the bulb?

[Chaps]

Come on you guys, unless you've electrically isolated your engine mount your hull is ground. Whats the big deal? Just don't use it to complete any circuits. What am I missing and/or where is the problem?

[ALUMINATOR]

My engine electrical system is isolated from my house bank and the diesel starter is the only ground to the engine and that is only connected when the started is turning. Then a solenoid disconnects the ground wire. So yes the hull is isolated and not used for a ground! I even have an isolated ground alternator. I was asking why I am getting dc voltage between the hull and neg side of the house battery.

[Chaps]

Somewhere in your boat there must be an electrical device that is attached to a bulkhead, panel or hull framing that grounds through its mount.

[ALUMINATOR]

Somewhere in your boat there must be an electrical device that is attached to a bulkhead, panel or hull framing that grounds through its mount.

I got busy with my real job today but maybe tomorrow I will start looking at each circuit until I find the bugger!!!

[capeannfisherman]

Is your boat in the water? At a marina? You should not read a voltage from your negative terminal to the hull. Is your motor attached to the hull with a metal connection? If so isn't the motor itself attached to the negative terminal of the battery?

[IronwoodIsland]

Hi Aluminator

Just as a matter of interest, voltage is actually something that you can measure without a complete circuit - it is really the electric field you are measuring. 12 Volts just means that each coulomb of charge has 12 joules of energy at that particular position where the leads of your voltmeter are.

To see this, if you have a really good voltmeter you can measure the electric field of the earth - you will get different voltages as you wave the leads around in the air without touching anything.

To some extent this is similar to what you may be seeing with your meter as you poke around in the circuits you are working on.

Although switches may be off, there is often still some leakage around the open contacts through or around the switch case. The 12 volts at the battery has to get divided out somewhere around the circuit. As a fictitious example, relative to the hull, you might have 6 volts developing over the resistance inside the open switch and 6 volts elsewhere, thus you could get some odd voltage on your meter, nothing to worry about as there is likely almost no current happening.

What is more informative to check is if all your loads are off, what is the leakage current from the circuits as they are connected to the battery.

The main reason we don't want multiple ground points in Al boats is that boats are wired with copper wire. At each grounded junction between the copper and the aluminum wire a small battery is formed if the joint traps water. The voltage generated at each of these "batteries" depends on the salinity, amount of oxygen etc etc. If the "batteries" all generated the same voltage they would all cancel and there would be no current flowing to cause corrosion at one end of the loop. In practice they generate different voltages so it does not cancel.

Another reason to avoid multiple ground points is that as loads increase in the circuits, more and more of the return current starts to shortcut between the hull grounds due to the resistance in the black ground wire where the current should be flowing. The unwanted hull return current can cause corrosion at the ground connection where the current leaves the aluminum (more positive terminal) if it goes on for a long time. This is called a ground loop.