Batteries and Voltage + hull

[timddo]

I was doing some electrical cabling on the boat and using my voltmeter i tested if the hull was earth.

Anyway. i put the positive terminal of the voltmeter on the battery and the negative on the hull

guess what 12.72 volts

Now is that normal.

Putting both red and black on the hull gives me zero

Putting black on the neutral on the battery and red on the hull gave me zero

[Travesso]

You have a negative connected to your hull somewhere mate. Putting both terminals on the hull will always give zero volts, whether or not it is earthed. I dont know if it is normal practice to run negative through the hull... I would have thought it would speed up corrosion and electrolosys?

[Travesso]

On the plus side, if your hull is supposed to be earthed like it is (connected to the negative terminal), you will only need to run half as many wires to place accessories / lights in areas remote to the switch panel! ;) All cars are wired in this way, but I definitely would not earth a boat hull... corrosion can be a cruel enough mistress as it is normally!

[FNQCairns]

Your outboard to run needs an earth, your earthed outboard is bolted to the hull, it's normal. Although conducting current proper is not, can turn a boat into an aspro.

measuring voltage from the hull doesn't mean there is stray current of the likes that will cause a problem....but it is possible.

cheers fnq

[DAVE_S]

timddo
Have a look at this link and it will tell you how to test your hull.
http://www.protroll.com/books/?id=5&p_id=4

[TheRealAndy]

On the plus side, if your hull is supposed to be earthed like it is (connected to the negative terminal), you will only need to run half as many wires to place accessories / lights in areas remote to the switch panel! ;) All cars are wired in this way, but I definitely would not earth a boat hull... corrosion can be a cruel enough mistress as it is normally!


You should never ever wire a boat like a car. You will end up with holes in it.

[Travesso]

You should never ever wire a boat like a car. You will end up with holes in it.

Nothing like a $30K aspro clear... :o

[finga]

You have a negative connected to your hull somewhere mate. Putting both terminals on the hull will always give zero volts, whether or not it is earthed.
Not quite right.
I have seen potential differences (ie voltages) in different sections of the hull. And quite large differences.
Not good to have and harder to resolve.
Equipotential bonding in boats is a must for boats that are moored or in the water a lot.

[PinHead]

Isn't putting you meter on the positive terminal of tbe battery and then onto the hull just making the hull the earth in that instance. You have provided the current via your meter.

I would have thought the test you would use, would be to use the ohmeter and from the negative bus bar to the hull to see if there is continuity there?

[finga]

Isn't putting you meter on the positive terminal of tbe battery and then onto the hull just making the hull the earth in that instance. You have provided the current via your meter.

I would have thought the test you would use, would be to use the ohmeter and from the negative bus bar to the hull to see if there is continuity there?
Basically an Ohm meter measures current (or compares what voltages goes in and what voltage comes out) but the scale is in Ohms. An Ohmmeter normally gets the voltage from the internal battery.
So in this instance the voltage is applied by either the boat battery or the internal battery of the meter......and you basically measure what comes out...but the units of measurement (ie resistance or voltage) is determined by where you get your voltage from.

Someone else might be able to explain better then me....I hope.

Basic thing is if you want to measure voltage or resistance you need some form of potential difference......unless if you want the reading to be zero, zilch, nadda

[Spaniard_King]

People get led astray when they testing Expotential differences in alluminium hulls

firstly we all forget thatn the outboard is bolted to the hull and the earth of the battery is bolted to the engine.

The dramma starts when the earth return for any accessories on the boat takes the Path to the negative through the hull to the engine and on to the battery.

Very important for all earth connections to be propper and maintained propper to ensure minimal current flow thru the hull

[8astripey]

the major problem with tinnies comming out of dealers today is they are wired to a price and by an auto electrician. i bought an alycraft 4.25 run-about in 1999 from a tweed dealer at the boatshow. great little boat but after a year in the shed the paint started lifting and every screw holding the floor down turned white. i had put a battery isolator in it at this point and extra battery and checked all the wiring. found they had bolted all the neg,s to a screw under the dash just like in a car. lifted all the foam out under the floor and found electroysis in 5 spots nearly through the jull which cost me a bit of time and effort to repair. mainly where the substandard floatation had been holding moisture against the ribs.
rang allycraft and was told they only supply the hulls to dealers who fit them out. strange as allycraft put out the broacher fitted out as purchased. contacted dealer who said i did not know what i was talking about and they wire all their tinnies and aluminium craft that way? again strange as i am a cathodic protection engineer, after getting nowere with allycraft and dealer who was only interested in reneging on any waranty, again strange how that came into the conversation also even tho i did not mention it.
anyway all the advise you get from these forums can only help mate. my advise is to put a battery isolator in and look for that one connection to your hull apart from the motor connection disconnect it and run one neg wire back to your battery. run all your accessories neg's back through this wire. lift your flooring and look at the hull where any moisture may accumulate . some of the ply they use in the floor will hold moisture and electroysis loves moisture. if you can, mount any of your accessoiries with brass screws and nuts isolated from the hull by fibre washers (ever tried to get a corroded stainless screw out of aluminium without destroying something, brass you can cut with a pair of pliers and drill out if corrosion sets in). if you have a bare hull under the floor and you have kids or anyone in the boat who dont care what happens when they drop a hook or sinker, lift the floor regularly to check for those bits of metal and if you are extra keen pain the hull with a good primer and white finish (you can pick up a lot of detail whith a white background). but whatever you do whenever you buy a new ally boat with a floor lift it and inspect it. then you will get an idea of how some well know dealers can be trtusted.

[timddo]

Can i do the following test
1. disconnect the negative from the outboard
2. test from battery postive to hull - readings should be zero

I'm pretty sure the boat is earthed back to the battery.

[Mister]

I was doing some electrical cabling on the boat and using my voltmeter i tested if the hull was earth.

Anyway. i put the positive terminal of the voltmeter on the battery and the negative on the hull

guess what 12.72 volts

Now is that normal. YES

Putting both red and black on the hull gives me zero. Voltage? So it should

Putting black on the neutral on the battery and red on the hull gave me zero. So it should

All good then.