Stray Current Corrosion . . .

[ianhef]

By "a circuit" do you mean that the multimeter was reading 12v with the probes on the hull and the + lead from the sterio?

No fortunatly its not that serious. I had the meter on resistance, the one that makes a noise when the two ends touch? Well it makes that noise when I touch the back of the stero and the bolt?

[noelex 77]

Originally Posted by ianhef

By "a circuit" do you mean that the multimeter was reading 12v with the probes on the hull and the + lead from the sterio?

No fortunatly its not that serious. I had the meter on resistance, the one that makes a noise when the two ends touch? Well it makes that noise when I touch the back of the stero and the bolt?

Try measuring (with the voltage scale on the multimeter) from the + supply of the stereo (with the CB on) to the hull..
Try this with the engine battery switch in both the on and off positions.

[ianhef]

Originally Posted by noelex 77

Try measuring (with the voltage scale on the multimeter) from the + supply of the stereo (with the CB on) to the hull..
Try this with the engine battery switch in both the on and off positions.

I've tried that and the result was zero. Is this good or is it bad?

[noelex 77]

Do you get continuity (the buzzer ) with and without the aerial connected?
Is the mounting hardware of the stereo touching the hull?

[ianhef]

After taking the thing to pieces the continuity was only between the negative and the bolt on the portlight, no continuity between the aerial or any of the other wires (positive, or speakers). Maybe it supposed to be like that? I'm not so good with electrics!

[noelex 77]

If there is continuity between the negative on the stereo and the bolt on the portlight you should see 12v when testing between the stereo + (when the stereo has power) and the hull.
I don’t understand how this is not the case, perhaps you could check that again. Sometimes a small amount of rust on a bolt will prevent the multimeter probe making proper contact, so you get a wrong reading.

[Boracay]

When I was measuring I was getting up to 0.04 Volts. However the only one I worried about was one that I could switch on and off...

The digital meters are very sensitive. I think that some of my "voltage" was galvanic difference.

[ianhef]

Ok folks, apologies. Tested again with the stereo turned on! Yes there is a small voltage between the stereo and the bolt. On the 2 V DC setting I get 0.085 with the stereo on and receiving, 0.035 with power to the stereo but on standby and 0.001 with the power turned off to the stereo. Mmmmm so what does it mean? Throw the stereo out?

[ianhef]

Tested again, disconnected the stereo completely and measured the voltage. This time its looking pretty grim. Between the positive and the bolt I'm getting 11.28, when I turn the breaker off for the stereo I get Zero. So any idea what this means?

[noelex 77]

OK that makes a lot more sense. The Stereo is providing the + and the hull is tied in to negative.
Some metal boats do tie the - to the hull so it is not necessary a problem.
What is always bad is wiring equipment up, and using the hull as the negative wire (like a car). This creates many points with a slightly different potential and can lead to problems.
You have to find out if the negative has been deliberately connected to the hull ( this is not ideal in my opinion, but many people consider it OK for a steel boat) or if it was designed to not have the hull at ground potential and a negative wire somewhere in the circuitry is accidentally touching the hull.
If the hull has been deliberately connected to the negative it needs to be done at one spot only. It is generally done at the engine. If you remove the battery negative connection to the engine and the voltage reading you are getting with the multimeter disappears then there is only one negative connection to the hull and the boat was probably built like this.

Warning electrical corrosion problems on boats are a complicated subject. Every owner of a metal boat should do his best to understand as much about the subject as possible. I am only a metal boat owner. There are others in the forum with much more professional knowledge, such as Brent Swain who always gives excellent advice. I am also much more familiar with aluminium yachts where the negative supply is usually not connected to the hull.

One final question are you having corrosion problems like the OP or is it just the multimeter reading that is concerning you?

[ianhef]

Noelex 77
Thanks for your words of advice. As far as I'm aware there is only one single earth and it is through the engine. My mission now is to make sure that this is the case and test it to make sure there are no stray currents. As far as I can tell there is not a problem with corrosion, it was just the fact that I was getting these readings on the multimeter and I didn't understand why. I'll try to get further into the boats electrical system and see if I can figure out properly how she is wired and learn a bit more about the electrical system and corrosion. Cheers.

[osirissail]

Here is what a marine corrosion/electrolysis meter looks like. Notice that the "green bands" are about midway on the meter scale. That means that you will be getting some milli-volts in the system when everything is "normal."
- - Also here is a link to what the various readings mean as to being over or under zinced.
Corrosion Quiz: Question 1

[ianhef]

Has anyone used this seabis system? Have a look at SeaBis Exclusively Prevents Electrolysis Corrosion - Guaranteed I'm not 100% sure if it is a really good system or just a fancy looking multimeter? Anything that helps the fight against stray currents is worth a look but its a bit on the expensive side, then again so are repairs!

[ianhef]

I had full day following wires and trying to figure out how the boat is wired. From what I can see the negative goes to two places, the starter motor and the alternator. From the starter motor there is a wire which goes directly to the negative bus bar, there is also a wire from the alternator that leads up to a meter, this same meter has a wire leading to it from the isolator switch. The positive from the battery goes to an isolator switch and from there to the starter motor and the buss board via a meter. Looks like its all earthed through the engine via either the starter motor or the alternator, does that make sense? Only issue is when I disconnect the wire directly from the starter motor to the buss bar I still get a reading across the two buss bars? I'm confused, its beyond my simple knowledge so I've got a marine electrician coming round on Friday to have a look and hopefully clear things up. I'll let you know what he thinks!

[osirissail]

The ground wire to the alternator is extremely important for protecting the alternator. The alternator's connection to the engine block is through some less than perfect electrical conducting bolts and brackets which frequently results in blown out alternators. The ground wire direct to the alternator is a very common solution.
- - However the main "fat" ground wire to the engine starter is probably connected to one of the starter to engine block main bolts. It all depends upon the physical location of the electrical panel's ground buss bars. From there the "fat" ground wire is routed to the nearest major attachment point on the engine - normally an engine mount bracket or other large bolt on the engine. Then another "fat" ground wire is lead back to the battery's negative terminal.
- - IMPORTANT -If you have a flexible coupling or "drivesaver" in your engine/prop shaft system you may have an "electrically isolated" prop shaft which means there is NO earth ground in your boat's system anymore. Some installations will have a wire from one side of the flexible couple/drivesaver to the other side to restore the electrical pathway. For the DC system this is not a problem as the batteries and DC loads are a closed circuit. But for the AC system you may have a serious problem. I have a separate grounding plate on the hull specifically for the AC earth ground so that the AC and DC never shall meet.
- - Normally the only thing interrupting this DC ground wire back to the battery would be a "shunt" used by amp-hour meters to calculate the total power used. Anything else would be highly suspect. Ampere meters are normally in the main positive feed wire between the battery switch and the load c/b's.
- - It is common to connect the AC system earth ground to the engine block and subsequently also to the DC system ground. A leaky AC appliance or receptacle or improper wiring connection can "leak" AC voltage to the earth ground and impress it also on the DC system which is not good for your delicate instruments. Of course, if you do not have any leaky AC components this is not a factor/problem.
- - Additionally, alternators are AC generating devices which are rectified to output DC. Failing diodes can allow AC to get into the DC system - but if your tests are done with the engine shut down, this would not be a factor.
- - Try disconnecting the AC system earth ground (and if you have a genset - also the neutral) from the engine or DC system ground buss and see what your readings are or if they change.
- - Remember there will be some milli-volts of readings in any bi-metallic system where sea-water is involved. The Corrosion Meter gauge is painted with the acceptable amount of voltage for each type of metal in the system. Also the chart in my post #27 shows the normal voltages encountered. Only if you are getting voltage outside these normal ranges do you have a "under/over" zinc'd problem.