Floating Ground on Aluminum Boat: Voltage Reading?

[colemj]

We also have a silver chloride half cell. I believe their function is to measure the anodic protection or over-protection provided to a piece of metal (the hull in your case), and not the current that may be flowing to something. So if the piece of metal has sufficient anodes, then the reading will be "good", even though there may be a problem when the anode runs out.

Mark

[SVSAVANNAH]

Hi all i have a similar question, not sure how all this works, should i post here or make a new thread?



Anyway , keep it simple for now:


I have an aluminum boat, i rewired when I bought it and am constantly upgrading and making changes. I have always done my utmost to maintain a floating ground


BUT


If i put a multi meter pos on my pos buss, and the negative on the hull I get about 5-10 volts (it randomly changes ever time i check). If i put a test light that normally draws 0.1 amps from pos to hull it does not light up. same for a 5mm led. With a meter on the 10 amps or the mA, or micro amp setting i get 0.000 amps-- no current flow.



If i do the same as above but from negative buss to hull I get a reading of 28 - 35 volts....

as above it randomly changes every time i check, test light does not light, meter on amps shows no current even on micro amps.



But if i take a bare wire and put it on the negative buss, and scratch the other end on the haul it makes a small orange spark..... and the spark gets less if i keep doing it in rapid succession. If i put a 5mm LED (anode to haul and cathode to negative buss) it very dimly lights up.



Also FYI I have a 24v system. I have solar panels that operat at 30+ volts, but it does this with them disconnected and or at night. My engines are grounded to the hull i guess but i have them on a physical switch to brake that circuit and it does this with them off, or disconnected, or once i jacked it up and had it on wood not touching the haul



I have been chasing this for almost a year now.



WTF





thanks

[wguinon]

Your negative battery connects to the engine block via the starter gound. The block connects to the sea water through the heat exchanger and the water pump">raw water pump and the sea water connects to the hull, now hear the word of the Lord:
The yellow metal in the heat exchanger or pump comprises an electrolytic cell, the source of the voltage.

[SVSAVANNAH]

Originally Posted by wguinon

Your negative battery connects to the engine block via the starter gound. The block connects to the sea water through the heat exchanger and the raw water pump and the sea water connects to the hull, now hear the word of the Lord:
The yellow metal in the heat exchanger or pump comprises an electrolytic cell, the source of the voltage.

I see what your saying, but I can completely take all the negative and positives for that matter off the start battery, or take the alternator and starter off and put them in a box and still get the voltage . I can see how all the different metals from heat exchanger, raw water pump, to zincs and shafts and props etc. all bathed in an electrolyte would create a battery, i guess i'm not a chemist but 34volts seams like a lot.

[slug]

Originally Posted by goboatingnow

without a circuit diagram its hard to fully understand

(A) I assume you have isolated starters and alternators , so that there is no unexpected paths to ground , This would also include engine senders.

(B) Remember a voltage meter is a very high impedance load, hence you can get "voltage" readings off almost anything. Whats matters is whether there is current flowing

so the 100ma would be a concern.




I suspect you have a ground return path which includes the hull ( well not suspect ,you do actually have a return path). You need to look at all the negative return paths from all DC devices.

you will be unlikely to eliminate all leakage current , but it should be possible to get the isolation reading into the mega ohm range which on a 12V system gets the leakage current into the microamps.


Areas to look at

Radio gear,
Devices with EMI filters with earth bonds to AC
AC devices with earth and DC - connections
Engine electrics
poor wiring causing leakage to hull


Its just a case of painstakingly going over the wiring and the devices.

Dave

Yah that sounds good

A very common leak is dc electric motors ....carbon brush dust buildup on the brush cage assembly leaking - to the hull

Isolate the motors like refer , auto pilot , winches and have a look

Electronics are also troublesome ...isolate and investigate

I’ve been messing around with metal boats for a long time , sometimes a leak springs up , I spend many hours chasing it down , the it goes away all by itself




Good that the OP is vigilant

[smac999]

Originally Posted by msponer

So far I've increased the resistance between the hull and ground from 0k to 800k ohms by disconnecting the VHF radio. I think there's some other issue to isolate, but am not sure what I am going for:

gernally the vhf antenna coax sheild is DC ground inside the radio. and when you mount a metal antenna bracket onto a metal boat. you just grounded the DC to the hull. a plastic bracket. or spacer of some sort to the metal bracket should remove that.