Hi Nick,

A boat electrical system has a lot of details, and
the devil lies somewhere in those details. If
you are going to find the problem, you need to be
able to draw a _detailed_ schematic of your boat
wiring including the 12V DC system, and the 120V
AC dock power.

Based on what you wrote below, I drew a basic
schematic of your 12V DC system, but it does not
have enough detail to determine where the problem
is. It did generate some questions that may help
you put a finger on where the problem lies.

1) Where is the "Safety Ground" for your 120V AC
dock power, and how is it hooked up to your hull?

2) Is your boat hooked up to dock power when it is
in the water?

3) Where are your 120V AC neutral and safety
ground connected together? And are they connected
to, or isolated from your 12V Ground?

4) If you disconnect your solar cells completely,
what voltage and resistance do you measure between
the 12V ground and the hull?


nick wrote:
Hi All.

I don't know if anyone can help with suggestions here but I'll give it a
whirl.

I hauled out my boat last week to do the bottom paint and discovered
substantial blistering of the paint. I attacked these with a high
pressure cleaner initially and that stripped most of the loose paint off.

In places bare metal was exposed, in others it was back to the primer or
undercoat. It was particularly evident around welds and through skin
fittings (metal ones only) and even around the anode bolts.

I had a shipwright and a very experience boat painter look at the hull
and both confirmed a stray current problem, albeit relatively minor.
Interestingly, both said I had TOO MANY anodes on the hull and that this
in itself can cause a problem.

I have since done some quite extensive reading on the subject on the net
and in some books and all confirm the diagnosis.

So, armed with a multimeter I set about doing some testing but first a
basic description of my electrical system.

The boat has a large house battery and a decent engine battery plus 180
watts of solar panels. The engine has a standard alternator (30A I
think) plus a heavy duty 120Amp alternator mounted on it.

The engine is isolated from the hull via rubber engine mounts.

All wiring that I can see is doubled with the positive wiring going back
to 2 main battery isolator switches with single pole appliance switches
in the circuit at the switch board. The negative wires all come back to
a bus then to the battery.

The only things that do not go through the main isolator switches are
the solar panels (via the regulator) and the bilge pump.

There is a Plasmatronics solar charge controller between the solar
panels and the house battery which appears to regulate the charge on the
negative wire...

There is also a sophisticated charging regulator for the heavy duty
alternator (can't remember the brand). Manages the boost, absorption, etc.

Back to the multimeter testing, when checking voltages, I am able to
measure a voltage between the +ve and -ve wires of approx 13.4V. I can
also measure a potential between the -ve wire and the hull of 0.6V with
the -ve being at a higher potential than the hull. This then gives a
voltage between the +ve wiring and the hull of 14V.

With everything double wired I would not have expected any connection to
the hull and thus not be able to measure any voltage between either the
+ve or -ve wires and the hull.

To add to this, when checking with the ohmmeter between the -ve wiring
or engine block and the hull, I can measure a resistance (indicating a
connection ) with the meter in one polarity but open circuit with the
polarity reversed.

I have a limited electronics background from many years ago and this
suggests to me that somewhere there is a diode passing current between
the -ve wiring and the hull. The 0.6 volt drop plus the dependence of
polarity in measuring a resistance between the hull and the -ve.

Sure sounds like a diode to me too. Now the
question is where is the diode.

I am suspecting something to do with my solar panels as their frames are
screwed directly to a SS bimini and hence the hull... (Can solar panels
leak to their frames???)

This may indeed be the problem. Disconnect the +
and - leads from the solar panels and retry your
measurements between -Ve and the hull. If the
solar cells have have anything to do with the
problem, this will show it.

This voltage was measurable both with the boat on the hard and in the
water.

What I don't understand is how the hull can be at a lower potential than
the -ve wiring. I also don't know whether there are any diodes
connected to the panels as the controller has a mosfet switch in it to
prevent battery drainage back through the panels.

The answer is simple. Somewhere your +Ve is
connected through a fairly large resistance to
your hull, and there is a forward biased diode
between your hull, and your -Ve. Draw the circuit
diagram I just described, and you'll instantly see
what I mean. The question is where is the diode
connection from hull to -Ve, and where is the
leakage resistance from the +Ve to the hull.

Solar cells are themselves just a diode, however
there are usually many cells in series to produce
a usable voltage output.

Is it possible that instruments (depth sounder, autopilot head, etc)
could be leaking to the hull?

Anything is possible at this point. _Something_
is causing the problem.

Sorry for the long winded post but hoping to get to the bottom of this
without paying a sparkie a fortune to do it for me...

Any suggestions?

Thanks for any assistance.
Cheers,
Nick.

Good luck with it, and let us know when you find
the problem.

Don W.