On May 17, 7:16 am, 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.

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 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.

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

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.

I've measured the stray current in my marina and found an average
voltage of .5 DC. Most marinas will have some stray voltage. Anyone
can take a meter and put one end to the ground terminal on your shore
power and one lead in the water and see if there is any stray
voltage.

What condition were your zincs in?

from this old boat:
******************
How much zinc

The amount of protection a zinc anode provides depends on its surface
area. The zinc surface area needed varies with the kind of metal being
protected and with the chemical make-up of the water, but you can use
1% of the surface area of the protected metal as a starting point.
Check the protected metal frequently. If it shows signs of corrosion
despite the zinc, you need more surface area.

Zincs should be replaced when about half of the anode has been lost to
corrosion. Ideally we want that to occur not more frequently than
annually. The longevity of a sacrificial zinc anode is a function of
its weight. When a zinc lasts less than a year, you need one with more
weight.

Normally, however, you are not faced with determining the appropriate
anode size (other than diameter for a zinc shaft collar). Rather, you
are simply replacing depleted zincs with new ones of the same size.
Check all zincs at least annually and replace all that are half
depleted.
******

Joe