Joe wrote:
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.

Nick it could also have been a bad priming job before or when the
barrier coat was applied. Paint over dew ect...ect.. The welds are
always areas that harbor moisture.

Could also be stray voltage from a nearby fiberglass hull effecting
your hull. You would be suprised how many idiots use car type battery
chargers without grounds to keep the batteries charged. I've seen
zincs boiling on glass boats before.

Are you zincs welded on or bolted?

Yeah, I saw the wiring on a fishing boat just up the jetty from me...
it was frightening, masses of wires just twisted together with no
insulation tape or terminal strips, etc... Who knows what he's pumping
out.. and there are quite a few fishing boats there and odds are that
others will be just as dodgy.

My anodes are bolted on. They are on studs welded to the hull.


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.

No it is not. It will make contact with the hull via the shaft and
shaft tube.

I looked into that and the shaft has a thick spacer made of some kind of
plastic between the gearbox and the tube which isolates the engine from
the shaft.

snip


Any suggestions?

As others stated, turn it all off and measure, power up one at a time
and measure.

Good luck, let us know what you found.

Joe

Well, I'm getting a bit confused now. I disconnected my solar panels
completely from the wiring, totally disconnected the batteries and
insulated the solar panels from the hull as an added precaution. I was
able to measure ~0.8v between the -ve wiring and the hull. I don't
understand where this can be coming from.

I suspect I'd be wasting my time looking for the source of this
potential and would be better off looking for the connection or path
between the wiring and the hull.

At least I know now that my solar panels aren't to blame...

I've just bought a book that covers marine electrical, stray currents,
corrosion, bonding, etc in great detail so I'm going to digest this in
the hope that I'll better understand what I'm looking for.

Will update this when I make some progress.

Thanks everyone for your input. Don, I don't use shore power and the
boat has no AC wiring. It's purely a DC think but thanks for the thought.

Cheers,
Nick.