On Mon, 4 May 2009 02:32:51 -0700 (PDT), joost
wrote:

(old stuff snipped)


Hi Bruce,

Thanks for that information. I'm happy to hear that I'm not the only
one with this problem. Ideally I would indeed like to isolate the
whole electrical system. Because what is happening now is that if
anywhere in the system a ground wire touches the hull, the return path
of the electricity goes through the sea water, to my prop, prop shaft,
engine and engine ground back to the battery.

Unfortunately, giving the starter and alternator a separate ground
connection is not an easy thing to do reliably. Therefore I'm now
thinking of connecting a separate wire from engine ground to the hull
(the engine isn't really ground. the resistance between hull and
engine measures 10 ohms). I think this is what is called 'bonding'.
WIth that extra wire, any leaking ground current can take an
attractive direct path back to the battery instead of going through
the sea water and creating expensive damage.

Any comments on this approach?

Thanks,
Joost



O.K. here we go.

First of all the DC electrical system should be insulated from the
boat. No grounding, no nothing! The reason is that there is no
requirement that the DC system be connected to the boat and if it is
it can cause problems.

Usually the engine mounts insulate the engine from the beams on which
it is mounted and a plastic "flex plate" can be used to insolate the
propeller shaft from the engine.

Radios usually need a "ground plane" that is essentially the "ground"
side of the antenna circuit. This can be isolated from the hull by the
using capacitors. See recent post on rec.boats.cruising by Larry, et
al.

Assuming that you have a steel boat that is built properly the only
dis-similar metals you have in contact with sea water is the propeller
so with a completely isolated electrical system galvanic activity is
kept to the minimum.

You do still need zincs but the need is less.

Bonding - as there is no dis-similar metals then you do not need
bonding.

Lightning protection - as the entire hull is in contact with the water
and the mast and stays are not insulated from the hull the boat is
effectively grounded for lightening strikes.

Radios - Most radios use a "single sided antenna circuit" in other
words a wire leads from the antenna terminal on the radio case to an
antenna. the other side of the circuit is provided by a "ground wire"
to some sort of ground.

The ground side of the antenna circuit can be isolated from DC ground
by connecting it through a capacitor to the hull. See above.

Note: that some radio antennas are "two sided" and both the ground and
"hot" connections are made to the antenna. A "case ground" does no
harm in this case.

A.C. Power - AC power is more complicated and I won't go into it in
detail, but basically you can totally isolate an on board AC system
and treat it similar to the DC system but if you use "shore power" you
have different circumstances.

Commercial A.C. electrical power has connections to the physical
ground (the planet/ground/earth/dirt) and thus to a boat, through the
sea water. Because of this connection using commercial power (shore
power) without a ground circuit is extremely dangerous as standing on
a steel deck with bare feet and touching a live AC wire could very
well result in death.

Resolving this situation is fairly straight forward but in the
interests of brevity I'll not cover it here as it gets a little
complex - Isolation transformers 'n all that...

For good electrical/electronic information you can ping Larry over in
rec.boats.cruising as he is really knowledgeable (albeit outspoken).
There is another guy over there and I am embarrassed to say I don't
have his name (sorry mate) so you can get good information there.

Hope this helps.

Cheers,

Bruce in Bangkok
(bruceinbangkokatgmaildotcom)