to earth or not to earth????

[ean_p]

am rewiring the boat this winter....she's steel and in fair condition.....the existing wiring is in two cable format and the neg side of all the batteries has been earthed to the hull and the engine block. The engine by the way is fitted with fully insulated alternator, starter and instument senders, so therefore there is no functional need to earth anything. Please just refresh my memory....what are all of the pro's and cons for earthing and not earthing the boat, the block and the batteries.......and yes I know its not stictly good english to talk of earthing the neg side, but you know what I mean...(I'll suffer for that!!!). Any thoughts would be well received....

 

[oldjohnnyb]

The reason for isolating the electrical system on yachts and big ships is to avoid corrosion of the hull.
Having said that the Blue Funnel Line operated a ship called the GUNUNG DJATI which not only had one side of the electrical system earthed but also used earth return!!
The vessel had been built as the German passenger ship PRETORIA in 1936.
The Germans were short of copper at the time and apparently perfected a method of building the hull so it wouldn't rust due to the current flowing through it.
Blue Funnel operated her for about three years taking pilgrims to Mecca.
She was scrapped in 1987 making her over 50 years old so the elecrical system doesn't seem to have done her any harm.
It will be interesting to see what others think of using an earth on the electrical system of your vessel. My advice would be follow the herd and don't earth your electrics as per the text books.
John

 

[HaraldS]

I see, you have only got one response so far (though very good) on this controverse and intersting subject. I have only spent substatial time working through what happens with metal parts on a non conductive hull, from all the aspects of lightning protection, corrosion, HF ground and more, since that's what my boat is like. Given your steel ship my response is less thought out and a quick shot. But maybe it helps.

(1) I think a steel ship is less troublesome. I can't think of a reason why your return couldn't go through the hull, like it does on any car.

(2) In theory a steel hull should do extremely well with polluted marinas that have a lot of current go through the water. But only if it had conductive connection to the water at most parts of the surface, depending on the underwater paint that may or may not be true. A conducting hull in the water would bring the surrounding water to the same potential and prevent stray currents through things like props and bronze fittings. A few zincs will easily take care of the rest.

(3) Naturally you shouldn't have any lower grade material in the water, like aluminum ruddershafts or the like.

(4) Lightning protection should be perfect as long as mast and rigging wires are well connected to the hull. You'll even have sort of a faraday cage. But if your devices are ground connected with minus at multiple spots, keep in mind that a partial, and in case of lightning quite huge current would go through those and fry them. The masthead VHF antenna is a typical culprit.

(5) HF has it's own way of what it thinks is a good counterpoise, but using the hull would certainly not harm other than potentially connecting DC ground with the hull. See item (6).

(6) Ideally I'd leave DC minus entirely unconnected from the hull, especially since you already got isolated engine electrics. But in practice you'll find that hard to do since many devices, like most VHF, will make that connection. You could separate thouse over DC-DC convertes like I did. But I had to also do it for voltage conversion, so the separation came for free. If you can't isolate it, I'd really do a solid ground connection near battery and engien in order to avoid a return current going through one of these devices because of a broken minus wire. Current for an anchor windlass would fry the PCB of your VHF. So I would say check your devices and if you can keep all isolated, go for the isolated installation. If the VHF is the only problem, you could use a DC-block in the antenna cable. Or, if you have a fancy active multi-band masthead antenna, you would need to isolate the mount.