Solar P.V. - Is it me or them?

[Gibbo]

Why? I would be real grateful if someone could give an in-depth explanation. I could see corrosion being speeded up in riveted boats, where high resistance could set in between the plates/rivits and therefore cause a potential difference between two parts of the hull. I can also see - when on shore power - that two or more craft can be connected together via the earth wire, and so could create a P.D. between different craft with a return path via the water.

 

I don't see how connecting negative to the hull could have any effect - there is no return path for the current via the water. How many of you are sure that your starter motor and alternator is 100% isolated from your hull? Honestly, I have never seen one. The only way would be to mount the engine on rubber and have a rubber flexible coupling to the prop shaft, and then go to great length's to ensure that all throttle cables etc. are 100% electrically isolated.

 

Alternatively, you would have to get specialised/modified alternator/starter motor fitted. From what I have seen, most boats appear to have standard, automotive, neg. ground equipment fitted.

 

I am genuinely not trying to be "clever" here. It's just the way I see it right now, and please do put me right. I usually miss something quite obvious!! I went on about it being pointless fitting an RCD on the 230v side of my inverter since it was floating anyway - no "live" or "neutral", so no shock hazard. A fellow on another forum pointed out that in the event of a wiring fault whereby one of other leg came into contact with the hull, there would be a very real danger of electrocution. I have since changed my mind, and have installed an RCD, also tied one leg down the hull and called it Neutral.

 

Constructive critisism is always appreciated.

 

Thanks,

Mitch.

 

(edit) Could there be something to do with creating a P.D., no matter how small, over the length of the hull? The batt's are fore and all the gubbins in the engine hole.

 

You basically got it in the edit. Sort of............

 

Let's not get into whether the DC systen should be connected to the hull. That's a different issue that always ends up in an argument.

 

But what you've got is the hull actualy being used a current carrying conductor. That's very bad for corrosion.

 

There are some people who think this can't cause a problem, their reasoning being "but the electricity takes the path of least resistance so it will all pass through the hull".

 

Take note of that "but the electricity takes the path of least resistance". It seems to imply that ALL the current will go through the path of least resistance. This is a complete fallacy. I struggle to work out where it came from.

 

You have an inverter connected to the hull at one end of the boat, the batteries at the other, the negative return is passing through the hull. Assume the hull has a certain resistance (of whatever value) between those two points. Your 3kW inverter at full hoot will pull 300 amps. Even if the resistance path through the water is 10,000 times higher than that through the hull, 1/10,000 of that current will pass through the water. That's 30mA passing through the hull into the water, then back into the hull again.

 

Absolutely lethal for corrosion.

 

Gibbo

Edited November 30, 2008 by Gibbo

[MikeV]

Many canalside pubs and hotels do exactly this, in fact, I discovered all this after reading about a pub guv being fined 5 grand for dumping un-treated waste into the cut, after being repeatedly warned that his treatment system had malfunctioned and he delayed having it repaired.

You need a licence to do this.

 

Our marina has only recently been completed and the company that provided the sewage system initially installed a sewage processing system with the outflow running into the canal. Someone, who new more about the legalities of this, asked the marina owner if they had a license from BW to do this. Apparently not! And when BW were contact they said - "not a chance!"

 

The original supplier has now had to do the work to provide a pumped system into the main sewer.

[Mitch - Soma]

Many thanks to you both.

 

Gibbo - now makes perfect sense, will run a couple of 70 sq mm cables front to back and get the neg. off the hull at the front ASAP. I absolutely get the point of NOT having a current flow through the hull, since there HAS got to be some residual flow through the water, but that there is scope for argument around simply having a P.D., without a closed circuit.

 

Mike V - O.K., so probably "historic", as in previous, installations have been licensed. It now just depends on how a B.W. official feels that day as to whether he grants a "new" licence or not!

 

I also heard a rumour that a certain company wanted to put their rain run-off into the cut, but B.W. wanted to charge so much for this that it was more cost effective for the business to install huge pumping equipment to put it into the sewer instead! Didn't make a whole lot of sense to me - I thought B.W. would welcome some extra water which was reasonably clean. Perhaps B.W. get's more water than they want when it rains, so they would just have "extra" to take care of?

 

Really pleased I got into this Forum - have never had so many questions answered so adequately in such a few day's.

 

Many thanks to everyone.

 

Mitch.

Edited November 30, 2008 by Mitch - Soma

[greywolf]

have you visited the blog smudging there boat is powered by solar panels.

[Alastair]

If I have a rope lying across the panel, shading at least one cell in each "half", I lose the whole panel.

that is a cruddy panel design.

 

I've lived off a single PV panel (very low power requirements). I took advice, and bought a panel that had lower maximum power output, but more consistent output. One feature of the panel was that it was many small cells in parallel. So small sharp shadows like ropes didn't knock out the whole panel.