Electrics/paint experts?

Eeyore

Eeyore

Well-Known Member
Joined
5 Oct 2007
Messages
835
Location
Kudat, Malaysia
www.yachtrhumbdo.co.uk
A friend has a steel boat which keeps shedding it's paint/anti-foul in big patches. The boat is currently on the hard and connected to shore power. A multi-meter reading between hull and ground shows an AC voltage fluctuating between 20 and 30 volts. Clearly something is not right and he's convinced that this 'earthing' is causing the paint loss. The boat is fitted with a galvanic isolator and shore power feeds through his inverter/charger.

Anyone got any ideas?
 
Sorry. I'm somewhat a numptie when it comes to things like this - and he (John) isn't much better! Do you mean...unplug the shore power at the boat end and check between the earth pin of the cable and actual earth?
 
Actual earth is of course the sea by the way, literally put the end of the probe into the water or better still use a rod with the probe connected. If you measure more than a trace voltage a few tenths of a volt the main supply isn't properly wired. Be aware that when you are making measurements of this nature it's quite easy to connect yourself across the mains accidentally. Very dangerous, take care, especially if the supply is indeed faulty.
 
I'd say that your best approach is to speak to the shore power supplier,the marina or whoever and complain that there appears to be an earth fault. Poking around in what sounds like a badly wired mains pole could be fatal.

I'm assuming that the paint flaking was taking place in the water and the boat has recently been put onto the hard to solve the problem, is that the case?

If there's 30v AC on the hull on the hard the whole hull is approaching the level of a hazard. Climbing aboard in the rain could be an electrifying experience /forums/images/graemlins/smile.gif

<edit>
Just seen you are in Turkey. Was going to offer some help if you were nearby but I guess that's out! Suspect that your mains supplier may be a little unreceptive to your approach. ( but then rain may not be a problem /forums/images/graemlins/smile.gif )

The most important thing is to ensure the ongoing safety and galvanic isolation of the hull especially before it goes back into the water. He needs to check out the bonding and grounding circuits on the boat and test the galvanic isolator.
<edit>
 
Mmmmm Dont like the sound of this at all. I would recommend you have it checked-out by a marine electrician.

Are you sure you have installed the GI properly? Sounds like an earth fault or interrupted earth. Possible case for an isolating transformer?

Anyway, do get it checked out by a professional electrician familiar with marine environments. If its allowing this level of leakage now, who knows what it will pass in the future! /forums/images/graemlins/frown.gif
 
Dont mess about with it: disconnect it from the mains - NOW. Please.

Unless you KNOW how to trace a mains fault safely - and you say you a numpty in these things - then disconnect it, make it safe and LEAVE IT ALONE.

The steel hull is clearly not earthed correctly: any fault that allows live to leak to earth will result in the hull become live at up to mains potential. It only takes a damp connector or damaged cable.

Electrocution is instant. You get no warning. We dont want to hear you or your friend has been killed next week.

If other posters think I am OTT, tough. I have seen it happen - it is horrible, and a very unpleasant way to die. I still hear the poor guy screaming his life out.
 
I do agree with previous post - however you might try taking the GI out of circuit by connecting its earth's together just in case it's faulty and interrupting the earth.

The only thing I disagree with previous post is:

"If you cant fix it, (DONT) get a bigger hammer... "/forums/images/graemlins/ooo.gif
 
Thanks to everyone's imput so far.

This is an ongoing problem.....out of the water last year showed the bare patches of hull and the paint primer was the main suspect then. Hull was shot blasted, primed and re-painted, also some of the anodes removed as a result of expert technical advice. This year, it's the same...bare patches on lift out. The paint just seems to drop off in big flakes in certain areas. The voltage twixt hull and the hard was discovered today, so now that is under investigation but of course he wouldn't have been connected to shore power during the sailing season.
 
Just guessing here and could be wrong but.......my experience with foreign power supplies is that quite often they are a bit cavalier with the polarity, witness the prevalence of reversible two pin sockets in abundance outside of the UK. I'm guessing that if the live is on your neutral pole and your boat is bonded throughout this could be the problem? If so, a simple neon screwdriver would show which pole on their/your hook-up lead was actually live?
 
Clivesheps test may not work with some continental supplies where both leads are at a positive potential above earth - usally 120 - 125 volts to earth, but 230 - 250 volts across each other. This is reckoned to be 'safer' - particularly in areas where installation and maintenance may leave something to be desired as a 125v a/c shock is more survivable. Unfortunately it can cause some problems for UK standard installations. This just may be the cause of OP's problems if his boat was wired to UK standards and is designed to take the full 250volts above earth on one side with the Neg return effectively at earth potential.
 
OK.....I give up....you've all lost me now! /forums/images/graemlins/confused.gif

Seems there are two problems then. 1) why is the paint being 'burnt' off (John's description, not mine) when in the water and not connected to shore power, and 2) why is there a meter reading of 20/30volts from hull to earth when out of the water.

I'm glad it's not my boat! /forums/images/graemlins/ooo.gif
 
Yes Eeyore; there seem to be two problems - the paint coming off may simply be that it wasnt put on right in the first place. If it was not primed properly, or the primer was put on in too low a temperature so that dew was forming on the steel, it would just fall off again. Possibly there may be galvanic problems too.

The 30 volt hull is because it is not earthed properly and IS DANGEROUS. PLEASE dont mess about - get it sorted.
 
Can't comment on the electrics but you seem to have plenty of advice there anyway. Paint, however, I do know something about. The boat is probably suffering from an extreme form of cathodic disbondment. This will be why you were advised to remove some anodes last year even though it was probably not the real cause of the problem.
Cathodic disbondment is caused when there are too many anodes (or too high high a current in an impressed current system) for the paint scheme. Hydrogen gas forms at the primer/steel interface and pushes the pain off the surface. It is most often seen as a series of blisters around an anode which, when removes leaves a shiny steel surface bare underneath. Obviously normally seen out of the water and when the blister is removed the steel quickly corrodes. The answer to the problem is to remove anodes and/or increase paint film thickness. When impressed current systems are specified on a steel structure it is normal to increase film thickness of anticorrosive scheme. With the voltage you are talking about I imagine the blistering is so bad the paint is coming off in sheets as there is so little left attached.
Answer sounds like 1) sort electrical system out. 2) blast steel to prepare surface 3) apply new paint scheme using a higher film thickness in case there is still any problem.
If blasting is not possible use the best preparation system available.
Might be prudent to look closely at areas where paint is still attached as its adhesion may be compromised so complete removal underwater is required.
 
You must log in or register to reply here.

Share this page

Top