Welding a steel craft whilst afloat - electrolysis?

Discussion in 'Metal Boat Building' started by LittleVlet, May 6, 2010.

  1. finnracing
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    finnracing Junior Member

    Thanks Gents

    Just to clarify. Shore side add RCD. On Boat fit 16 Amp isolating transformer with 16 amp cabling. Fit 30mA 16 Amp RCD after the transformer on the boat side with suitable breakers for ring main.
     
  2. jonr
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    jonr Senior Member

    Oh, I would also double check (with an ohm meter) that the case of the isolation transformer does not touch the boat ground. That would be easy to do and would negate much of the benefit. If you have the model without a case, I'd skip the ground wire from shore entirely (like CDK does).

    I think you will have a very safe, state of the art electrical system.
     
  3. finnracing
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    finnracing Junior Member

    Gents

    Many thanks Once again.
     
  4. powerabout
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    powerabout Senior Member

    WHy dont you just put a gen set on the boat and connect the welder to that?
     
  5. powerabout
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    powerabout Senior Member

    RCD
    sold with all 2 wire double insulated garden tools in Australia for about 20 years...
    no earth available I think thats why they invented them.

    I couldn't take what the UK does as any sort of standard..lets face it anyone can call themselves an electrician..no mandatory trade qualifications in the UK.
    The only reason flying is not dangerous is the FAA make the rules on who can fix them
    Why is all the plumbing on the outside of the walls...no-ones trusts a plumber either

    Just ask yourself why UK plugs have fuses....
    so they can run one wire around the house and put a single 50 amp in the box...thats so cheap and nasty!!!
    No power in bathrooms...why because UK components are not safe....
    Funny how there are power outlets and switches in bathrooms in the rest of the world and all ships?

    ( Ok in the last 5 years or so they have discovered mega-ohmn testing..on commerical properties..ok a few years and maybe it will be homes too)
    ( rules too on the ring main..not how long it is just how far in a striaght line the furtherest outlet is from the fuse box..nice!)
     
  6. finnracing
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    finnracing Junior Member

    Hi Jonr

    We have not upgraded the shore power to the boat as yet. We will probably do so as soon as time and money allow. We were cruising of the west coast Scotland this weekend. On arriving at our destination my buddy and i decided to conduct some tests on the anode. Check to see whether it was doing its job. I disconnected the cable from the one side of the anode and placed my Fluke 79 in circuit. This cable is the one that leaves the engine block. The tests conducted and results were as follows:

    Batteries off at panel & No shore power connected 25mA oscillating a bit.
    Batteries on at panel & No shore power connected 25mA oscillating a bit
    Batteries off at panel & Shore power connected 19mA & oscillating a bit
    Batteries on and shore power connected 19mA & Oscillating

    I conducted the same tests on the cable on the other bolt and current read 7mA & oscillating. The oscillation by and large increased in uA's

    Anode bolts and nuts are shiny with no corrosion.

    The boat is a recent aquisition and 21 years old.

    Do these results seem normal to you?
     
  7. CDK
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    CDK retired engineer

    I assume you are referring to the hardware attaching the electrodes, not the zinc itself, which should look badly corroded unless it was installed yesterday.

    The 25 mA suggests there is a considerable area of metal to protect. Make sure the current flow is negative at the electrode.
     
  8. finnracing
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    finnracing Junior Member

    CDK

    Thanks for the reply.
    Yes that is correct. The nuts and bolts are in excerlent order but the anode itself is approx half the size it was when installed. I will check whether the current is negative the next time on the boat and get back to you. We were a little concerned with the extent of some of the corrosion on nuts and bolts in the engine compartment. Especially the ones on the salt water inlet filter. There is also some corrosion below the paintwork in several places on the binnicle. How can i check for galvanic action on the binnicle?

    Once again thanks for the advice.
     
  9. CDK
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    CDK retired engineer

    If half the zinc is eaten there is no need to check polarity because it does what it is supposed to do: sacrifice itself.

    English isn't my native language: binnicle is no part of my vocabulary, but perhaps you meant binnacle (some sort of box).
    If that is the case, it is an inside area which you cannot protect electrically, only with a paint system. Once corrosion has started under the paint I do not need to tell you it is a nasty job to repair. But that's another topic.
     
  10. finnracing
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    finnracing Junior Member

    CDK

    Thanks for the reply. The binnicle is metal assy/stand that the wheel and engine throttle is attached to in a sailing yacht. A couple of questions:

    How do i treat the corrosion of the bolts and nuts in the boat and minimise the re-occurance thereof? (Would a clean with a wire brush and smear with general purpose grease be ok.

    How would you measure current on the Binnicle/wheel assy housing?

    Thanks
     
  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Galvanic action requires the presence of an electrolyte, being seawater or even condensation, so the important thing is to keep metal surfaces dry.

    General purpose grease is a temporary solution only because in a saline environment grease is partially converted into soap and washes away.
    I would use one of the products specially formulated to provide protection against corrosion, like Tectyl or Waxoil.

    Measuring current between metal parts that are in contact with each other is not possible with an ordinary instrument because the current is only flowing in the contact area which has a very low resistance. It would require a coil around the area, a calibrated AC source and a very sensitive amplifier.
     
  12. jonr
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    jonr Senior Member

    For above the water line items, avoid different metals touching each other and don't worry about anodes. I have found LPS3 to work very well as a corrosion spray - order a gallon of it and keep a spray bottle handy.
     

  13. finnracing
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    finnracing Junior Member

    Thanks Gents

    A great help once again.
     
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