Welding a steel craft whilst afloat - electrolysis?

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

  1. CDK
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    CDK retired engineer

    Yes, they are so stupid they can only design stuff like radar and computers and satellites.

    I'll look around for a nice frame and nail that to the wall!
     
  2. capt littlelegs
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    capt littlelegs New Member

    That is my point, designing stuff like radar, computers and satelites does not qualify you in any way for electrical installations, it's a different field entirely! You stick to your job and I'll stick to mine!
     
  3. capt littlelegs
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    capt littlelegs New Member

    AC current by its very nature (flow-return) cannot corrode your boat. Galvanic isolators (AKA zinc savers) do not stop stray currents in the water. What they do is help to reduce galvanic (the water and metal parts act like a battery) currents that are created by completing the circuit when connecting a metal boat or parts of a non metal boat to the shore safety earth. In doing so they reduce the erosion of sacrificial anodes which are fitted for this purpose.

    A GI should be approved and rated for the proper fault current (can be 5000 amps) to prevent it failing and leaving the boat with no earth. That can result in the boat becoming live and the stray current to earth through the water can kill any swimmers as well as anyone stepping ashore, not to mention the danger on board.

    A voltage is no indication of current flow, you need an ammeter and a circuit for that. Try it on your disconnected car battery. The terminals will show a voltage but the only current flow is through the voltmeter that only you have created!
     
  4. Wynand N
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    Wynand N Retired Steelboatbuilder

    So true.
    Recently found that although the battery showed 12VDC it could not turn a small motor that uses very little current.
    I thought the motor was shot until someone told me to check the current flow from battery and found it was actually "flat" although still delivering 12VDC on the multimeter.
     
  5. capt littlelegs
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    capt littlelegs New Member

    That doesn't sound right unless the voltage was disappearing under load and if it wasn't then there wasn't a complete circuit because of the motor or wiring, assuming the motor could run on 12 VDC.
     
  6. jonr
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    jonr Senior Member

    > A voltage is no indication of current flow {in water}.

    A voltage difference will always cause current flow in any natural body of water. You won't find a case where it doesn't. How much current? - that varies.

    > do not stop stray currents in the water. What they do is help to reduce galvanic currents.

    That's contradictory, at least for my definition of stray currents (ie, stray currents includes galvanic currents).

    > AC current by its very nature (flow-return) cannot corrode your boat
    Change that to "AC current with no average DC offset" and we may agree on that. Of course small DC offsets are common.

    Connecting your boat to a ground wire is a good idea while you are using shore power, doing it without a galvanic isolator (ie, a blocker of any low voltage current flow) is a bad idea.
     
  7. capt littlelegs
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    capt littlelegs New Member

    You still can't check current with voltage.

    Stray currents can be anything anywhere around the hull parts and to other boats, can still be galvanic but galvanic current through the earth wire is the only one that a GI reduces.

    AC supply current is pure sinewave, you are getting carried away! In any event a properly wired boat will have no more than a few milliamps AC leakage current flowing in the earth wire until an earth fault occurs. If you disconnect the shore earth as has been suggested then all that AC leakage will go continually through the water and if you are still suggesting AC current is harmful then this will make it worse, even more so if the boat has a number of electronic devices that require leakage to earth! These AC currents also defeat the GI and it becomes ineffective for DC.
     
  8. jonr
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    jonr Senior Member

    > AC supply current is pure sinewave, you are getting carried away

    The supply (voltage) is generated as a symetrical sinewave, but the load may draw from only one side of the zero line - ie, the load (like a simple and quite common rectifier) may draw only positive current - all of the circuit current then flows in one direction! If you assume that the current balances, then you could be in for a disappointing surprise when it comes to corrosion. Don't put AC (or DC) through your hull and water and assume it will be ok.

    On a different subject, the most interesting case of electro-corrosion I've heard of was where the whole marina had some flow from one side to the other. This current found metal boats to be an easier path and so would enter at one end and exit at the other. The result was pinholes at one end of the boat - no matter what you did with grounding wires, isolators or zincs. Solution for this was an impressed current system - you can actively generate more current than the stray currents and neutralize them.
     
  9. CDK
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    CDK retired engineer

    WRONG AGAIN!

    AC current can and will corrode a boat, albeit slower than DC.
    The negative period makes the boat an electron emitter sacrificing metal ions that do not return during the positive cycle. This is not a reversible process.
     
  10. capt littlelegs
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    capt littlelegs New Member

    You've never done eloctroplating then, pulse reverse plating? I never have a problem with AC leakage anyway. Supposing you are right why do you advocate disconnecting the earth and then create this situation?
     
  11. capt littlelegs
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    capt littlelegs New Member

     
  12. CDK
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    CDK retired engineer

    Ahh, an easy question! I like that.

    Let us call the shore power primary and the boat's wiring secondary.
    In a sound installation, there is only capacitive coupling between these two, mainly in transformer(s) and noise suppressing filters.
    That may result in a few mA of AC current for a setup containing at least one large transformer or several smaller ones.
    It can never be more than 5 mA because that would trip the ground fault interrupter to indicate something is wrong.

    Let us also assume we are talking about a hull of 5x2 meters with a surface area of 10 sq. m. or 100.000 sq. cm.
    The ionizing (corroding) current is limited to 2.5 : 100.000 = 25 pA (pico amp), a value which cannot even be measured with a good multimeter.
    It is safe to assume that there will be no substantial damage.

    Now we take the same hull, firmly connected to shore ground.

    The conductivity of the water is between 50 uS/cm for clean drinking water and 53 mS/cm for ocean water (see http://www.eutechinst.com/techtips/tech-tips25.htm).
    For our 10 sq.m. hull that amounts to a value between 5 S and 5300 S or in Ohms between 0.2 Ohms and 0.19 milli-Ohms.

    A 1 volt offset on the shore ground terminal, a not unusual value, would cause a current between 5 A and 5200 A (!!!) if that would not be limited by the cable resistance and the much poorer conductivity of the ground electrode.
    Even the value of 5 A for clean drinking water results in 50 uA/sq. cm and will cause serious paint damage and corrosion.

    The resistance values of a 5 meter long hull also prove that electrocution of swimmers near a boat with faulty wiring is a fairytale, with the possible exception of a GRP hull with the stern drive or outboard connected directly to the power grid without fuse. Such stories originate in Hollywood.

    So beware of boats with lots of foam, heat and a 50 Hz buzz at the stern.
     
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  13. jonr
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    jonr Senior Member

    I don't disagree, but it would be instructive to repeat the analysis with a galvanic isolator (diodes) on the ground line.
     
  14. CDK
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    CDK retired engineer

    In that case there would flow no AC or DC current as long as the fwd voltage drop of the diodes isn't reached. In installations like I am connected to (and the waterfront sockets as well) it wouldn't make a difference: the excursions of "shore ground" vs real ground are there day and night and presently swing between +17 and -8 volts.
    The ground still is relatively humid because there was an extremely wet winter, in July/August everything will be bone dry so the ground electrodes in the power grid do nothing at all.
     

  15. jonr
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    jonr Senior Member

    With an unusually poor ground like that, I agree - don't connect it to your boat - rely on the ground fault circuit interrupters and circuit breakers to protect you until the grounding system is fixed (both ends - where is the voltage coming from, why isn't it tripping a GFCI and why doesn't the earth connection have less resistance).


    Has anyone looked at a DIY impressed current protection system?
     
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