Stray current detector

Discussion in 'Boat Design' started by Katoh, Jul 12, 2011.

  1. MikeJohns
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    MikeJohns Senior Member

    Bataan did you read this?

    "AC is Alternating Current so dissolved particles will be replaced every half cycle (Hertz or 1/60 or 1/50 second). So AC cannot cause electrolysis corrosion."

    This is absolutely wrong ! There is so much miss-information and confusion in the marine industry with respect to metallic corrosion. This is a classic case.

    Your own eroded shaft pic is from AC induced electrolysis, AC causes more harmful and much faster material erosion. Ac prevents the protective corrosion products from slowing the rate and strips them very effectively.


    [Added]

    It makes me quite angry to see material on commercial sites written by ignorant people who have an abysmal understanding of the subject. Pretend experts are a bane to everyone, he obviously doesn't have any proper understanding at all.

    It's a very good example of how confused people can get.

    I just read through some of the remainder of that site: He makes so many serious and obvious mistakes it's not funny, the poor layman reading that has no hope of separating the rubbish from the truth.

    It gets worse the more you read, there is a total lack of understanding as to what causes electrolysis he is presuming its from internal leaks from the DC system to the hull and this is quite wrong.
     
  2. BATAAN
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    BATAAN Senior Member

    "In theory there's no difference between theory and practice but in practice there always is."
    -Yogi Berra
    Of course in theory it'll work, but in practice every dissimilar metal connection to an aluminum hull causes a problem unless it's stainless as I remember, and even those are suspect, there can be no salt water anywhere near, ever, to get between the SS and the AL to cause crevice or poultice corrosion, so what connects to the SS, copper? All these convenient connections to the hull are SS bolts through a web frame? And that connects to a copper wire?
    I hear sizzling.
     
  3. BATAAN
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    BATAAN Senior Member

    In the boat yards we used to call guys with ideas like this 'job security', as they laid awake nights thinking up new ways to throw money at their POS boat, convinced they understood the whole thing better than any experienced boat yard peon ever could. Kept me in potatoes and steak for 30 years...
     
  4. MikeJohns
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    MikeJohns Senior Member

    Yes self appointed experts with no formal training in the area they pontificate about. They sell their ideas through charisma and I'm sure he's a great bloke but what I see on that site is simply fraudulent.

    As for the LED lighting circuit the connections could be done properly we can seal a cavity to the deepest ocean floor and keep it bone dry. Can certainly seal a wiring connection easily to the dry protected deckhead. But the more I look at the physics of this the more I think it could actually be quite feasible to use an alloy hull safely as a negative DC bus. I have just never seriously considered this before.

    I'll have a chat to AdHoc he's just designing a new alloy Swath maybe we could make that a testbed ;)
     
  5. viking north
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    viking north VINLAND

    Ac Stray Current

    Exactly as per the Wooden Boat article--Hope you can enlarge to read--Geo.
     

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  6. BATAAN
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    BATAAN Senior Member

    Of course it'll work if perfectly realized. That's the problem. Also entropy in general. What is the design life? Does this reduce the design life? Why do it and what is the saving? Usual risk/reward stuff.
     
  7. BATAAN
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    BATAAN Senior Member

    Reads fine and is just the first rule in more words and applied to AC on boats.
    1. Don't be stupid.
    2. **** happens (especially if you violate rule 1 above)
    3. Bring beer.
     
  8. MikeJohns
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    MikeJohns Senior Member

    There's a bit of a misnomer in that article.

    Ion exchange into the seawater as the metal is oxidised doesn't mean the same ions will be deposited in the other cycle. Any available ions will do, not necessarily the material that was stripped off even though its in close proximity. That random ion reduction coating is then stripped off more easily (since it's just a thin metal salt coating not the solid metal again) along with new base metal in the next cycle and so on...

    It's a complex subject.

    The continual net balance of oxidation and reduction at the site would only ever balance if the fluid were a pure ion solution of the base metal.
     
  9. BATAAN
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    BATAAN Senior Member

    Any old ion will do.
     
  10. MikeJohns
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    MikeJohns Senior Member

    Exactly !

    Try putting seawater in your car battery, and argue that pure lead will somehow be the only substance deposited back from the seawater in the charge cycle :)
     
  11. erik818
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    erik818 Senior Member

    Mike,
    I’ve been travelling so I haven’t had the opportunity to comment. I did the math for the magnetic field and come to field strengths that are relevant for the magnetic compass. The field strength I get is in the order of µTessla, not femtoTessla. This is how I calculate:

    Assume that the boat has a one wire system. The wire passes 1 m from the magnetic compass. The return current is left to find it’s way back via the hull best as it can.

    The field strength from the DC current in the wire is:
    B = µ0*I/(2*pi*r)
    µ0= 4*pi*10^-7 [Vs/Am]
    r = 1 m (for the sake of the calculations)
    I = 100 mA for lighting diodes, 1 A for a lantern, 10 A for a searchlight and 100 A for an inverter or charging current to a battery group.

    If I = 1 A we get
    B = 4*pi*10^-7 [Vs/Am] * 1 [A] /(2*pi*1 [m]) = 2*10^-7 [Vs/m²] = 0.2 µTessla.
    If the current is 10 A we get 2 µTs and 100 A gives 20 µTs.
    The earth magnetic field is 50 – 100 µTs so 1 A will give a maximum angular error of Tan-1(0.2/50) = 0.2° and 10 A will give 2.3°.
    A current of 100A will give a maximum angular error of Tan-1(20/50) = 22°.

    The return current through the hull might do anything between increasing the field to partly cancelling it. A have assumed no effect. 1 m from wire to compass is realistic but doesn’t account for excessive stupidity when doing the wiring. My conclusion is that the effect on the magnetic compass must be considered when using a one wire system in a boat, with a minimum distance to the compass depending on current.
    Magnetic field distortion is one drawback with using the hull as a return (if my calculations are correct). EMC problems resulting in reduced radio range is another. I could continue listing risks with a one wire system, but I would have to work hard to list any significant advantages. Routing one, two or three wires is about the same amount of work.
    Erik
     
  12. MikeJohns
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    MikeJohns Senior Member

    Erik
    Thanks for your thoughts on this. The unbalanced magnetic effect of the single conductor as you have calculated is a good point. The femto tessla or less was what I was predicting in the hull itself not the feed wire, you'd have to be careful where you routed high current cables particulalry in an alloy boat unless it was in one of those steel flexible sheaths.

    The advantage could be half the wiring weight and cost but really I'm interested in exploring the relationship between current through the hull and the idea that it causes corrosion as is so often suggested. I may be guilty of playing devils advocate myself here.
     

  13. Katoh
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    Katoh Senior Member

    Gentlemen
    I have been away for a few days and see the discussion continuing.
    I must state I do not come from an Electrical background, but an engineering background in Civil/Structural but play around with a lot with CNC machinery ,and actually build them. I have been being trying to gather as much information from the experts as possible.
    A lot of what I preach comes from Charlie Wing's book Boat-owners Illustrated Handbook, and from posts and replay's written by Kevin Morin, very highly regarded aluminium boat builder. Both will tell you that good bonding is essential, and the hull should never be used as the negative feed for any electrical device.
    Eric I agree with you 100% with all that you have said, and I also believe that if you want to go down the path of a single wire eletrical system in an aluminium boat, make sure your handy with the welder because your going to be patching holes, one after another.
    I wont Quote but the assumptions were made that wet foam , or even dissimilar metals from trailer where the boat was sitting could be a contributing factor. Yes that could be if were the holes formed there was foam which there was not, and the boat sits entirely on poly rollers, giving it a fair bit of isolation to the trailer.
    Lets take an example, if we a two wire system to say the fuel tank sender unit, and the well that the tank sits in over time accumulates with water to the point where the tank and now its wiring are under water. The well is sealed and there is no way of telling that this situation has occurred. Every time you place a current to those sender unit wires after a while they are going to start haemorrhaging current into the hull. I cant see that being a good thing.
    I will post one other picture This is a shot of the hull inside directly over one of the holes, note there is no foam, nor had the hull being touched at this time. The discolorations in the aluminium is a mixture wax dirt and water from were I flooded the hull initially to see how many holes it had. the aluminium internally is smooth with no pits or signs of corrosion. The first pic is a bit alluding so I have added another same area just a bit further forward.
    Katoh
     

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