Stray current detector

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

  1. BATAAN
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    BATAAN Senior Member

    Did some one put the wrong (non-aluminum compatible) non-fouling paint on this boat once in the past possibly? Here is a blurb from an aluminum boat mfg. It could be over-zinced too.
    Also aluminum will corrode if it has salt water and no oxygen, such as under failed places in the paint under some conditions.
    Foaming stuff in an AL boat is asking for trouble.
     

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  2. MikeJohns
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    MikeJohns Senior Member

    Bataan
    That's severe electrolysis, nothing to do with current flowing through the hull. Most likely due to failure to use isolation.


    But if you consider the electric field I posted about before that must be present for corrosion to occur you should be able to see that ABYC views or not it's all simple physics.

    And an alloy hull is a very low impedance conductor. So effectively if you ground the hull anywhere and have less than several thousand amps flowing through it ( not hard) you are going to effectively have the same polarity over the entire hull.

    We still haven't added in the anodes. They are a low impedance connection to the sea water and in reality any corrosion would occur on the anode closest to the battery even if you managed to get sufficient current through to raise the voltage above a corrosion threshold.

    So I'm wondering out loud here as to whether we really could actually use the alloy hull of many classes of vessel quite safely as the main negative bus. And if not what real explanation can be given from a metallurgical or physics perspective.
     
  3. viking north
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    viking north VINLAND

    Mike I don't disagree with you on the hull handling a return current without damage --an example would be using alum. wire instead of copper in a D.C. situation. Other than possibly some extra I squared R losses no damage occurs to the actual conductor. However you have hit the nail on the head regarding the wet and dampness conditions on smaller vessels. That combined with dis similar metals and salt water we have the perfect conditions for problems.I think the ABYC wiring codes have more to do with maintaining a reliable safe power source within the vessel rather than long term dis similar metal corrosion although over the years thru customer pressure consumer protection against even this could have also played a role.
     
  4. BATAAN
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    BATAAN Senior Member

    As I understand it, in theory the field is equal but in practice it varies due to many factors, setting up local eddy currents. Hull return is simply not done except in the very cheapest and most amateur build in my 30 year professional boat yard experience. I have seen steel fishing boats destroyed by doing this sort of slap-dash thing, in combination with a host of other electrical no-nos. You will not find hull return on a nuclear submarine, but you will find a sophisticated system to monitor and neutralize all hull eddy currents.
    ABYC is a product of the insurance industry and they are a bunch of hard nosed SOBs who only value what works and don't like paying out anything they don't have to, so their stuff is usually pretty well engineered and researched.
     
  5. erik818
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    erik818 Senior Member

    Katoh,
    I agree that it´s good and common practice to connect all negative battery sides firmly together and then connect to hull at one point only. The problem is that you want to measure stray current which require that you measure the current on the negative side. You want to do something that isn´t common (but in my opinion good practice) and that is not compatible with wiring all negative sides firmly together. The positive battery side should already be separated with diodes anyway, as good practice.

    You can measure the voltage drop over a permanently installed shunt resistor instead of inserting an ammeter in the negative lead. If you separate the battery groups and insert a shunt resistor between the negative sides for measuring the stray current, you have to beware of the voltage difference between battery groups if there is current in the shunt resistor. The maximum voltage difference can be limited with a pair of counter-directed diodes in parallel with the shunt resistor. You should also short circuit the shunt resistor with a relay when the generator is charging. The only current through the shunt resistor when not charging is stray current. Keep the shunt resistor as small as practical depending on the sensitivity of the voltmeter and desired resolution. My suggestion is 1 ohm.

    Voltage difference in the negative side wouldn't be problem with the old engine I have. The only components connected to the motor battery would be the starter motor and the generator. Modern engines have electronics, but as long as there is no communication between motor electronics and all other electronics, there shouldn’t be a problem. If there is communication but a modern digital communication is used, e.g. CAN, a voltage difference less than a volt shouldn't be a problem anyway.

    I guess it comes down to that you have to know what you are doing. You’re trying to develop a new functionality so you have to foresee that there will be few unforeseen problems.

    My background is electronics engineering, but I've been working with military land vehicles and not boats. Our concern is EMC, not stray currents as such. Stray currents through the hull will ruin the EMC performance so we don't allow that. In our vehicles the 24 V DC network is separated from the hull, and connected at one point only, at the engine. As a rule you want the power lead and the return as close as possible, a twisted pair is the ideal. Using the hull for return will provide an enormous area encircled by the current. No current in our vehicles is ever routed through the hull or engine block; the hull is a shield not a conductor. I don’t have any opinions on the corrosion that will be caused by stray currents on aluminium hulls. In my world, stray current through the hull will radiate electronic noise and thus degrade the range for the radios. A stray current is an indication that something is wrong and should be fixed, so I think that it’s a good idea to supervise the stray currents like you want to do. Measuring insulation resistance is one way, but you will miss the possible problems that occur as a result of the electronics being energized.

    Erik
     
  6. viking north
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    viking north VINLAND

    The best article i've read on this whole complicated subject as related to marine craft was one written in the march/april 1990 Wooden Boat. To all not familiar with electro/chemical theory i highly recommend a read. I will do some further research on a direct link and post it here or once I figure out a way to magnify the extremely small print in the subnotes I will start a new thread and post the entire article of some 18 pages. Yes those were the days when magazines contained 75% good info 25% advertisement verses the opposite today,--Geo.

    P.S. One interersting fact I learned from the article is the possibility of a metal eating itself as a result of impurities setting up surface areas of potential differences on the same sheet. Just a piece of teaser info to catch your interest :)
     
  7. MikeJohns
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    MikeJohns Senior Member

    Typical problem around welds particularly with slight differences in the weld and base metals. But most metals can form their own corrosion cell on the surface that's a common cause of pitting.
     
  8. MikeJohns
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    MikeJohns Senior Member

    I suspect as Viking has pointed out that the main concern is actually causing corrosion inside the hull in the areas around the dissimilar metal fittings. I can still see no reason for reasonable currents returning through the hull if the internal connections are robust and protected.

    No ship at sea could destroy itself through using a reasonable current hull return path to negative. If there is no electric field then there is simply no mechanism for this to occur. Once it gets to the dockside and plugs in to a non isolated supply then there's a different story.

    Eddy currents are completely different and are induced in the hull from close proximity AC circuits not from DC return paths.
     
  9. MikeJohns
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    MikeJohns Senior Member

    But an enormous area adds up to an absolutely miniscule current density and then we are back to electric field potentials from the voltage drop along the return path. No voltage drop then no electric field is possible in th ewater and no problem can exist.
     
  10. viking north
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    viking north VINLAND

    Yes Mike, and cases of severe pitting could indicate poor quality control in the manufacturing process especially in the alloys and all the more importance to purchase from a reputable company where quality takes priority over price. It's a hard call in todays competitive world and the struggle to keep the company above board but i do suspect more than one realizes, many severe corrosion problems are the result.
     
  11. BATAAN
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    BATAAN Senior Member

  12. BATAAN
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    BATAAN Senior Member

  13. viking north
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    viking north VINLAND

    Battan-- with you all the way on the ABYC two wire system and codes and as I stated earlier it's just more reliable, safe and trouble free-- basically it eliminated one big problem associated with using the hull as electrical returns --Floating grounds. By the way that wooden boat article is an excellent one in relating to wooden hulls RE alki delignification .--Geo.
     
  14. erik818
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    erik818 Senior Member

    Mike,
    If you encircle an area with a current you get a magnetic field. If it's a DC current a voltage drop isn't necessary to uphold the field, but you get a static magnetic field that will influence the magnetic compass if the current is large enough.

    If it's an AC current, like the noise components in the DC supply current, energy will be transmitted and there will be a negligible voltage drop to accommodate for the microwatts that are transmitted. Transmitted noise will reduce the sensitivity of the radio receivers on board with reduced range as consequence.

    It’s bad practice to use the hull as a conductor. I don’t know if it’s common bad practice with boats or not. On top of that, it’s tricky to get a good electrical connection to aluminium unless it has a surface treatment for that purpose, so there are more reasons to not use the hull as conductor.
    Erik
     

  15. MikeJohns
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    MikeJohns Senior Member

    Erik

    Its the level of the current density that's the issue. If it's miniscule (which I showed that it is) then so is any magnetic field, add to that the inverse square law and any suggestion that it would effect the compass is not feasible. The generated field would be in the order of femto tesla at the greatest magnitude for normal equipment return currents in the hull so I don't think that's an issue.

    So although I'm bound up with best practice like everyone and wouldn't change without a very thorough investigation I still cannot see from first principles what the problem would actually be. If for example on your 50 foot alloy hulled sailboat the dc LED lighting circuits used the hull as the negative return ( robustly and sensibly connected) I'd like anyone to suggest a mechanism from an electrical and metallurgical point of view as to how this could actually cause any problem.

    I don't want to have current regulations regurgitated, that's what I'm interested in; whether we can actually change those regulations.
     
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