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Prop shaft material

Discussion in 'Ask All Sailors' started by Ken13559, Jul 20, 2017. Add this thread to a FAQ

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  1. SG

    SG

    Joined Feb 11, 2017
    356 posts, 37 likes
    J/Boat J/160
    US Annapolis
    Ah, one is electrolysis and the other galvanic. (galvanic requires bi-metallic preference). Thanks for reminding me, Charles.

    http://www.pcmarinesurveys.com/AC DC electrolysis.htm

    An excerpt from the cite (and site) above:

    2. Stray Current corrosion : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.

    In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate.I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.

    Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.


    Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.


    Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes the water more conductive however, polluted fresh water can be even more conductive with the right contaminants.

    With our aging fleet of pleasure craft it's likely that at some time, less than expert hands have played with your electrical system. If your vessel is suffering from any electrical faults or unusual corrosion consult with an American Boat and Yacht Council Certified marine electrical technician with specific corrosion control training or give me a call and I will try to set you up with an expert in this field.​
     


  2. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    821 posts, 59 likes
    Nor'Sea 27 - "Kiwanda"
    US Portland/Anacortes
    That term 'electrolysis' is not a favored term because it also refers to the dissociation of water molecules - but pcsurveys is a Canadian so we give him a little latitude -- heh heh.

    Charles
     


    Last edited: Aug 12, 2017 at 12:51 PM
  3. JamesG161

    JamesG161

    Joined Feb 14, 2014
    1,561 posts, 203 likes
    Hunter 430
    US Waveland, MS
    Well...
    The damage is not caused by the volts, but the amps,
    and AC can be semi-rectified to DC very easily in salt water or through a badly wired boat.

    The simple reason for pure AC not being a big corrosion driver is it Alternates between Positive and Negative. Not so with DC currents.
    _______
    @Ken13559 if this is a new problem to your boat and you have not made any recent electrical changes, then I would suspect new arrivals at your marina as the source.

    Jim...
     


  4. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,810 posts, 92 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    But when the AC is "going forward" it does damage so 50%
     


  5. Ken13559

    Ken13559

    Joined Oct 29, 2005
    1,771 posts, 38 likes
    Hunter 326
    SG Singapore
    As a precaution for the moment, I've removed shore power connection when I leave the boat. I'll measure for voltage between shaft and seawater. How far down must the anode be lowered? Same level as prop shaft?
     


  6. JamesG161

    JamesG161

    Joined Feb 14, 2014
    1,561 posts, 203 likes
    Hunter 430
    US Waveland, MS
    Submerged and Attached per zinc instructions to the shaft.

    Sea water is the the other electrical pathway for galvanic protection.
    Jim...
     


  7. JamesG161

    JamesG161

    Joined Feb 14, 2014
    1,561 posts, 203 likes
    Hunter 430
    US Waveland, MS
    :plus: for sure.

    But galvanic corrosion is a battery or DC flow. AC is rapidly changing ± and the galvanic chemical reaction is slow to start, so it is somewhat less than DC which is slow and steady.
    Tortoise and Hare effect.:)
    Jim...
     


  8. JamesG161

    JamesG161

    Joined Feb 14, 2014
    1,561 posts, 203 likes
    Hunter 430
    US Waveland, MS
    That is not true.
    Pure water is NOT an electrolyte!

    If you dissolve salt in water, the salt will ionize and become an electrolytic [passes electricity].
    This is another reason AC amps is less of a problem since the water ionization by AC is slow and is suddenly reversed. It is the salt, not the water that passes the amps.

    That dang Hare[AC] thought he would win, but no one should ignore him.
    Jim...

    PS: A lightning strike can ionize pure water.:yikes:
     


  9. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    821 posts, 59 likes
    Nor'Sea 27 - "Kiwanda"
    US Portland/Anacortes
    Not critical - over the side and a couple of feet from the surface works just fine.

    Charles
     



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