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923 millivolts galvanic reading too high?

Discussion in 'Technical Discussion' started by Capt J, Jan 8, 2015.

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  1. Capt J, Jan 8, 2015
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    Capt J

    Capt J Senior Member

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    I have a boat that has some electrolysis going on, mainly on the trim tabs. Found out they weren't bonded to the boat, but do have zincs on them. Anyways, we're getting a solid 923 millivolt reading on everything (metals) under the waterline. Which is higher than normal, but my marine electrician says it's not high enough to do any damage. Boat has a galvanic isoltator. Is fiberglass. Checked shorepower cord and all underwater fittings. Getting the same reading with the shorecord unplugged. So do I not worry about 923 millivolts? I hear that normal is 550-650 millivolts?
  2. Capt Ralph, Jan 9, 2015
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    Capt Ralph

    Capt Ralph Senior Member

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    That's .923 volts. Almost a whole volt. 11 more and you can start your outboard.
    I would be concerned.

    Get off the dock and measure again.
  3. jhall767, Jan 9, 2015
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    jhall767

    jhall767 Senior Member

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    Actually .90 to 1.05 volts is the correct reading for an Aluminum hull so you shouldn't be having a problem. If the tabs are isolated from the rest of the under water metal then it is just between them and the anodes. Is the .923 volts between the tab and the water? What is happening to the anodes on the tabs?

    BTW since it is Capt J I am assuming stainless tabs and the correct anodes per the manufacture.
  4. Capt Ralph, Jan 9, 2015
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    Capt Ralph

    Capt Ralph Senior Member

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  5. jhall767, Jan 9, 2015
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    jhall767

    jhall767 Senior Member

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    I know the hull is fiberglass. Doesn't really matter since the tabs are totally isolated from the rest of the underwater gear. .923 volts indicates more protection than needed. But not that much more. The key is having the correct anodes. If they are correct than there is another problem.
  6. Capt J, Jan 9, 2015
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    Capt J

    Capt J Senior Member

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    The tabs have zimar zincs on them. Bennett just recommends that their tabs have zincs on their s/s tabs in saltwater (doesn't specify anything other than zincs). There is no specification. The zincs wear down gradually over a 10 month period on the tabs. As do the rest of the zincs. BUT, the zincs have never been changed as an entire set, just as needed.....for example.....one month trim tabs and rudder..........4 months later a transom zinc........ 4 months later shaft zincs. All of the zincs are lasting about the same time though 10 months +/- Some people have said that you have to change ALL of the zincs at the same time? Should I go to smaller zincs on the rudders and trim tabs.....like from R6 to R4 (for example).
  7. Capt Ralph, Jan 9, 2015
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    Capt Ralph

    Capt Ralph Senior Member

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    If all are bonded, the size of the zincs may not matter. There is one site that suggest total weight of zincs per alloy boat. http://www.boatzincs.com/anode_weight_calculation.html I did not find anything that offers any other size rules for plastic boats.
    I've always put on the biggest that will fit the tabs, shafts, platform supports and thrusters if larger is available. No such thing as too much (maybe rum,,, naa).
    Even though you disconnected the shore power service, I would still be interested in a reading away from the dock.
    We were working another post a few months ago. A WestMarine meter was in the conservation. Anybody remember the readings (if any) during or after that post? I have not looked for it yet.

    Is there a plus or minus in these numbers? With shore service disconnected and readings did not change, what direction is the current flowing? From or to the ship? I still think an away from the dock reading could be interesting. Also with all breakers on and all breakers and battery switches off.

    Yep, I'm asking a lot, but still learning from your efforts.

    ,rc
    Last edited: Jan 9, 2015
  8. Capt J, Jan 9, 2015
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    Capt J

    Capt J Senior Member

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    Perhaps the electricity in the water is a little higher than normal at this dock, but everyone seems to say .923 is a little high for a fiberglass boat but nothing to be worried about. We also started the engines and generator and ran all systems with the shorecord disconnected and the reading did not change. It stayed .923 with everything we tested and all fittings.

    There is a 46' Bertram at the dock behind the boat at the next house but it's bow is about 15' from the Silverton and supposedly this Bertram hasn't been cared for or left the dock in 4 years. It probably has no zincs left on it. Also the bilge pump kicks on fairly often on this Bertram.......
  9. Capt Ralph, Jan 9, 2015
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    Capt Ralph

    Capt Ralph Senior Member

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    You would have to say Bertram. Think I need to start a Bertram rescue pound.

    AND, Ole Berts with leaking chargers (old Raritan), old water heaters (missing internal anodes), leaking (DCv) pump and float switches, butchered ACv & DCv ad addons, Yada yada, can ruin a good neighbor and his running gear. Go turn his batteries off and unplug the dock ACv service. Then another reading.
    Don't forget to turn da stuff back on before it sinks.
  10. jhall767, Jan 9, 2015
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    jhall767

    jhall767 Senior Member

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    Sorry if I missed it but are you reading 923mv from the tabs or the general boat system? Silver chloride reference electrode in the water through a volt meter to the trim tab? The tab without anodes on it should read around 300mv depending on the grade of stainless. 923mv is normal with anode.

    If you are not reading from the trim tab it is entirely possible that there is stray current at the back of the boat that is stronger than the rest of the boat. However I would expect this to eat the anodes quickly.

    Zinc anodes should not overprotect stainless steel. The other extreme is that there is no protection being given by the anodes. Confirm continuity between the anode and the tab.
  11. Capt J, Jan 9, 2015
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    Capt J

    Capt J Senior Member

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    The diver is good about cleaning under the zincs before installing new ones. I have used the same diver almost 10 years and also on another 8 boats........ No issues on any of the other boats. Trim tabs were read and read the same as the rest of the boat as far as I know. 3' of the shorepower cord was in the water when I went to the boat, not sure if this adds anything to the mix. But corrosion seems to be focused on stern of boat as I had to have the swim platform s/s screws replaced when we hauled out in October. The shorepower cord plugs in at the stern inside a storage locker. The galvanic isolator is also mounted on the stern. This is a aft cabin motor yacht so that area is dry.
  12. NEO56, Jan 10, 2015
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    NEO56

    NEO56 Member

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    Capt. J, Is it possible to go hit another marina away from the Bertram, and take another set of readings? I doubt that you'll get readings that high at anchor, so a Marina is probably necessary to eliminate a lot of variables.
  13. Capt J, Jan 10, 2015
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    Capt J

    Capt J Senior Member

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    Well it would just be a costly pain in the butt......between my time, an electricians time, and on and on......From what I'm hearing from everyone, .923 is a little high but nothing to be too concerned about.
  14. NEO56, Jan 10, 2015
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    NEO56

    NEO56 Member

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    Yep, probably so...I'd be willing to bet it's that blood sucking Bertram behind you:D
  15. Capt J, Jan 10, 2015
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    Capt J

    Capt J Senior Member

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    Probably, but it's behind the former mayors house of my city and I happen to live 5 houses down.....
  16. Capt Ralph, Jan 10, 2015
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    Capt Ralph

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    Their usually nicer after they are OUT of office.

    Cruel, Cruel. It's a lost, poor, abused Bertram. She don't know better.
    Call the owner names, (former mayor, he's used to it) save da Bertram.

    Accepting donations now for the Save Abused Bertram Shelter.
  17. jhall767, Jan 11, 2015
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    jhall767

    jhall767 Senior Member

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    I think it is pretty clear it's not the boat but something in the water around the boat. There is corrosion on parts of the boat that are properly protected and completely isolated from the boats electrical system. Probably a neighboring boat but it could be electric lines in the water. From your description it seems like the issue is under the dock or on the other side of it. Can the boat be docked bow in? Maybe just turned around when the owners are using it?
  18. Capt J, Jan 11, 2015
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    Capt J

    Capt J Senior Member

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    The owners haven't seen the boat in 4 years. Seriously. It never moves except to go to the yard for a bottom job every 3 years. Yes, 3 years and only because it needs a hull survey for insurance. I do have a diver on it monthly. I will look under the dock and see if I see any wires hanging underneath the dock. But the measurements were during the day when the docklights were not on.