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Thread: Motor anodes

  1. #31
    Ausfish Platinum Member
    Join Date
    Mar 2015
    Location
    Kalbarri, WA

    Re: Motor anodes

    Andy's idea of an equipotential bond would certainly be worth trying--you don't appear to have an"issue" that requires immediate rectification, just something to keep an eye on.

  2. #32

    Re: Motor anodes

    Quote Originally Posted by ranmar850 View Post
    You have two separate , isolated systems, each running through a single isilation switch so are effectively running it as two single outboards rather than a twin setup? Are you running a House battery--sounds like you don't have a House, just running your electronics through the Start battery on one motor, ie, the one showing the greatest anode loss? You don't have a Parallel (Both) switch in case you need to start one motor off the other battery?
    Just getting a mental picture of your setup.
    Correct, two independent systems, if I had a flat battery, I would have to either swap batteries, or use jumper leads, no connection between motors, or switch with "both" no electrical connection except via the water when launched.

  3. #33

    Re: Motor anodes

    Quote Originally Posted by ranmar850 View Post
    Andy's idea of an equipotential bond would certainly be worth trying--you don't appear to have an"issue" that requires immediate rectification, just something to keep an eye on.
    Yes, if I only owned either motor (not twins) I would never suspect anything.

  4. #34

    Re: Motor anodes

    Quote Originally Posted by Noelm View Post
    OK, I am back online after a horrendous time with our fabulous NBN and offshore support, can't bare to think about it because I want to smash things.....
    LOL...I tried telling people not to sell Telecom but they still voted for Johnnie.
    If you did then serves you right. If you didn't then cry in your beer like me. Its appalling and....you were warned.

    Sent from my SM-G900I using Tapatalk

  5. #35

    Re: Motor anodes

    Anodes are designed to be sacrificial, so rather than looking for why one is corroded, it might be worth instead looking for why one isn’t corroded?

    The corroded one might be simply doing its job as designed. The other, maybe it was not contacting the block cleanly (high resistance between anode and the item it’s protecting renders the anode practically useless).

  6. #36

    Re: Motor anodes

    Quote Originally Posted by The Woo View Post
    Anodes are designed to be sacrificial, so rather than looking for why one is corroded, it might be worth instead looking for why one isn’t corroded?

    The corroded one might be simply doing its job as designed. The other, maybe it was not contacting the block cleanly (high resistance between anode and the item it’s protecting renders the anode practically useless).
    I did kind of think about this, but, each motor has 3 internal anodes, one external on the gearbox, and one big one on the transom bracket, all are displaying the same condition on the one motor, it would be pretty unlucky for all of them to be suffering bad connections, but not impossible I guess.

  7. #37

    Re: Motor anodes

    Sorry, missed where you said they were affected similarly.

  8. #38

    Re: Motor anodes

    Just a though, measure the current between the 2 negatives, not voltage. Its quite possible that the currents are small enough that you cannot measure them with a standard multimeter though.

    I am not a betting man, but I would almost put money on this one. I am dying to get on the water and do some tests now. How sad is it that my main motivating reason to put the boat in the water right now is to satisfy my own curiosity about my theory.


  9. #39

    Re: Motor anodes

    OK, I have access to equipment to measure small current, not exactly sure how there could be current between the negs though, but, it's early in the morning and I need to think about it for a bit.

  10. #40
    Ausfish Platinum Member
    Join Date
    Mar 2015
    Location
    Kalbarri, WA

    Re: Motor anodes

    You can have differences in potential between two nominally identical potentials, although the difference should always be small. Any difference will cause the potential for current flow, and this will depend on the magnitude of the difference and the resistance between those points. It's still all Ohms law. I've measured +/- 3 v between the hull of my aluminium fishing boat and the water, alongside a jetty. it changed to less when away from that jetty, using the same multimeter.
    When you use an anode to protect anything underwater, its purpose ( obviously ) is to be sacrificial, ie, to be eaten away rather than the piece of metal ( motor, hull ) be eaten away in its place. So for it to be eaten away, there needs to be a current flow. Now, whether this current flow is initiated by purely cathodic action ( two dissimilar metals in a conductive solution) or by electrical issues within the installation ( which are accelerating cathodic action)is what we are really speculating about here. At least, this is my understanding of it. Personally, I am always happy when I see anodes disappearing at a rate I would consider acceptable--it just means that they are doing their job. Never happy to pull an internal anode and find nothing but the brass plug, you have sent some time unprotected. We had multiple anodes bolted to the hulls of our allloy crayboats, usually two on the transom, and at least two on the keel. Plus, we would have shaft anodes clamped on the protect the manganese bronze props. All of these weighed at least a kilo each, would be replaced annually and inspected when you came out halfway through the season for a hull clean and paint. You made sure that they were bolted to a clean surface, and, if they wwere just going white and crumbly, it simply meant that they were of the wrong composition, probably not pure enough zinc. The chandlers would quickly pick up on this from complaints and only stock those which eroded and kept clean as they did it.

  11. #41

    Re: Motor anodes

    Quote Originally Posted by Noelm View Post
    OK, I have access to equipment to measure small current, not exactly sure how there could be current between the negs though, but, it's early in the morning and I need to think about it for a bit.
    The more I think about this, the more I think I am wrong (re measuring current). There will be some sort of current flow, but I am guessing that with most measuring equipment that the impedance will be too low to actually measure any current flow. The problem then arises when you get some sort instrument with a high impedance the background noise becomes a problem. Its certainly possible to measure current flow in that situation, but I think its totally impractical.

    Its been a while since I studied this stuff, but I have been staring at a few equations and tables. I have also been staring at my old text books (which I think have been more useful than the internet). I need to state here that I am making assumptions based on both the material that the outboard and the anode is manufactured from, as well as my very lacking memory on the subject of material and corrosion science.

    Anyways, assuming cast aluminum for the outboard, and assuming zinc for the anode, the potential if measured between the outboard and anode is only going to be in region of 0.3V if I got my calculations correct. The actual potential difference between the 2 outboards is going to be significantly less. I have my doubts that its going to be possible to measure any voltage difference between the 2 outboards with any degree of certainty without the use of some sort of specialised equipment.

    So I think both you and I are wasting our time trying to measure any difference. However I am certain that having two isolated chunks of metal in seawater will still have sort of potential difference. How much, I have no idea. Its still the most likely reason that both of us (and others) have experienced the same level of increased corrosion on one outboard over the other.


  12. #42

    Re: Motor anodes

    Yep, I get where you're coming from, everything I measured was seemingly OK?

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