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Thread: New Boat Grounding Question

  1. #46

    Re: New Boat Grounding Question

    Quote Originally Posted by swof63 View Post
    Again, it’s primarily a corrosion issue, and possibly also ensuring that any wiring faults blow fuses rather than smoulder and burn.
    An electrical shock that will seriously damage or kill you needs to be above 30mA ( and in reality normally above 75mA) and below 200mA directly across the heart. About the only way to do that with 12v is to have two needles directly inserted into the myocardium and introduce the current directly across the tissue. In normal practice it’s going to be external hand-foot or hand-hand exposure. Even wet skin has a minimum resistance of about 1000ohms or so ( dry up to 100,000ohms) and then the typical hand-foot body resistance of about 500ohms ( hand-hand similar ) would give you about 5mA assuming both hand and foot wet. I started out as a biomedical technician - isolation, leakage, macro shock, micro shock were daily grist. In external situations anything below 32v dc was low voltage.
    Reality check people: how many times have you touched an installed car battery positive terminal with either your hand or a tool, whilst another part of your body was touching metal or ground? I bet you didn’t die.


    Sent from my iPad using Ausfish forums
    granted you may be right about the amount of current to kill butyou have to allow a certain safety margin. My experience with electroncs is that if it can happen, it will. You have to allow for the odd bod who doesnt need much to kark it. Earth leakage devices operate from 10mA to 40mA. So you have to assume a certain margin. Its no good later saying " that's never happened before".
    so after saying that, i went back to the research and can say that its higher than 75mA even, but it all depends on circumstance. AC or DC current, skin condition, place of contact ie hands or chest etc etc . My take is keep a safe margin when i calculated these things.
    i do agree with your opening line.

  2. #47

    Re: New Boat Grounding Question

    Quote Originally Posted by Noelm View Post
    I must admit I am bamboozled by all the to and fro, I fully understand what goes on and why, but for some reason, it's getting twisted around, I think Telwater are correct by the way! measuring from the battery positive to any ground/earth/bond/negative will always show 12V.
    I have been following this thread with much interest, but remain somewhat confused by the various expert opinions. I have recently taken delivery of a new 6.2M platey and decided to attach a zinc electrode block to the hull on the left spare transducer bracket. My thinking, after owning a second hand tinny for 5 years with multiple corrosion spots over the hull, was that a sacrificial anode directly attached to the alloy might prevent a similar problem in the new hull. Being an electrically challenged individual, I was hoping this would prevent any galvanic corrosion of the hull, hypothetically being the sacrificial anode would corrode in favour of the aluminium. I am not sure whether the electrical circuits are grounded or bonded to the hull and motor, as all electrical were done by the local boat builder and as mentioned, I have no idea of circuits other than how to turn the system on an off via the isolation switch. There is no evidence of corrosion or pitting on the hull to date after 130 hours in salt water and I do a thorough wash down after each trip. Any help and explanation is greatly appreciated. I just want to be sure I have done the right thing (or more importantly, not the wrong thing in tis case). Cheers SS

  3. #48

    Re: New Boat Grounding Question

    Quote Originally Posted by seastrength View Post
    I have been following this thread with much interest, but remain somewhat confused by the various expert opinions. I have recently taken delivery of a new 6.2M platey and decided to attach a zinc electrode block to the hull on the left spare transducer bracket. My thinking, after owning a second hand tinny for 5 years with multiple corrosion spots over the hull, was that a sacrificial anode directly attached to the alloy might prevent a similar problem in the new hull. Being an electrically challenged individual, I was hoping this would prevent any galvanic corrosion of the hull, hypothetically being the sacrificial anode would corrode in favour of the aluminium. I am not sure whether the electrical circuits are grounded or bonded to the hull and motor, as all electrical were done by the local boat builder and as mentioned, I have no idea of circuits other than how to turn the system on an off via the isolation switch. There is no evidence of corrosion or pitting on the hull to date after 130 hours in salt water and I do a thorough wash down after each trip. Any help and explanation is greatly appreciated. I just want to be sure I have done the right thing (or more importantly, not the wrong thing in tis case). Cheers SS
    Thats what they do on large vessels, a large zinc anode attached to the hull.

  4. #49

    Re: New Boat Grounding Question

    The zinc won't hurt but once the boat is on the trailer it will do precisely diddley squat. On larger boats the zinc is there as a sacrificial anode to take care of galvanic corrosion due to disimillar metals used in consruction. Some makers also go to great lengths to ensure the hull is isolated with many vessels having DC "earthing" fault indication systems. Ship's chief engineers in particular can get quite animated when you inadvertantly install a "negative earthed" accessory. A lot of this has more to do with electrolysis faults induced if differently wired vessels are plugged into the same shore power network in close proximity to each other - effectively making one boat dissolve.

    On my last tinnie I ran a dual pole isolator so that everything was totally disconnected when the boat was in the shed. It still got pin holes starting up in various random places. Keeping salt out of areas that you can't wash properly and making sure you wash everywhere you can with something to break the salt down (fresh water isn't good enough on it's own) is key to keeping an alloy boat corrosion free.

  5. #50

    Re: New Boat Grounding Question

    Quote Originally Posted by scottar View Post
    The zinc won't hurt but once the boat is on the trailer it will do precisely diddley squat. On larger boats the zinc is there as a sacrificial anode to take care of galvanic corrosion due to disimillar metals used in consruction. Some makers also go to great lengths to ensure the hull is isolated with many vessels having DC "earthing" fault indication systems. Ship's chief engineers in particular can get quite animated when you inadvertantly install a "negative earthed" accessory. A lot of this has more to do with electrolysis faults induced if differently wired vessels are plugged into the same shore power network in close proximity to each other - effectively making one boat dissolve.

    On my last tinnie I ran a dual pole isolator so that everything was totally disconnected when the boat was in the shed. It still got pin holes starting up in various random places. Keeping salt out of areas that you can't wash properly and making sure you wash everywhere you can with something to break the salt down (fresh water isn't good enough on it's own) is key to keeping an alloy boat corrosion free.
    Yes its all about understanding the concepts. Its then easy to work out why.

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