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Bonding/electrolysis ?

Discussion in 'Musings With Maine Sail' started by Sefuller, Oct 29, 2017. Add this thread to a FAQ

  1. jviss


    Joined Feb 5, 2004
    2,870 posts, 332 likes
    Tartan 3800
    US Westport, MA
    Yea, that's interesting. I'll have to check mine, too. I think it should be isolated.

    I'm glad you brought this up. I had never considered it before. I wonder how many others have.

    Here's mine. It looks like the shield is connected to the bracket, which is connected to the mast.


    Last edited: Dec 2, 2017
    Sefuller likes this.
  2. dlochner


    Joined Jan 11, 2014
    2,246 posts, 827 likes
    Sabre 362
    113 US Fair Haven, NY
    As I read the article, the issue isn't connecting the DC ground to the mast via the VHF antenna for lightning protection. You are correct lightning is going to do a lot of damage and will arc over any small gap and make a mess of things. Angry electrons are nasty fellows.

    The issue is backdoor connections via the lightning protection system. The shield on the coax cable is connected to the VHF radio's metal case which is connected to the DC negative which is connected to the engine block and the AC ground. If you should remove all bonding, i.e., the wires connecting all the underwater metals, but do not isolate the VHF antenna, then you have provided a back door, the keel connects to the mast, the mast connects to the VHF antenna, the VHF antenna connects to the coax shield, which connects to the VHF radio's case and so on. In a rather roundabout way in this case anything connected to the keel would be functionally bonded to the other metals on the boat. Assuming that all the through hulls are electrically isolated, the metal bits to worry about are the SS prop shaft and the bronze prop which are connected to the AC and DC ground systems. And we know to isolate the AC ground with a galvanic isolator or an isolation transformer.

    Stan also mentions a couple of other avenues for stray current, the fuel sensor gauge in a metal tank being a prime candidate. The standard float switches from Moeller ground the tank to the DC negative. The WEMA gauges do not use the tank as a ground.

    I'm curious about a couple of conditions that may also connect metal to the DC ground, especially in saltwater. How conductive is that water in an engine's cooling system? It seems that the bronze through hull is connected via the saltwater in the cooling system and that it would be necessary to bond the through hull with the engine to have them remain at the same electrical potential. The same might happen with air conditioning cooling systems, water cooled refrigerator systems, or desalinators. If that is the case, then those through hulls would need to be bonded.

    So, if I understand bonding correctly, the goal is to completely isolate metal bits or to make certain that all metal bits that might be connected are at the same electrical potential. Wrestling angry electrons is certainly a challenge. :what:

    Sefuller and jviss like this.
  3. Skipper


    Joined Oct 9, 2008
    1,589 posts, 267 likes
    Bristol 29.9
    US Dana Point
    Regarding corrosion and lightning:
    It's my understanding that a proper bonding system has 2 purposes:
    1. For corrosion, to bond the chainplates for the stays and shrouds, the mast base, thru hulls, and engine. Then the shaft anode protects the whole boat. You can also hang a cabled anode off a stay for added protection. The mess starts when the shaft anode is neglected, and there is no other protection. Especially re the aluminum parts on the engine, plus rigging, spars, etc, all of which will succumb to the bronze thru hulls.
    2. For lightning, it's not to disperse the strike, although it may to some degree. It's to keep the metal bits on the boat at the same electrical potential as the ocean water, thereby reducing the lightning's preference for the mast.

    All of this may be wrong. :)
    Feel free to beat me senseless.
    Only discussion.

    Last edited: Dec 3, 2017
    JamesG161 and Will Gilmore like this.
  4. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,686 posts, 518 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME

    As always AC, DC, Lightning, SSB Radio and galvanic bonding/grounding are often very misunderstood. I will simply highlight some points Stan makes in hopes that it may "pop" and an ah-ha moment may happen...

    DC Ground:
    Every light or appliance should be wired with its own DC return wire. Never use the mast, engine, or other
    metal object as part of the return circuit."

    What this is very simply saying is that a bonding circuits green wires or the mast or any other part of the vessel should not be used to carry on-board DC current. I see this rule violated far too often and nearly every boat out there, with a VHF, is putting DC current through the spar. DC current won't just stick to the coax some will return down the spar and some through the coax unless you isolate it. This puts stray DC current right onto the lightning & galvanic bonding system on many boats. If I had a dime for every navigation light I saw use the spar as a DC current carrier I'd be retired by now. I also routinely see folks just run the DC black or yellow wire to any green wire they can find thinking this is ok. IT'S NOT..

    Isolating the braid of your coax from the spar is not very difficult..

    "AC Ground
    The best solution is a heavy and expensive isolation transformer."

    Bingo!! A galvanic isolator, what most boaters use, can only protect your boat against "galvanic level" (dissimilar metals voltage & current potentials) damage. It can not protect you from electrolytic or what is commonly called stray current corrosion. The term electrolysis is what hirsuit women do to their upper lip.

    An isolation transformer (properly installed) is the only good way, short of unplugging your boat, to keep you electrically isolated from other boats in the marina, but they are no inexpensive.

    "The acceptable solution (for the rest of us) is to install a light and inexpensive Galvanic Isolator"

    A galvanic isolator should be the absolute bare minimum if you plan to plug into a marina. Be sure to only install "Fail Safe" galvanic isolators that meet the ABYC standards.

    It is also a good idea to use a Ground Fault Interrupter (GFI) in your AC wiring.

    Not only is is a good idea, GFCI's for outlets are required in head, galley, weather decks and engine spaces under the APYC safety standards. Whenever GFCI receptacles can be used, they should be used. The ABYC standards set a bare minimum, not a maximum. Not only are 5mA GFCI's required but also a 30mA ELCI is required as the vessels main breaker. These changes make boating much safer. I often prefer to go a step beyond the minimum requirements and use GFCI's where ever I possibly can. This includes water heaters, air conditioners, reverse cycle heat, battery chargers etc. etc.[/B][/I]

    Lightning Grounds
    Connect a 4 AWG battery cable from the base of your aluminum mast to the nearest keel bolt fromexternal ballast. If you have internal ballast, you should install a lightning ground plate. One square foot is recommended for use in salt water; fresh water requires much more. Do not rely on a thru-hull or a sintered bronze radio ground (e.g. Dynaplate) for use as a lightning ground.

    Lightning bonding is intended to help minimize damage to the hull, that is it, that is all it does. If done well, it can certainly help to minimize hull damage. Lightning bonding is not, nor was it ever, intended to help you avoid a strike or protect your electrical equipment. I do lightning investigation & repair work and I have yet to see a single lightning system prevent a strike. In fact, I see "fuzzy bottle brush boats" get struck more often rather than less often. I also see lightning bonded boats hit slightly less than non-lightning bonded. I also see lighting bonded boats suffer less hull damage. All we can do is try to provide a low impedance path to Earth in order to help minimize hull damage so you hopefully don't sink when thousands of tiny holes are blown in the hull as lightning takes a more difficult path, through you hull, to reach ground..

    RF Ground
    Your VHF doesn't need to use the ocean as a counterpoise, so here we are dealing only with the ground needed for your HF/SSB radio.

    Seeing as most boat owners do not have an SSB I will let the article explain this, and it does so quite well. An easy and very inexpensive fix to isolate the VHF whip from the spar is shown above.

    "Bonding and Electrolytic Corrosion Due to Hot Marinas
    Do not bond any thru-hulls or other immersed metal that can be electrically isolated. Specifically, Figure 1. Conductors running from the external keel or ground plate to the mast, stays and to the metal fuel tank will protect against a lighting strike, and there will be no DC connections to the engine or to the electrical system. Keep your metal keel/ballast, your metal rudder shaft, your engine/prop, and all thru-hulls electrically isolated, from each other, and from the engine."

    Another excellent statement.. If the resistance between your bonded underwater metals provides a lower resistance path for stray current your boats bonding circuit becomes a path for it. The current path will flow from one immersed bit of metal, through your vessels bonding wires, and exit the vessel again at anotehr point in the bonded circuit. It is this exit point that will suffer corrosion. By not providing a path for stray current it will not use your boat. Bonding underwater metals only serves to slow the rate of DC corrosion, which almost always originates on-board an owners vessel, even when they are vociferously claiming "I know it's the new guy at the end of the dock causing my rapid anode erosion." It rarely ever is and almost always originates on the owners boat.

    "Keep your metal keel/ballast electrically isolated from all other bits of metal. If you have the misfortune to have an external iron or steel keel, however, mount a zinc directly on it to reduce the rate of corrosion. Leave lead keels/ballast isolated."

    If you don't have an SSB isolating the keel or external lightning bonding plate is easy to do and it will yield a boat that is less likely to suffer from hull damage in a strike. If you have an SSB read the points in Stan's article. Just be certain you mast is de-coupled from the rest of the vessel DC system electrically, including nav lights and your VHF whip.

    Sefuller and Will Gilmore like this.
  5. Plumbean


    Joined Jun 3, 2015
    4 posts, 0 likes
    S&S 34
    US Port Washington

    Thanks as usual for the thoughtful replies. I have a followup question for you (and anyone else who cares to jump in), as I am trying to implement the recommendations in the article but running into some difficulty. My current setup has a 4 AWG wire running from the base of my keel stepped mast to the nearest keelbolt (which is silicon bronze) on my external lead keel. I've run into some corrosion problems and the consensus is that the VHF is the culprit (although now you have me worried about my bilge pump too).

    In order to remedy this, I have fabricated a new VHF mount for my masthead out of some scrap G10 that I had lying around, so once that is installed this spring I will have the VHF mount at the masthead isolated. So I think I have that in hand.

    However, Stan also suggests installing an "inner/outer DC block" in the VHF system. I have not been able to find a commercially available inner/outer DC block, but Polyphaser does make a block that prevents DC current on the center pin of the VHF cable -- See

    I recently purchased one of these to install, but the instructions indicate that you have to ground it for it to work. I spoke to tech support at Polyphaser, and they said they recommended a 6 AWG wire in order to shunt any lightning strike, tied into the existing lightning protection system. In other words, they want me to wire this to the 4 AWG wire running from my mast base to the keel. My concern with this is that if I wire it like this, the coaxial shield would then seem to be connected again to my mast/keel, leaving me with the same corrosion issues. Am I thinking about this correctly? Or, if I have isolated the antenna at the masthead, should I just call it good and not bother with the DC block? Alternatively, should I install the DC block but not ground it?


  6. Plumbean


    Joined Jun 3, 2015
    4 posts, 0 likes
    S&S 34
    US Port Washington

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