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Old 11-04-2010, 06:53   #1
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Electrical Bonding of Thru-Hulls, etc.

One on my thur hull fittings corroded all the way sometime in the recent past. I posted another thread a while back that the only thing holding it in was caulking. When I replaced that fitting and valve, along with another that was frozen next to it, I noticed that the bonding wires to both were in bad shape. So I put replacing the bonding wires on my project list. Since the orginal owner kept the boat in a marina I kind of was blaming the corroded valve on the bad bonding wires and stray current in the marina.

I was on the West Marine site looking for something else and got distracted by the article in the below link (getting distracted is becoming a problem). This article also suggests my corrosion problem was stray current. But that the bonding itself as the problem and to NOT bond thur hulls together etc. While I had kind of accepted the bonding everything, my Navy engineering training (ungrounded systems) was kind of causing a mental battle.

So I think I'm going to keep the bonding on my project list, but it's to disconnect things that have no need to be connected.


West Marine: West Advisor
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Old 11-04-2010, 07:21   #2
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We only have the anode, rudder shaft and P bracket joined (together with the earth buz bar). The rest including engine intake and the heads are all stand alone with no bonding. Boat came that way, so I'll not change it.

What was worrying when working in the engine bay I moved the P bracket earth and the earth cable fell off, due to corrosion. Now repaired and the cable checked with the meter.

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Old 11-04-2010, 08:14   #3
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We have two main mechanisms at play here:
Galvanic Corrosion
When we have dissimilar metals in an electrolyte, we have the potential for a primary cell. The farther apart on the galvanic scale they are, the better the cell. Copper and Zinc will give you about a Volt, Copper and Stainless Steel about 0.25 volts. Depends on the electrolyte a bit, this is in brine. This voltage will just sit there, nothing happening, until we electrically connect them together. At this point, current will flow, and metal ions will leave the most electronegative metal, into the sea - corrosion, the metal is eaten away. The most electronegative metal is hopefully your Zincs. For common metals, only Magnesium is more electronegative.

Bond the through hulls together, and you will get increased depletion of your Zincs.

Ocean currents
A current flowing though the water past your boat will go in though one through-hull and out through another if they are bonded together, as it is a lower resistance path than the water. This will corrode one of the through-hulls, irrespective of the metal, as metal ions will be dragged into the water.

Conclusion: Do not bond the through-hulls together.

I like MIL-STD-1250A for this kind of thing. It is downloadable free from https://assist.daps.dla.mil/quicksearch/

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Old 11-04-2010, 08:39   #4
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There is no single answer on which everyone will agree.
There is only one truth here - when you connect two (or more) metals in the presence of water, you just made another battery.
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Old 11-04-2010, 09:03   #5
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Yes, it is a continued source of disagreement. We all made batteries in school with a couple of dissimilar metals and a light bulb between them, of course.

How the boat comes from the factory is probably a good indication of what the professionals think. Cheaper not to bond initially of course.

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Old 11-04-2010, 09:29   #6
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I'm squarely in the no-bond camp. I've always thought that the bonding wires create a great path for lightning to blow out the seacocks.
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Old 11-04-2010, 09:41   #7
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That's a vision I prefer not to have.

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Old 11-04-2010, 17:07   #8
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I was in the bond everything camp. But after the the head overboard valve completely lost it's connection and was only held in by caulk I've changed sides.

Will still bond the mast, chainplates, and engine.
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Old 15-04-2010, 22:33   #9
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There are good technical arguments for both sides of this issue. There is an overriding safety concern that predicates bonding. That being said, if the vessel is bonded, the boat owner must perform periodic maintenance on the bonding connections to ensure that each connection is < 1 ohm. Since most owners will not perform this maintenance, my recommendation is to not bond.

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Old 15-04-2010, 23:08   #10
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Charlie, can you describe this maintenance, first I ever heard of this. I have a bonded boat (from factory), are we just talking about cleaning the connection? Now that I think about it that does sound logical to clean the connections, item number 367 on the to do list (man oh man more work!). Do you put a connection grease on the fittings, I don't know the name of the stuff but it is used for improved transmission (can ya tell electrical is not my strong suit?)

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Old 16-04-2010, 05:24   #11
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OG-You need to establish and maintain a very low resistance connection in a, generally, very hostile environment (your bilge). The ABYC specs < 1 ohm and, in fact, should be as low as possible. Remember, we are talking millivolts of potential when working in the galvanic world. Stripping off insulation and then clamping the bare ends to a through hull with a hose clamp will not provide an enduring, low resistance connection!!

Initial installation

ABYC also specs AWG 8 wire (larger if part of the lightning ground). I use AWG 8, tinned, Type 3 (Ancor wire) with green insulation. I use open barrel ring terminals and crimp them with a box or hex crimper. I then solder the connection and apply adhesive line heat shrink.

Clean the area where the connection is being made and bolt/fasten the ring terminal on the end of the "whip" to the fitting. Spray the connection with CRC's Heavy-Duty Corrosion Inhibitor.

All "whips" from various underwater metal are then brought to a small bus bar. Don't daisy chain because one failed connection wipes out the protection for the downstream fittings. Tie the bus bars together and then tie the whole bonding system at one place to the DC vessel ground. NEVER use the bonding system as a current carrier.

Periodic Maintenance

With a good DMM, measure the resistance from the bus bar to the underwater metal fitting. If it is <1 ohm, spray a bit of CRC on the connection and you are done. If it is >1 ohm, correct the problem.

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Old 03-05-2010, 16:07   #12
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So in the end I ran some new grounding wiring. Took all the bonding wires off the thur fittings (half were bonded).
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Old 03-05-2010, 17:04   #13
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An interesting post. I have been wondering what to do about the steel stern tube on our steel yacht. It is full of seawater (designed to be that way) and not painted internally. I have a carbon brush to earth the s/s propshaft to the hull. Would I be better not having it earthed?
Regards, Richard.
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Old 03-05-2010, 17:05   #14
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thru hulls

I was taught to un bond and isolate or bond and protect.I think the un bond and isolate is based on the idea that a thru hull that is not near any current source is not subject to stray current corrosion since there is no path to ground unless it is immersed in an electrolyte like salt water.
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