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Old 09-11-2019, 08:01 PM   #1
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Another bonding/grounding conundrum

When my trawler was hauled for the first time since ownership recently, an area on the starboard hull around a Dynaplate showed corrosion. The area was about 1' x 2 ' with loss of bottom paint down to bare gelcoat in places. I have the following comments/questions:
1. There are 2 hull anodes; the Dynaplate on the starboard hull and an AL anode bolted to the hull near the stern. There is no SSB/HAM radio installed on the vessel. A galvanic isolator is installed on the vessel.
2. What is the cause of the hull corrosion around the Dynaplate?
3. Why is the Dynaplate being used for the bonding/grounding system when the stern anode could be used to bond/ground the entire vessel?
4. If the hull corrosion was caused by the bonding/grounding wires connected to the Dynaplate, should I remove those wires and connect them to the stern anode?
5. Pictures of the bonding/grounding system are attached to this post. Is there anything else wrong with the installation?
Thanks
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Old 09-11-2019, 08:17 PM   #2
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What type of bottom paint? Is it high copper content?

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Old 09-11-2019, 11:14 PM   #3
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What type of bottom paint? Is it high copper content?

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Interlude Micron CSC - cuprous oxide 37%
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Old 09-12-2019, 08:26 AM   #4
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At least some of the thruhulls pictured use a hose clamp to clamp the bonding wire to the side of the thruhull. This never works for more than a short time if ever. Not only that, but the way the wires are captured under the hose clamps in 2 different locations WILL also add to the resistance. The bonding wires need to be connected to the same point at each junction. Since the whole thing appears to be daisy chained, as soon as you have high resistance at one point, everything else downstream will not be bonded. Check resistance from the main ground on your boat to each of the thruhulls and I bet you'll find high resistance. To operate properly, you need a MAX of 1 ohm between your main ground buss and every thru hull and zinc.

I would also check the galvanic isolator just to be sure its ok.

Ken
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Old 09-12-2019, 09:22 AM   #5
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Following KC's excellent observations, I'd recommend a fresh start. Engage a well experienced marine electrician to properly redo the vessels bonding system. As well, go through the 110 systems and bring the ship's ground, individual grounds and neutrals up to ABYC recommendations and new GFCI code.

On your vessel this should not be a huge job provided reasonable access is available.
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Old 09-12-2019, 09:32 AM   #6
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At least some of the thruhulls pictured use a hose clamp to clamp the bonding wire to the side of the thruhull. This never works for more than a short time if ever. Not only that, but the way the wires are captured under the hose clamps in 2 different locations WILL also add to the resistance. The bonding wires need to be connected to the same point at each junction. Since the whole thing appears to be daisy chained, as soon as you have high resistance at one point, everything else downstream will not be bonded. Check resistance from the main ground on your boat to each of the thruhulls and I bet you'll find high resistance. To operate properly, you need a MAX of 1 ohm between your main ground buss and every thru hull and zinc.

I would also check the galvanic isolator just to be sure its ok.

Ken
OP, a question for the group, but I think it relates to your situation..

I assume daisy chaining is allowed per code, but a single wire to each thru hull is the better solution?
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Old 09-12-2019, 11:06 AM   #7
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When I removed the SSB radio from my wooden hull, I left the sintered bronze ground plate in place and connected a heavy copper cable from my lightning rod to it, nothing else. The burning away of the paint around yours suggests some unwanted current flow. You might want to carefully hammer check you underwater through hulls while you are following the advice about checking resistances.
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Old 09-12-2019, 02:24 PM   #8
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The burning away of the bottom paint suggests you have a stray current problem vs a galvanic cell problem. The stray current could come from your boat or a neighbor boat or even a dock. If the corrosion rate is high, the stray current is likely DC. Check all your bilge pumps and switches for exposed positive DC wires and in general for any wires in the bilge.

If you are handy with a multi-meter you can measure voltage and resistance between your underwater metals. They should all be at the same potential.
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Old 09-12-2019, 03:50 PM   #9
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In looking at the photos it appears that there bare bonding wires that are clamped to the through hulls using hose clamps. That is certainly not recommended. If indeed you are to use hose clamps to connect a bonding wire to a through hull then a barrel connector should be installed on the end of the bonding wire not just bare wires under the clamp. Or you can drill the hose clamp and bolt a ring connector to the clamp.
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Old 09-12-2019, 07:02 PM   #10
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Originally Posted by Fletcher500 View Post
OP, a question for the group, but I think it relates to your situation..

I assume daisy chaining is allowed per code, but a single wire to each thru hull is the better solution?
Daisy chain wiring is fine as long as the 2 wires are mounted to the same location. When you start putting wires on different points of the same item, you introduce multiple ways for increased resistance between the wires.

Ken
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Old 09-12-2019, 08:58 PM   #11
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OP, a question for the group, but I think it relates to your situation..

I assume daisy chaining is allowed per code, but a single wire to each thru hull is the better solution?
Yes daisy chaining is ok, but it is better to have a home run, so to speak, from each item. And it isnít a code but rather recommendations.
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