To bond, or not to bond?

[gunnarsilins]

On my Moody 42, built -77, has a yard installed bonded system with all underwater metals, like prop shaft, through-hulls, rudder stock and stern glands interconnected to each other, battery minus and a limpet anode.
As far as I can judge this system has been fine, and during my ownership I have introduced a bowthruster with its own anodes, also bonded into the system without any problems (these thruster anodes are used up to 80 % in a year)

But last year I introduced two additional things.
First a new fridge compressor with keel cooler, with its own anodes. This cooler has a ground lug which I connected into the bonding system, which also is according to what´s written in the installation manual.

I also introduced a shore powered 2 kW water heater for the cabin heating system. The system also has a heat exchanger plumbed into the engine cooling system.

Now things begin to happen. After three months in water the anodes on the keel cooler has been almost entirely eaten up, but all other anodes (bowthruster, propeller and main limpet anode) show only normal wear.

My immediate thoght was that there is something happening with the keel cooler - but what? It cannot be stray currents - they should be shorted out to battery minus.

But suddenly a more scary thought occured - the AC powered water heater has its housing grounded, and this AC ground has a connection (?) through the circulated anti-freeze mixture to the engine plumbed heat exchanger, which in turn has a connection (?) through the engines coolant to the engine block, and further to the sea water?

Is this scenario possible, and thus introducing small voltage differences to another shore powered boats at the marina?

And would a isolating transformer be the best solution?

Thoughts and ideas anyone?

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[Evadne]

Just a thought, does the shore powered 2kW heater have an earth connection to the supply earth, in which case its chassis will be at this potential, and is this isolated from the boat's earth, in which case this could be at a different potential? Your thought of completely isolating it from shore DC potentials sounds like it's in the right direction.

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[LORDNELSON]

Is it possible for you to cease using the 2KW heater(and electrically disconnect it) and just have one or two of the of the usual thermostatic 500watt heaters plugged in. Then fit a new keel cooler anode and see what happens after say three months? This would enable you to decide with reasonable certainty whether it is the heater that is causing the problem. Has anything changed near your boat that would affect the electrical side of things around your boat - a different boat in the next berth, some changes to the electrical services in the marina?

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[lumphammer]

Have you fitted any galvanic isolation between your ac earth and the boat ground. With ac power you are connected via the earth lead to the ground of all the other boats in the marina connected to shore power, creating all sorts of galvanic problems.

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[bedouin]

If all your through-hulls, anodes and so on are bonded together then each item will be protected by the one with the least resistance. So it is possible that the anode on your keel cooler is also protecting other metal fittings in the area. This would be particularly likely if dirt, corrosion or a bad connection was increasing the resistance of the connection to the main anode.

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[gunnarsilins]

Dave White

Yes, it´s chassis is grounded to shore AC, but not to the boats ground.

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[gunnarsilins]

Lord Nelson

In theory yes, but now it´s really the season when I want to use the heater.
But I might give it a try.

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[gunnarsilins]

lumphammer

No, there is no galvanic isolator between the two grounds, because they are not connected together.
But my concerns were that the anti-freeze mixture itself in the heating system and engine could create such a connection. If so, a galvanic isolator wouldn´t help I beleive.

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[AndrewJ]

I found one thing that works for me. I took a zinc anode and connected it to a wire and dropped it over the side. I took an
amp meter and connected one lead to the wire (connected to zinc) and the other amp meter lead to a bonded shroud and
measured the current flow. I then disconnected all AC shore power to the boat and measured the current. I just used this to
isolate whether the difference in potential, and hence my zincs being eaten up, were caused by DC or AC. I found mine was caused
by the AC. I then got a "polarity checker", (small cost at a hardware store) and put that into an ac socket on the boat. I found
the wires were reversed, (the shore power lines were apparently reversed), so I just exchanged the hot and common leads on my boat,
which corrected the problem. (naturally when I get to another marina, I'll have to recheck the polarity.)

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[gunnarsilins]

bedoin

I understand your point, but when installing the keel cooler I also rewired the whole bonding system, with new wire and soldered terminals everywhere. So I beleive the connections are really good.
Also, if this were true, the chances that only the keel cooler anode would show excessive wasting must be minimal. Apart from the limpet and keel cooler the bowthruster and propeller also have their own anodes, and these had signs of normal usage. So I find it rather hard to beleive that all ground wiring is bad apart from the one to the keel cooler.

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[gunnarsilins]

AndrewJ

This looks interesting!
Basically I just could disconnect the wire to the main anode and perform the measurements?
I´ll give it a try!

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[Evadne]

That does sound interesting. I like the design of your of the "anode current detecter" as well. I'd hope that an isolating transformer between the shore power and the boat would obviate any polarity problems. I suspect that there may also be a problem caused by having the shore earth to the heater and the boat's earth for everything else. Solving that is not as easy as it sounds. As has been said, I don't think joining the boat's earth to the shore earth is a good idea, you may get galvanic corrosion due to an earth loop with adjacent boats anyway. Removing the earth from the heater entirely is not usually recommended, especially in writing, although it would cure that problem. If it were possible to make sure that the 2kW heater earth was not in contact with the boat's earth or the bonded system, that might cure it and be cheaper than an isolating transformer.


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[bedouin]

Re: bedoin

I'm not expert on this but a few further thoughts:

Physical distance also matters - so the bow anode is unlikely to interact with those near the stern - I've heard is said that an anode needs to be within about 1m of the metal it is protecting to be any use.

Is it possible that the keel cooler anode is a different material than the others (e.g. Magnesium based, not Lead) - that might also explain it.

Similarly is it the case tha keel cooler anode is physically closer to (for example) through-hulls than the limpet?

I think you need to consider two separate questions (a) The rate at which anodes are eroding and (b) the comparative rates of errosion. These may or may not be related

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[pvb]

Here\'s your easiest solution..

Your problem is certainly the result of the shore power connection, and your cheapest solution is to fit a galvanic isolator.

For safety, your AC earth must be connected to your DC ground. If it’s not, and if a fault allows a short between AC live and the boat’s DC system, there wouldn’t be enough fault current to trip the breaker on the pontoon. Instead, your boat would become live and would be a serious hazard - both to you and to others.

The problem is, connecting the earths means the boat is vulnerable to galvanic corrosion. The grounding through the shore power wire makes the underwater metal parts of your boat into one plate of a voltaic cell, with the water as the electrolyte and any other grounded metal (other boats or metal pontoons, etc) as the other plate. The earth wire in the shore power lead connects the two plates together, and a small current will flow - causing corrosion.

The solution is to fit a galvanic isolator in the shore power earth wire, just after it enters the boat. A galvanic isolator uses diodes to block the tiny currents causing galvanic corrosion, whilst allowing the larger fault currents to pass, so as to ensure that the shoreside breaker will trip safely if an AC fault occurs.

You can make a galvanic isolator yourself, or buy one fairly cheaply. <A target="_blank" HREF=http://www.yandina.com/galvanicIsolator.htm>Click here</A> to see an explanation of how to make one on Colin Foster’s excellent website.


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[gunnarsilins]

Thanks all...

...of you! Now I got enough ideas and suggestions to keep me busy a couple of days, measuring leak currents and building galvanic isolators!

Wish you all fair winds!



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[andyball]

Re: Here\'s your easiest solution..

regarding the "make your own" link....maplin,RS etc all sell bridge rectifiers similar to those shown. The beefiest maplin one is 35 Amp & costs a huge £1.49 . Bear in mind that the how-to article is based upon US 110V supply so the currents mentioned are around double those found in a european "220V" supply.

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