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Old 24-12-2012, 11:12   #1
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Throughull corrosion and galvanic burning on hull

I have read the excellent posts in this forum on this topic but not found any that specifically address my situation so I am asking for assistance.
I have a fiberglass boat with 4 unbonded bronze throughulls. 3 have minor green patina growing from the bottom up. The fourth is behind the head and is heavily corroded all over. The threads are all gunked up with green as well, but after cleaning with sandpaper, the metal appears to be bright yellow and there is no pink or white flaking. Any ideas why there is so much corrosion on this one?

When I hauled the boat last week, there was a 12" square patch of galvanic burning on the hull around the bronze strut. It is my understanding that the grounding plate is part of the strut where it is through bolted into the hull. It was blistered and the paint bottom paint had flaked off in that one area. The yard foreman said it was galvanic burning and he asked me if there was a green wire connected to the prop shaft. We sanded off the paint and applied new bottom paint.

I checked and found a connector attached to a bolt on the grounding plate, but there wasn't a wire attached. I also checked the grounding lug inside the shore power circuit breaker box and discovered that the green wire for the AC ground had been disconnected. I re-connected it, but have not yet installed a green wire going between the AC ground lug a bolt on the grounding plate bolt. I bought Don Casey's book on Simplified Sailboat Electronics and read up that the green wire on the prop shaft connects your boat to other boats in the marina and is an entry point for stray current.

When I hauled the boat 3 years ago, there was no sign of any galvanic burning on the hull. Since I hauled the boat, I installed new batteries and and a battery charger. Now the boat is connected to shore power when it is at the dock. I also just bought a little heater for mold control. I am wondering if being plugged in all the time s the reason for the galvanic burning and the advanced patina on the one through hull?

I did some of the tests that Casey suggests in the book. I did not get a reading when I tested for stray current in the water. I did get a reading of 1.8v when I tested for AC current between the ground and neural sockets on the boat. I then tested the 30 amp socket on the dock and got the same reading, so the dock wireing seems to be the source of that problem. I am not sure what to do about that or if it is a problem.

Possible solutions that I have come up with are:

Unplug the boat from shore power and use solar panels to charge the batteries. Plug in the heater I use to prevent mold directly to the shore power cord using a pigtail adaptor, OR install a Galvanic isolator, which would be more convenient and useful for when I wanted to recharge the batteries quickly or run the electric heater when my wife is on board.

Buy or make a "Zinc Fish" and connect it to the ground plate.

Install a green wire between the ground plate and the AC ground.

I would be interested in hearing from anyone who can address these issues. I have learned a lot here over the years and always appreciate feedback.

Happy Holidays to All,

Larry
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Old 24-12-2012, 11:29   #2
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Re: Throughull corrosion and galvanic burning on hull

The head thru hull always gets that way if it's near the head. Probably from the Urine/ fumes. Good bronze turns green for sure. If you haul out, pick at the thru hulls aggressively and you'll find out if they are deteriorated.
re:The galvanic issue.... they are quite complex. Is the strut bonded with a wire on one of the bolts inside the hull? If it is maybe it would be better not bonded if you are actually getting burning.... A/C power is definitely your enemy in many marinas. Some people say to ground it to machinery and some not. I guess it's a bit of a risk either way. Maybe someone more electrically inclined will answer that for you.
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Old 24-12-2012, 16:47   #3
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Re: Throughull corrosion and galvanic burning on hull

CONNECT THE SAFETY GROUND WIRE FROM SHORE TO THE VESSEL'S AC SYSTEM. THIS IS A CRITICAL SAFETY ISSUE! REALLY.

"Galvanic burning", aka, "haloing" is generally a sign of high galvanic potential interacting with a high copper percentage bottom paint. How many anodes do you have installed and are they zinc or aluminum alloy?

The hull potential measured against a silver-silver chloride reference cell for a FG boat should fall in the range of -550 to 1100 mVDC. If you are high in this range, removal of some of the anodes (surface area > voltage) should drop your hull potential lower in the range and mitigate the haloing.

The 1.8 VAC potential between N and G at the pedestal is indicative of undersized wiring feeding the dock and is not at all unusual. We like to see it close to 0.0 VAC but rarely do in the real world.

You would be well advised (and IAW ABYC Standard) to connect the AC safety ground bus to the DC negative bus in one and only one location, typically behind your main power panel.

Stray current is not really the issue with the safety ground wire. It is cathodic protection current being generated by your anodes leaving your vessel via the safety ground wire and protecting other metal bits on shore or other vessel's that are not diligent in maintaining their anodes. This loss of cathodic protection current can be mitigated by an isolation transformer, the best and most expensive option, or by a high quality galvanic isolator.

There is quite a bit too this and your vessel has several inter-related electrical and galvanic corrosion issues. If your reach exceeds your grasp, hire a professional, preferably an ABYC Certified Electrician and Corrosion Technician to help you out and train you.
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Old 25-12-2012, 09:14   #4
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Re: Throughull corrosion and galvanic burning on hull

Hi Charlie and all the Sailors 'Round the World this Christmas Day,
Thanks for your informative and thoughtful response.

Are you saying that I have too many zincs on my prop shaft and strut?
My prop, shaft and strut are in good condition. Looking nice and shiny when I hauled last week .The shaft has always had at least one zinc on it. It is bronze and has 2 donut zincs on it. I rememberer that another zinc was added to the strut in 2009. The strut was through drilled and a round disc shaped zinc mounted on opposing sides of the strut. I also added a trickle charger in 2009. In 2011 I installed new batteries (2 6 volt, 140 AH Trojans and a full sized battery charger.
I don't mind spending money on preventative measures and will install a high quality galvanic isolator. I am going to buy the Pro Mariner 30 Zinc Saver.
Do you think there is any need for a zinc Grouper hung over the side? if the life lines are connected to the DC ground is that adequate for protection or should the zinc be connected to the engine or prop shaft?

Thanks for your help.
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Old 25-12-2012, 09:48   #5
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Hi


I had the same galvianic burning problem after I replaced one of my hull anodes with another that was too big, it was only 1/2 lb bigger but that was enough to tip the system over the edge.

Did a test in the water and it showed over protection on the meter, simply swapped things back to initial size and all is well now.

I got a little hung up on protecting the hull metals and went to far !!!

I fitted a victron isolation transformer too, that gives me total peace of mind when plugged inti shore power now.

Try looking at reducing an anode a size or two?

Charlie.
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Old 25-12-2012, 10:41   #6
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Re: Throughull corrosion and galvanic burning on hull

A Galvanic Isolator is only about $100 and is an essential line of defense against electrolytic corrosion if you are using shore power.
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Old 25-12-2012, 11:01   #7
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Re: Throughull corrosion and galvanic burning on hull

i went the simple way. no grounding at all. no shore power at all. i run entirely on 12 volts and a solar panel keeps the batteries charged up, even in a slip. after many years my seven bronze thru-hulls still look perfect.

and yes, i use high copper content bottom paint (pettit trinidad).
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Old 26-12-2012, 09:46   #8
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Re: Throughull corrosion and galvanic burning on hull

You may cause a serious potential electrical hazard if you start disconnecting the boat manufacture's ground system that meets ABYC rules for keeping your boat electrically safe from lightning and AC electrical shorts. Professional Mariner has a great simplified work book that explains corrosion problems, and walks you through the corrosion protection process. The corrosion test meter is well worth the money, and you will feel much better being able to monitor your boat's zincs with the meter.
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Old 26-12-2012, 09:52   #9
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Re: Throughull corrosion and galvanic burning on hull

Quote:
Are you saying that I have too many zincs on my prop shaft and strut?
Yes. Increasing sacrificial anode surface area increases cathodic protection potential (voltage).

Quote:
Do you think there is any need for a zinc Grouper hung over the side? if the life lines are connected to the DC ground is that adequate for protection or should the zinc be connected to the engine or prop shaft?
As it appears that your vessel is over protected (too high a potential) now, the addition of more anode material would not accomplish anything useful. As I mentioned, determine your vessel's hull potential. If it is high (more negative) in the protected band as I suspect, then remove an anode an try to move the hull potential towards the middle of the -550 to -1100 mVDC band and see if the haloing is mitigated.
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Old 26-12-2012, 22:47   #10
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Re: Throughull corrosion and galvanic burning on hull

Thanks for responding Charlie. So which test should I do to determine the "hull potential" ? I would like to measure that, remove a zinc and re-measure.

Thank-you,

Larry
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Old 28-12-2012, 08:51   #11
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Re: Throughull corrosion and galvanic burning on hull

Hull potential is typically taken with a silver-silver chloride reference cell amidships submerged to approximately 1/2 of the vessel draft and plugged into the negative of a high quality DMM. The positive lead of the DMM is than touched to the underwater metal being tested and the potential (negative millivolts DC) is recorded. The "hull potential" is the potential recorded when the ship's ground (B- bus) is tested.

Other metals can be used as "reference cells" but their use necessitates the use of conversion tables to convert the readings to the Ag-AgCl standard.

An ABYC Corrosion Tech will have this equipment and will be qualified to help you.

As in my previous post; your vessel has some inter-related electrical issues and some of them are quite dangerous if not corrected. I strongly advise you to enlist competent help in correcting these issues. The "haloing" of your bottom paint is the least of your electrical problems.
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Old 29-12-2012, 08:14   #12
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Re: Throughull corrosion and galvanic burning on hull

Measuring and creating a contour map of the electrical potentials in the water around your boat is tedious but not difficult.

Analyzing and drawing conclusions from that data takes a lifetime of experience.
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Old 29-12-2012, 11:08   #13
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Re: Throughull corrosion and galvanic burning on hull

Quote:
Measuring and creating a contour map of the electrical potentials in the water around your boat is tedious but not difficult.

Analyzing and drawing conclusions from that data takes a lifetime of experience.
All true but absolutely not germane to the OP's original questions/statements.
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Old 30-12-2012, 10:21   #14
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Re: Throughull corrosion and galvanic burning on hull

You provided information on a silver-silver chloride reference cell for use in measuring hull potentials. Was that germane? If so, the original OP might want to consider my experience with them before investing.
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Old 30-12-2012, 10:48   #15
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Re: Throughull corrosion and galvanic burning on hull

I drilled a hole thru the strut and put a zinc on it, and got the same haloing effect. Took the zinc off, and it went away.
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