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Old 03-08-2018, 10:11 PM   #1
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Exclamation Anodes and Bonding

I was recently required to have my GRP vessel surveyed for insurance purposes, I currently have a shaft end anode and an anode on my shoe for the rudder.
The surveyor suggested I remove these anodes and fit one anode on studs on my transom. From the inside bond the shaft and rudder to it. To this end I taken an 8mm copper wire from the internal anode to a BUSS Bar then a wire to my rudder stock, rudder gland and tube. another wire has been taken to my propeller shaft via a heavy carbon/copper brush head string loaded on an arm, the shaft gland and tube.
He recommended I did not bond the motor!!!??? gearbox or ROCO filters etc.
All my through hull fittings which are bronze and with the exception of raw water intake are usually above the water line unless fueled and watered 100% which is unusual. It is my intention to replace all through hulls to Survey approved plastic units on my next haul out. I am on a marina however vary rarely plug in to AC, I do not have a genset but a 2000 w inverter, my system is fully 12 volts
I am being given so much contradiction feed back on bonding here in NZ my query is:

Does this bonding system appear sound or lacking
Should the engine room be bonded.

The system is now fully installed less my Transom anode which I intend to do within the next week or so.
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Old 03-09-2018, 10:18 AM   #2
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I performed a corrosion survey yesterday on a Bertram that had just had shaft brushes installed. The hull potential on the wire to the brush wiper arm was -940mVDC. The hull potential on the shaft, next to the brush, was -630mVDC. Shaft was still protected but that is a dramatic voltage drop. I rotated the shaft by hand and got no real improvement. The takeaway is that "shaft brushes" do not do nearly as good a job of distributing cathodic protection current as a shaft anode. The only exception is the very nicely engineered and manufactured brush rigging assembly from ElectroGuard.

Hull anode install sounds good. I would suggest using aluminum anodes. Higher potential (50mVDC), last 50% longer than zinc, and there is no cadmium, a heavy metal that works its way up the food chain to be deposited in us.

Bonding system backbone is well constructed. Don't daisy chain from one component to the other. Crimp and solder the ring terminals on the 8mm wire and then use a fastener (not a hose clamp or cable tie!) to attach to the various components. The connection should have < 1 ohm of resistance so the connection has to be clean, bright and tight. Spray with CRC Heavy Duty Corrosion Inhibitor to prevent corrosion deterioration of the connection.

Regarding bonding to your motor, ROCO raw water filter etc.:

The bonding system should be attached to the vessel ground, usually the engine block or its buss.Bonding the sea strainer does not gain much benefit as galvanic or cathodic protection current will not travel up a hose more than a couple of diameters.


Bronze through hulls have been around a long time and are reliable. However, changing them to a Forespar (Marelon) type component certainly takes away the possibility of corrosion. I would not put a "plastic" fitting anywhere where it could be stepped on or otherwise mechanically stressed.

Hope this helps.
Charlie
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Old 03-09-2018, 10:47 AM   #3
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Good points. Nice to see a pro relate real world marine experiences. Now a question, did the Bertram you mentioned have shaft anodes?
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Old 03-09-2018, 12:04 PM   #4
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I use the so called diver’s plate anode mounted on the stern and that is all I use. I started with a 6x12x1/2” aluminum but had to increase the mass to a 1” thick anode, again aluminum.

Why would a surveyor not recommend bonding connect to the engine? I don’t agree with that recommendation at all! Nor do I agree that you should use 8mm diameter copper wire to bond things together. The only advantage of large wire sizes is for mechanical protection, not electrical. The currents if any in a bonding circuit for galvanic protection will be in the miliampere range, not hundreds of amperes.

Shaft brushes! There are commercial brushes available but I know nothing about them. Your surveyor’s voltage reading between a shaft and the brush indicates the brush is not making good contact onto the shaft.....could be caused by oxidation.

I made my own brushes. Mine swing up and down by gravity holding the brush against the shafts. The pivot point is a brass door hinge, 1/2 mounted to a structure in the boat and the other half silver soldered to a naval brass bar about (from memory, not at my boat) 2x14X3/16”. My first experience used a hunk of bronze attached to the end of the bar that rode onto the shaft. That didn’t work! After a short boat trip, the hunk of bronze noticeably wore because of friction. So I replaced that with sintered oilite bronze. That worked.

A suggestion—- be sure to bond the stuffing box! I found verdigris growing on my shafts from the unbounded stuffing boxes. This verdigris found it’s way under one of my brushes making it inoperative. I found this with one of my system tests. After bonding and cleaning the problem is gone.

The most important thing IMHO is to purchase a half cell and do your own measuring a few times during a season to actually know if your system is working. Mine is a silver-silver chloride half cell I stole (because of cost) on Ebay. These things are specialty items and most people have no clues or needs to use them.
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Old 03-09-2018, 12:23 PM   #5
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Old 03-09-2018, 01:54 PM   #6
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The engine is already connected to the main ground and the bonding system should be also. What the surveyor may have been saying is not to ADD a wire from the engine to the bonding system and that is correct. Only ONE connection between the bonding system and the main ground buss. Otherwise, there can be different amounts of current in the various legs of the bonding system and that alone can set up galvanic corrosion between items that should normally be at the same potential.

Ken
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Old 03-09-2018, 02:44 PM   #7
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Originally Posted by kchace View Post
The engine is already connected to the main ground and the bonding system should be also. What the surveyor may have been saying is not to ADD a wire from the engine to the bonding system and that is correct. Only ONE connection between the bonding system and the main ground buss. Otherwise, there can be different amounts of current in the various legs of the bonding system and that alone can set up galvanic corrosion between items that should normally be at the same potential.

Ken
So, if I understand this correctly, if you have twins only one engine block should be connected to the bonding system (which is, in turn, grounded to the DC negative bus)?
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Old 03-09-2018, 03:07 PM   #8
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The engine is already connected to the main ground and the bonding system should be also. What the surveyor may have been saying is not to ADD a wire from the engine to the bonding system and that is correct. Only ONE connection between the bonding system and the main ground buss. Otherwise, there can be different amounts of current in the various legs of the bonding system and that alone can set up galvanic corrosion between items that should normally be at the same potential.

Ken
OK, guess I am being picky. Could you explain why/how two wires or more for that matter can result in galvanic corrosion? OK, if the wires are made of different metals and submerged yeah, I guess a corrosion issue could evolve but only between the wires made of different metals, and only maybe depending on the galvanic voltages differences between the metals.

And the currents.... I doubt if one used #22 wire that there would be any significant voltage drop across that wire from current flow due to galvanic corrosion. The current is too small, milliamperes or less. And there would only be a current flow if there is a voltage difference between the start and end of that wire.

The idea of a common bonding point is solid for traceability but there is nothing, nada, ziltch wrong with bonding in as many places with as many wires as possible. Not that anybody would want to do so.
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Old 03-09-2018, 04:01 PM   #9
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Phil
I performed a corrosion survey yesterday on a Bertram that had just had shaft brushes installed. The hull potential on the wire to the brush wiper arm was -940mVDC. The hull potential on the shaft, next to the brush, was -630mVDC. Shaft was still protected but that is a dramatic voltage drop. I rotated the shaft by hand and got no real improvement. The takeaway is that "shaft brushes" do not do nearly as good a job of distributing cathodic protection current as a shaft anode. The only exception is the very nicely engineered and manufactured brush rigging assembly from ElectroGuard.
I guess those silver slip rings do something! Stainless has some 40 times less conductivity than copper, so that small brush contact area appears to be the issue. And, being a new install, the brushes may not have yet worn in.
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Old 03-09-2018, 05:13 PM   #10
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OK, guess I am being picky. Could you explain why/how two wires or more for that matter can result in galvanic corrosion? OK, if the wires are made of different metals and submerged yeah, I guess a corrosion issue could evolve but only between the wires made of different metals, and only maybe depending on the galvanic voltages differences between the metals.

And the currents.... I doubt if one used #22 wire that there would be any significant voltage drop across that wire from current flow due to galvanic corrosion. The current is too small, milliamperes or less. And there would only be a current flow if there is a voltage difference between the start and end of that wire.

The idea of a common bonding point is solid for traceability but there is nothing, nada, ziltch wrong with bonding in as many places with as many wires as possible. Not that anybody would want to do so.
How it happens is no two wires are identical in their resistance. So you have current flowing in two wires that have different resistance, you will have differing amounts of voltage dropped across them, so if you have 2 zincs in the water, they should both be at the potential of about -1V but whichever one has the wire with slightly more resistance will be at a slightly higher potential than the other zinc. Whenever you have 2 metals at different potentials in conductive water there will be current flowing between them which will cause one to lose metal. So one zinc would lose metal to the other zinc.

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Old 03-09-2018, 05:14 PM   #11
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So, if I understand this correctly, if you have twins only one engine block should be connected to the bonding system (which is, in turn, grounded to the DC negative bus)?

That could work. Usually its easiest and best to just have one common ground point where all grounds come together.
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Old 03-09-2018, 06:29 PM   #12
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Per the applicable ABYC Standards:
> Wires in the bonding system are to be a minimum of AWG 8 which corresponds to 8mm, coincidentally.
> All engine blocks (propulsion and genset) are to be electrically connected together and to the vessel ground; aka, the B- buss, by a conductor as large as the B+ supplying the starters.
> The cathodic protection bonding system is to be connected at one location to the vessel ground.

Shaft brushes, IMO, are just another system that needs to be maintained and are totally unnecessary if the vessel has properly installed shaft anodes.

DiverDan: The hull potential readings on the port shaft were -940mVDC on the brush to wire solder joint and -938mVDC on the shaft. The shafts had been run about 10 hours so they were "seated".
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Old 03-09-2018, 06:55 PM   #13
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How it happens is no two wires are identical in their resistance. So you have current flowing in two wires that have different resistance, you will have differing amounts of voltage dropped across them, so if you have 2 zincs in the water, they should both be at the potential of about -1V but whichever one has the wire with slightly more resistance will be at a slightly higher potential than the other zinc. Whenever you have 2 metals at different potentials in conductive water there will be current flowing between them which will cause one to lose metal. So one zinc would lose metal to the other zinc.

Ken
NOt trying to sink or rock your boat but that just doesn't make any sense at all.

From your post:

"What the surveyor may have been saying is not to ADD a wire from the engine to the bonding system and that is correct. Only ONE connection between the bonding system and the main ground buss."

First of all, a properly bonded boat will have the bonding electrically tied to the grounding system which includes the engine. And by the way, the engine's electrical starting motor returns uses that common ground/bonding to get the starting current back to the battery.

So back to your premise that currents will flow if more than one wire connects the bonding to ground current will flow. No, there will be no voltage difference between the wires which are essentially in parallel. And the only way that there could be a voltage difference measured on a bonding bus would require current to flow on the bus, a lot of it.
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Old 03-09-2018, 07:13 PM   #14
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Per the applicable ABYC Standards:
> Wires in the bonding system are to be a minimum of AWG 8 which corresponds to 8mm, coincidentally.
> All engine blocks (propulsion and genset) are to be electrically connected together and to the vessel ground; aka, the B- buss, by a conductor as large as the B+ supplying the starters.
> The cathodic protection bonding system is to be connected at one location to the vessel ground.

Shaft brushes, IMO, are just another system that needs to be maintained and are totally unnecessary if the vessel has properly installed shaft anodes.

DiverDan: The hull potential readings on the port shaft were -940mVDC on the brush to wire solder joint and -938mVDC on the shaft. The shafts had been run about 10 hours so they were "seated".
If I had a $ for every time I was called Dan.

Anyway, it's great to see some real data on this board.
My story with shaft brushes is that I was running a boat with shafts turning at around 2500rpm at cruise, so I was loosing shaft zinc between haulings. Lots of water erosion. Anyway, i put on brushes to fix that issue. So, this last post indicates only 2mV drop across the brush to shaft interface?
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Old 03-09-2018, 07:19 PM   #15
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Per the applicable ABYC Standards:
> Wires in the bonding system are to be a minimum of AWG 8 which corresponds to 8mm, coincidentally.
> All engine blocks (propulsion and genset) are to be electrically connected together and to the vessel ground; aka, the B- buss, by a conductor as large as the B+ supplying the starters.
> The cathodic protection bonding system is to be connected at one location to the vessel ground.

Shaft brushes, IMO, are just another system that needs to be maintained and are totally unnecessary if the vessel has properly installed shaft anodes.

DiverDan: The hull potential readings on the port shaft were -940mVDC on the brush to wire solder joint and -938mVDC on the shaft. The shafts had been run about 10 hours so they were "seated".
Charlie--

I stand corrected pertaining to the bonding wire size! I was looking at the amount of current that such a bonding wire would carry and completely neglected ABYC's recommendation.

And while at it, ABYC also recommends:

Items to be Bonded Exposed, metallic non-current-carrying parts of the following items shall have provision for, and be connected to, the bonding system.

(1) Propulsion and auxiliary engines. It is recommended that this bonding conductor be connected to the engine negative terminal.
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Old 03-10-2018, 12:22 AM   #16
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(1) Propulsion and auxiliary engines. It is recommended that this bonding conductor be connected to the engine negative terminal.
Which extends to the buss bar connected to the negative terminal where we also land the conductor between engine blocks. I think we are in violent agreement ;-)

Diver"DAN"
Sorry!
Quote:
So, this last post indicates only 2mV drop across the brush to shaft interface?
Correct; the port shaft while stationary showed only 2mVDC voltage drop. The stbd shaft, though, showed 310mVDC voltage drop. Same assembly, installed at the same time with the shafts operated the same number of hours. Port shaft/propeller were at the top of the recommended protection range, stbd shaft/propeller were towards the bottom of the protection range, but still very much protected.

(Opinion) Shaft "brushes" are not as efficient, by a long shot, at providing cathodic protection to the shaft/propeller system as a properly installed shaft anode unless an assembly such as the very well engineered shaft brush system from ElectroGuard, or equivalent, is installed.
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