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  #1  
Old 10-11-2008
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Question Bonding/galvanic corrosion

I own a Marshall 18 catboat with a Yanmar 1GM Diesel engine and a standard 12-V. electrical system. The boat is kept in Wellfleet Harbor (Mass.), lovely but very "electrically active." In August, galvanic corrosion caused the bottom pintle and gudgeon to disintegrate; I replaced them and added a zinc plate bolted onto the gudgeon. Someone from a local boatyard told me I should also attach a bonding wire from the cooling water through-hull to the (grounded to battery negative) engine block to protect the through-hull, which at the time seemed to be in good shape. The alternator and starter do not have isolated negatives, but are "negatived" to the battery through the engine block. Since then, I read a recent column by Chuck Husick stating that the through hull should NOT be bonded. Did I do the right thing? The wire has been in place for about 2 months now. I'd sure hate to lose the through-hull in Cape Cod Bay.....
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Old 10-12-2008
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Nobody's answering in case they get it wrong and you sue!
To be safe, get an experienced MARINE electrician to check for stray electrical currents, it is the only way to be positive (!) about the state of things.
For starters though-
do you have a proper hull anode bolted through the hull into the bilge? This bolt should be connected with heavy cable direct the the engine, and direct to the through hull skin fitting- but- there is a case if the pipe to the fitting is plastic or rubber for not connecting the fitting at all. It all depends!
You may also need to connect the anode to the propshaft via carbon brushes.
The yanmar engine if salt water cooled will also have one or two anodes within the engine block which need to be checked regularly and renewed if necessary, on some boats as often as every month if there are problems...
Try connecting a multimeter on DC volts between the engine block and the hull fitting- if there is a voltage, there is a problem.
I will not attempt a detailed diagnosis at long range, there are too many variables to guess it, and the consequences too dangerous if its not right.

It can be a bit of a black art diagnosing electrolysis, but doing nothing is not an option in your case, it can get very expensive if its wrong!
Get expert advice locally, soon.
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Old 10-12-2008
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Galvanic issues

Dear Northup: Thanks for the reply! As an experienced refrigeration tech, I have good digital meters-that was an excellent suggestion to check for potential between the fitting and an anode suspended in the water. Soon as I get back to Wellfleet, I'll do so. Regarding a bolt through the bottom, there is already a zinc anode on the propeller shaft nut; since the bonding wire is connected to the engine, will it communicate with the anode or will the gearbox interrupt the current flow (if any)? I guess the first step is to determine, as you suggest, whether there is a problem by checking for voltage. This all started when a guy from a local boatyard, who saw me replacing the gudgeon, suggested I bond the through-hull. Actually, the through-hull displayed no overt sign of deterioration, so maybe I should have left well enough alone...
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Old 10-12-2008
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Dear NorthUp: I forgot to mention-my Yanmar engine (1GM) has a single anode; I change it every spring and usually find modest deterioration.
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Old 10-12-2008
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Seawater is seawater. There is no such thing as some seawater being more "electrically active" than other seawater.

If you connect your thru-hull to the boat's electrical system that has zinc protection, the thru-hull should be protected too. However the connections must be really good because you are dealing with very low voltage here.

If you don't connect the fitting to the boat's electrical system it can't be damaged by stray currents. But, it can still be corroded from seawater.

Stray currents from other boats arrive on your boat via the shore power connection cord. Specifically the ground wire. A galvanic isolator can protect you from this problem. However a wiring problem on your own boat can also create stray currents. You will probably notice this because your zinc will fail quickly if you have this problem.
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Steve :

Salinity and temperature matter a lot with this argument. Warm, salty seawater will tend to be more aggressive than the cooler, brackish stuff. Current speed matters too. Salt water that is moving tends to need more defences. I believe it tends to sweep away any defending electrical field.... "depolarising" I think they call it.

Oil rigs have to fight these battles all the time. Saline, fast-moving seawater often demands enormous power imput from the "impressed current" cathodic protection circuits on an oil rig. Less saline, slow-moving seawater is more manageable.

I am blessed right now to be in fresh water. Corrosion virtually stops. In the canal here, there are old boats over 100 years old, and made from rivited steel. All look ok, even now. Fresh water tends to freeze though, as the engine found out once.

Last edited by Rockter; 10-12-2008 at 10:47 PM.
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Quote:
Originally Posted by Rockter View Post
Salinity and temperature matter a lot with this argument. Warm, salty seawater will tend to be more aggressive than the cooler, brackish stuff. Current speed matters too. Salt water that is moving tends to need more defences. I believe it tends to sweep away any defending electrical field.... "depolarising" I think they call it.
Correct. Well said, Rockter

This is why, in any typical mooring field, some boats can have serious corrosion issues and some boats have none at all!..

Quote:
Originally Posted by Rockter View Post
Oil rigs have to fight these battles all the time. Saline, fast-moving seawater often demands enormous power imput from the "impressed current" cathodic protection circuits on an oil rig. Less saline, slow-moving seawater is more manageable.
I've often wondered if anyone is using an impressed current CP system on their boat??

You'd need a pretty big (presumably steel) boat to justify the expense and the systems I've seen tend to chew the juice a bit, so perhaps not..

Cameron
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Old 10-13-2008
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Hartley :
I don't think they would let you go for a powerful system as it is likely to feast on the other boats nearby. Some pour soul's boat is likely to get fizzed.
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Old 10-13-2008
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My point was that the concept of stray current lurking around in water is just plain wrong. The stray current damage is created either in your boats electrical system, or on another boat. If it's another boat the problem gets to your boat via the shore cord.

You boat is floating in water. It all at the same electric potential. It's like a bird on a wire. The exception is the electrical system on your boat or another one, or dissimilar metals that cause the problem. It's not that some marina's water more electrically active than another one (with similar salinity).

While there may be some conductive difference in brackish water verses ocean water, all ocean water is going to be pretty close in conductivity. Moreover, the point is if you are going to protect your boat from stray current or galvanic corrosion your approach is going to be the same regardless if the water is slightly more or slightly less salty. Here's the omst important part - the methods for protection are the same (for ocean water and brackish water) until you get to freshwater.
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Old 10-13-2008
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Even the purest of metals will set up little anodic and cathodic sites within the surface of the material. Steel would tarnish and given time, would pit.
Very quickly, the ion exchange will set up an electrical field that resists future ion exchange, and slows the reaction. In steps saline water, and tends to conduct away the electrical field. The effect is worse if the water is very saline, and/or flowing. This is one of the reasons why salted roads beat hell out of chrome. I believe it's known as "depolarisation".

This electrical field is typically a few millivolts, and so the reaction is not normally very fast. Indeed, it can be slowed radically with the use of a sacrificial anode, that counter-currents the system, tending to hold your expensive bits cathodic (-) with respect to the zinc (+). The zinc does the pitting instead.

Leaking currents are often very damaging as they will arrive with a much more aggressive electrical field that simply swamps your anodes, and they are defeated. Sometimes the voltages measured get beyond single volts.

Saline water encourages the effect as it is far more conductive. The best situation is my beloved fresh water, that makes the propagation of the electrical field much more difficult. In ultra-pure form, the electrical field cannot get from one ship to another. Even the ion exchange within a ship is greatly reduced. The near-absence of chlorine is welcome too. Stainless tends to live longer. Bronzes are too noble to notice anything in fresh water, really.

It freezes though. Yes, it freezes.

Last edited by Rockter; 10-13-2008 at 09:50 AM.
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