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#457132 - 04/05/09 09:43 PM Electrolysis/Measuring stray current at dock?? ****  
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power and sail Offline
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Have a boat across from me at dock who lowered a 25lbs load of zincs or other sacrificial metal off his transom to protect his sterndrive. It is attached with a wire to the engine block.
We have not had any problems with electrolysis or anode wear or pitting etc so far.
I was going to measure the situation for piece of mind and started wondering how? Between shore power ground and boat metal (bonded)? or where should I look for it?

I have a galvanic isolator on the boat so it may be that I am secure already with that but if this boat now some 50ft from mine has a load of sacrificers, would those help me too?


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enjoying both

"Aquadesiac"
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#457183 - 04/06/09 01:11 PM Re: Electrolysis/Measuring stray current at dock?? [Re: power and sail]  
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25 lbs seems like a lot of zinc. the presence of electrolysis is measured by the sudden disappearance of body hair on the fairer sex.


Bruce Toran
Former Owner of a 1996 Carver 320 Voyager
-----------------------------------------
"Don't Eat Anything Bigger Than Your Head"
#457187 - 04/06/09 01:38 PM Re: Electrolysis/Measuring stray current at dock?? [Re: BToran]  
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BoatingABC.com
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#457247 - 04/06/09 10:46 PM Re: Electrolysis/Measuring stray current at dock?? [Re: Admin]  
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power and sail Offline
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HaHa, how do you get close enough to measure it??
So no guidance to where to dip the probe for measurement?


power and sail
enjoying both

"Aquadesiac"
#457266 - 04/07/09 02:43 AM Re: Electrolysis/Measuring stray current at dock?? [Re: power and sail]  
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In theory, there is no difference between theory and practice. In practice, there is.

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07 SeaDoo Wake 215, 1996 SeaDoo GTX, 05 HD Dyna SuperGlide
#457271 - 04/07/09 04:06 AM Re: Electrolysis/Measuring stray current at dock?? [Re: Titanium]  
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Al Offline
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Al  Offline
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Vagabond Wanderer from Mich.
Galvanic current typically flows along the ground path to the pedestal ground. A galvanic isolator is placed along the ground path to interrupt the current. This means you would have to disconnect the ground to make the measurement.

If you could do it safely, you might be able to make a test cable that plugs in between the shore power and pedestal, and only provides the ground leads to test points where you could insert an ammeter.

Normal galvanic current usually has less than 1.5V of potential, and in the milliamp range, so the typical multimeter should measure it.

To rule out any serious wiring issues, I'd not connect the AC hot or neutral wires in the test cable, so that your boat remains powered down.

Still, this might not be totally risk-free as the ground connection itself is capable of conducting much higher currents, and I cannot anticipate every bad wiring scenario that might exist on the boat that could cause a high current to flow through the connection.

My last thought is that I can think of two reasons for corrosion to exist on the boat. First, it could be normal galvanic corrosion that is the result in dissimilar metals in contact with each other immersed in water. This can be thought of passive corrosion. A galvanic isolator can be effective in eliminating this corrosion.

The second, and more serious condition can occur if there are a combination of problems on the boat; say bilge water is present, and somehow a battery positive cable (with exposed wiring - not insulation) is in contact with the bilge water, this could result in an active corrosion problem, as it is being supplied by the battery, with a much higher current and voltage than normal galvanic corrosion. This could "melt" metals within days or hours.

A galvanic isolator will not fix this problem as it is only designed to work at voltages below 1.5VDC, which is where you will find normal galvanic corrosion. If the corrosion is being fueled by the battery, its not unlike an industrial plating process, but you are trying to plate the marina with your outdrive, or whatever is melting.

It might therefore be that your dockmate is hanging zincs like crazy because he has an active corrosion issue, caused by faulty wiring. If this is the case, a boat-load of zincs won't solve the problem.

The symptioms of the two types of corrosion are the same, in that less noble metals are corroded. However, galvanic corrosion could take a long time to damage metal, whereas active corrosion could only require a few hours or days to do damage.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#457390 - 04/07/09 10:49 PM Re: Electrolysis/Measuring stray current at dock?? [Re: Al]  
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power and sail Offline
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So if I understand you right you say that the potential between boat ground and shorepower (outlet) ground could be 1.5V at a low current level.
So I could use the converter I have from dockside outlet to normal house 3 pronge female and stick probe in ground and other one on boatside ground (inside of isolator).
I assume shorepower ground is same as metal dock structure.

The difference between active and galvanic is clear, we are not a galvanizing/anodizing plant:) I hope.

h


power and sail
enjoying both

"Aquadesiac"
#457396 - 04/08/09 04:32 AM Re: Electrolysis/Measuring stray current at dock?? [Re: power and sail]  
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Al Offline
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Vagabond Wanderer from Mich.
Galvanic current is normally a low current level. The voltage associated with it is typically more than 0.7VDC and less than 1.5VDC.

A galvanic isolator is essentially two diodes in series (in each direction), placed in the ground line. The diodes each require about 0.7VDC to conduct; in other words, voltages less than 0.7VDC won't pass through the diodes. Since two of them are in series, any voltages lower than 0.7x2, or approx 1.5V won't pass.

That is how the galvanic isolator works, it blocks all voltages lower than 1.5VDC, which is where voltage from galvanic action is at, but allows higher voltages/current to pass, which essentially is any catastrophic voltage/current from a ground fault or other malady.

What makes this work is that the diodes are rated for a higher current than the 30Amp service, so even if a dead-short were placed on the ground line, the breaker would blow rather than the diodes. I think the ABYC specifies 30% or more, so the diodes should conduct about 50Amps or there about before being fried.

Measuring power at the ground point on the dockside connector is only going to tell you part of the story, but at least its the "good" part.

If you read voltage at the dockside, you know that your boat is generating galvanic current, and your galvanic isolator is bad, so some attention is warranted.

But if you do not read any voltage, either your galvanic isolator is good, or you are not generating any galvanic activity on the boat, or some combination of the two. The only issue here is that your galvanic isolator could be bad, but your boat is not generating any galvanic current, but I doubt that will be the case.

Here is why.

When a boat is isolated from shorepower, the galvanic action it produces is minimized (hopefully). While there may be some galvanic action - it is typically handled by the on-board zincs. Rule of thumb - the zincs are normally only supposed to handle the boat's own natural galvanic activity.

However, when attaching to shorepower, it is unsafe to have an ungrounded boat, as if there is a bad connection on board, AC can find its way into the water, which is dangerous in its own rite, or a potential difference can exist between the boat and dock, and you could get shocked when grabbing onto a railing while stepping off the dock, etc.

So to prevent this, the boat's ground is grounded to the shorepower ground.

But then this essentially results in a metal-to-metal contact through the shorepower ground, to every piece of grounded metal in the shorepower system, including other boats, to all of the grounded metal in your boat. This results in the entire marina, along with the water, becoming a huge battery.

As your boat is grounded to this system, it does its part by supplying current to the battery via galvanic interaction with all that grounded metal. Of course, the boat with the most noble metal wins.

How much galvanic activity occurs depends on several things, the composition of all of the metals on the various boats, on the shorepower grid, whether other boats have galvanic isolators, and so on.

To prevent the bad effects of grounding your boat to the shorepower grid, a galvanic isolator blocks all galvanic current, but yet still allows the safe discharge of current as I described above.

Here is a scenario. Say you have a boat with I/Os, and an inboard (power or sailboat) comes into the slip next to you. Since the inboard boat typically has higher noble metals exposed to its galvanic system; stainless steel shafts and struts, often there is less galvanic activity on an inboard - either power or sail - than a boat with aluminum outdrives.

And the closer two metals are to each other, the less active the corrosion is - so a boat that has a lot of stainless underwater and nothing else won't be as galvanically active as an outdrive, say with a stainless prop and aluminum outdrive.

For that reason, inboard boat owners sometimes neglect to maintain their zincs.

But say your I/O boat doesn't have a galvanic isolator (or its bad), and that zinc-neglected inboard (also missing a galvanic isolator, or maybe its bad as well), parks in the slip next to you, and you both hook up to shorepower.

Now we have trouble.

The I/O's aluminum outdrive is connected by the boat's ground, through the shorepower system to the inboard's stainless shafts and other underwater fittings.

Electrically, its as if all of these components were in contact with each other.

Since the inboard has a huge amount of stainless, and the I/O has a less-noble aluminum outdrive, a severe and significant attack on the aluminum outdrive will occur as it is dissipating the galvanic current generated by the inboard stainless-to-I/O aluminum circuit.

So the I/O's aluminum housing in essence is sacrificing itself to protect the inboard's stainless steel.

Since the ground connection on the shorepower cable is not interrupted by the breaker, it matters not if the shorepower is active or not, it only requires the cable to be connected.

But, if either boat had a good galvanic isolator in the ground side, there would be no metal-to-metal contact, and no interaction (well, at least through the shorepower grid). But if one boat had a bad galvanic isolator, it still could sacrifice its metals to protect any metals on the dock connected to the ground grid.

Now, to this point I have focused on shorepower-connected systems as they can present the most significant source of galvanic current. However, conditions can exist that merely close presence to another boat can cause some galvanic activity.

Remember that there are two requirements for galvanic current; 1. metal to metal contact (or electrical contact) between two dissimilar metals; 2. the metals in contact with an electrolyte. The elecrolyte is conductive and provides a return path for the "circuit".

One little known fact is that water itself is an insulator, not a conductor of electricity. Pure water that is. But lake or ocean water have varying degrees of conductivity due to the materials in suspension. Saltwater has a higher conductivity than fresh water for instance, due to the salinity of the water. Too, polluted water can have various degrees of conductivity, depending on what is in the water.

That is why salt water is usually responsible for more corrosion damage than fresh water - its more conductive.

So then, if you are parked in a marina with other nearby boats, even though you are not connected to shorepower, you could setup a galvanic condition, especially in salt water, as the salt water could be conductive enough to act as the metal-to-metal electrical circuit, and the electrolyte return path.

Remember, the whole marina is in a sense, one large highly complex battery. For this reason, strange stuff can happen. In fact, batteries are sometimes referred to as "galvanic cells".

The guy with the zinc fish may either have a galvanic issue, or he is a bit over-cautious.

This is all theory, but it makes for an interesting story.

An excellent book on corrosion that I highly recommend is "The Boatowners Guide to Corrosion" by Everett Collier. Some of it is above my head, but understand enough of it that it makes some sense to me. I highly recommend this book for your library.

Forgive me if I made any mistakes in this discussion; its 4AM, looks like it is snowing outside (in April!), and I'm depressed.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#457553 - 04/09/09 05:35 PM Re: Electrolysis/Measuring stray current at dock?? [Re: Al]  
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GoFirstClass Offline
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GoFirstClass  Offline
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Ahhhhh, another graduate of the Vista Al School of Brevity! laugh laugh laugh

Just kidding Al. Nice explanation. Even I could follow along with what you were saying.

Now, where did I put my copy of "Electrolysis for Dummies!"?


"Beachcomber" 1995 Sea Ray 550 Sedan Bridge


Anchor's down......Bottoms Up!
#457573 - 04/09/09 09:50 PM Re: Electrolysis/Measuring stray current at dock?? [Re: GoFirstClass]  
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power and sail Offline
Admiral
power and sail  Offline
Admiral

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Great, Don't worry about snow in snow country, we had some here a couple of days ago. Today its again 70+.

Allright I will find out what is going on with the new neighbor and so far all good as we have not had any corrosion problems.

Interesting enough it does not take much salt in the water to change the situtaion. I was brought up in brackish water and we had a lot of wear on the sacrificial anodes, here in fresh water virtually none. Maybe I should invest in more sacrificing metals but so far so good.

thanks Al as usual a comprehensive wikipedia worthy write up.

h


power and sail
enjoying both

"Aquadesiac"
#457628 - 04/10/09 02:50 PM Re: Electrolysis/Measuring stray current at dock?? [Re: power and sail]  
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Al Offline
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Al  Offline
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Grand Poobah

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Vagabond Wanderer from Mich.
GFC; better hope I am not bored in retirement, or I'll liable to get a bit "wordy"... grin

d

Yipee, only three more Fridays at work...

(but I still have 11 more Mondays)

I am taking Thursdays and Fridays off beginning the first week of May, until the end of June when I retire.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#457810 - 04/12/09 10:04 PM Re: Electrolysis/Measuring stray current at dock?? [Re: Al]  
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power and sail Offline
Admiral
power and sail  Offline
Admiral

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SE
Well, you are getting close. My retirement got moved some 10 years recently:)
NOw I just have to learn to take out my vacations so I can play with my toys..


power and sail
enjoying both

"Aquadesiac"
#457918 - 04/14/09 01:36 AM Re: Electrolysis/Measuring stray current at dock?? [Re: power and sail]  
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SNUPY Offline
Admiral
SNUPY  Offline
Admiral

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Northern Ca
A long these same lines...has anybody hear of the Volvo Penta "Active Corrosion Protection System" I had my boat in the water (Ca delta) the first six months no problems, pulled, cleaned, waxed, put back in water. Another six months pulled,etc. ended up with several blisters on out drive. VP dealer recommended the Active Corrosion Protection System

Any thoughts??
Thank you


George
2008 Reinell 220 LSE
Slow like a Pro... Not fast like an A$$



#457923 - 04/14/09 05:49 AM Re: Electrolysis/Measuring stray current at dock?? [Re: SNUPY]  
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Al Offline
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Al  Offline
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Grand Poobah

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Vagabond Wanderer from Mich.
Corrosion protection systems such as Volvo's and MerCruiser's MercAnode attempt to neutralize the current flow of galvanic activity with an equal and opposite charge.

Recall I mentioned that galvanic activity is somewhat akin to a battery? You can then think of an active corrosion protection system as a second battery setup such that it is equal and opposite to the galvanic action, resulting in no current flow.

No current flow, no corrosive damage.

The active corrosion systems then require the ability to sense the current flow so that it can meter the precise opposite.

While I don't have one, I have not heard from anyone that didn't think it worked well enough, but since it is an active, rather than passive system as anodes are, it is prone to failure.

Of course, sacrificial anodes must be maintained as well - especially zinc, as over time, it can develop an (electrically) insulating film over its surface which means you have to clean them occasionally.

In a sense, the active corrosion systems work to prevent galvanic activity, while anodes divert galvanic activity to a sacrificial piece of metal.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#457939 - 04/14/09 09:23 AM Re: Electrolysis/Measuring stray current at dock?? [Re: Al]  
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Rocnat4 Offline
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The first year I had my boat in a slip, I had noticeable surface pitting on my outdrive even though my anodes were in excellent condition.

The second season, I installed a galvanic isolator and changed my anodes to magnesium. This seems to have solved the problem.

Last edited by Rocnat4; 04/14/09 09:23 AM.
#457946 - 04/14/09 10:50 AM Re: Electrolysis/Measuring stray current at dock?? [Re: Rocnat4]  
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SNUPY Offline
Admiral
SNUPY  Offline
Admiral

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Northern Ca
Thanks Al

One more stupid ?. On the V P web site they say nothing about fresh water, just salt water. I would think it would make no difference, but I am new to all this .


George
2008 Reinell 220 LSE
Slow like a Pro... Not fast like an A$$



#458012 - 04/14/09 10:06 PM Re: Electrolysis/Measuring stray current at dock?? [Re: SNUPY]  
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power and sail Offline
Admiral
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Admiral

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Salt waater is more aggressive but the system should work the same way in both.


power and sail
enjoying both

"Aquadesiac"
#458019 - 04/14/09 11:24 PM Re: Electrolysis/Measuring stray current at dock?? [Re: Rocnat4]  
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SNUPY Offline
Admiral
SNUPY  Offline
Admiral

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Northern Ca
Thanks

I think I should have searched some of the other forums!!

I was looking in the Engines, Drives & Props forum and there was a good thread about the VP ACP system.


George
2008 Reinell 220 LSE
Slow like a Pro... Not fast like an A$$



#458030 - 04/15/09 05:29 AM Re: Electrolysis/Measuring stray current at dock?? [Re: SNUPY]  
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Al Offline
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Al  Offline
Nautical Alchemist
Grand Poobah

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Vagabond Wanderer from Mich.
One side issue, if you are in polluted fresh water (the Cuyahoga River comes to mind), it can be like salt water.

It all depends on the "conductivity" of the water, which is a result of what is in the water.

Some pollutants can cause fresh water to be as conductive as salt water.

So the fresh water guys don't totally get a Mulligan on this.


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180

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