Stray electricity in the marina ??????????

[woodster]

yesterday i helped a dock neighbor take his boat across the bay to a haul out for a bottom job ....we pressure washed the hull and scraped the barnacles off the hull and prop and strut ......after looking close the strut was in good condition ...still looking a strong bronze color ...the prop was pitted and peppered with copper color and no where on the prop could be found bronze color and when hit with a hammer it gave back a very dull thud sound .....so the prop is most likely toast....oh and the zincs were gone from the shaft .......
my question is how do i test the water in the marina for stray electrical current with my volt ohms meter.....or do i need a special device to do this ......

 

[Don S/V ILLusion]

To have any meaningful results, you need a Silver/silver chloride reference electrode.
Your VOM meter is a useful tool on a boat but not for this.

Note that the vast majority of boat related electrolysis, corrosion, erosion issues are caused by a bad, faulty or multiple grounds ON the boat, not near it.

 

[Charles Erwin]

You do not need a silver half cell to find out what you need to know. Instead, solder a pencil zinc on the end of some #14 wire. See the diagram attached. Very reliable and very cheap.

As for the condition of the propeller it is impossible to say what happened because we cannot see it. However, the absence of a sacrificial zinc is a very bad sign.

The majority of sailboat propellers are made from manganese bronze - which is not really bronze but more brass. This material It has very high zinc content so if there is no zinc protecting the shaft/propeller assembly the zinc in the propeller metal recipe gives up instead. The short of it is that if you do not keep up the zincs - the zinc in the manganese bronze propeller wastes away protecting the stainless shaft. This process is what gives the pink color associated with
de-zincification. The strut is probably in better shape because it is probably not connected (wired) to the motor/shaft and is likely made from an alloy that is more corrosion resistant than the propeller.

You can get bronze propellers made from material with relatively low zinc content too but they are very expensive. Overall - if you keep the zincs fresh - manganese bronze propellers work just fine.

Charles

ps After you get a measured number suggest your friend install a galvanic isolator.

 

[JamesG161]

To have any meaningful results, you need a Silver/silver chloride reference electrode.

I have one of these that might be on board my boat, if we sail your way.
The reason Ag/AgCl2 probes are used is they are "standardized" and "referenced".

But...

how do i test the water in the marina for stray electrical current with my volt ohms meter.....or do i need a special device to do this ......

The answer to both is YES.

I plan on making one for our Marina after a session of new boat problem strays. last year.

Parts list...
1) DVM with range of millivolts DC
2) 6 foot wood 1x1 as a "spreader"
3) a piece of copper and a zinc.
4) a small boat , low to the water as the "Dect-O-Craft" to putt around the marina waters.
5) Sandpaper

This kinda like a radio signal direction finder(RDF). Now called Stray Current Direction Finder (SCDF).

You hang the copper(-) in water at one end of stick and the zinc (+) in water on each end of spreader. Watch your DVM as you patrol the marina. Drive slowly by each boat and put the boat in the "sights" (middle of your spreader stick). If there is a SC, your DVM (SCDF) will react to electric interference between your submerged metal probes.
BTW the greater the difference on the Noble chart metals the more sensitive your detector. i.e., Magnesium for Zinc and Brass for copper ( a strand of silver solder) .
Jim...

PS: The sandpaper is to polish your probes for better sensitive.

 

[JamesG161]

@Charles Erwin nice attachment. @woodster just attach the copper probe to where the "Bond" thingy is on that pdf.
Jim...

 

[SFS]

James - what would typical voltage readings be for a finding of no stray current? What level of voltage would confirm the presence of a stray current?

Edit: just looked at the voltage ranges in Charles's diagram, looks like that is my answer.

 

[JamesG161]

James - what would typical voltage readings be for a finding of no stray current? What level of voltage would confirm the presence of a stray current?

Hard to be very specific for you.
Water temperature, salinity, distance between probes, cleanness of probes, etc.
But you can do the same as if it were on your boat. If you use the Ag/AgCl probe for the "copper" probe, then you can use a the standard Noble series chart voltages.

What @woodster asked for was for a total "Marina" scan and not just His boat.

I bought this, best "peace of mind" tool I bought.

http://www.boatzincs.com/corrosion-reference-electrode.html $130

I do a complete survey of MY boat 2 times a year and when a new boat berths near mine.
Jim...

 

[TFrere]

Great information James. Thanks

 

[JamesG161]

James - what would typical voltage readings be for a finding of no stray current?

Wow you made my brain hurt, you owe me one dry martini!

In the Tampa Bay waters Spring to Fall water temps, using a clean zinc and plumbing pipe copper, you should "calibrate" your SCDF in an area of known ZERO stray currents. It should be approximately -600 millivolts (+600 if you cross the probes on the DVM).
If you get say a 20% change, during your slow speed survey, BINGO! Now hone in on the source.

Now here is the big question. What can YOU do about it? Point out your findings and perhaps the marina can hire a marine electrician to prove it.

BTW the pencil zinc alone, CANNOT DETECT, a stray current.
You have to do a series of tests for just YOUR boat. They show a "Bond system" check on @Charles Erwin pdf. My boat has no BOND zinc.

Clear as mud!
Jim...

 

[woodster]

thanks guys for the info i will give all this some thought and see where it goes

 

[Charles Erwin]

Maybe the term 'stray current' needs to be defined. For those of us who do this kind of thing regularly the term 'stray current' is universally used to describe hull potential measurements above about 1.1 vdc - which means what is going on is not galvanic activity. If the hull potential is below this value then zinc(s) and a galvanic isolator will protect your underwater metals both from your own (normal) galvanic activity and - if your neighbor is not well zinced - then the islolator will protect from his boat using up your zincs.

A zinc reference measures the difference in potential between the thing tested and the pencil zinc - exactly the sames as a Silver/Silver chloride half cell. Place the probe in contact with shaft and the VOM measures the potential of the shaft/propeller assembly relative to zinc. The point is any reference probe detects 'stray current' as long as you define stray current to mean measurements above possible normal galvanic potentials.

It is a fairly simple matter to understand that anything over 1.1 vdc is stray current - that is a potential produced by something other than normal, plain vanilla galvanic activity.

Charles

Jim - there is no 'bond zinc' shown in the diagram I sent Woodster and the diagram shows a reliable test for any underwater metal - not just the bonded ones.

 

[JamesG161]

One thing I love about this forum and this post, in particular is it makes me think.
I went to my boat today to spring service my diesel engine and this post reminded me, it was time to do an electrical survey with my Ag/AgCl probe.

My double zinc shaft anodes installed in late Oct (verified -980mV and photos) are now Gone!
I KNOW that my boat is NOT the problem by doing the series of tests that takes about 1.5 hours and compared against last year. I immediately put a drop zinc in the water and the -420mV today immediately jumped to -720mV and was slowly climbing to the normal -980mV zinc protection, which is now provide by my 8 lbs drop zinc.

I will change my terminology @Charles Erwin to mean a increase in galvanic activity not ascribed to MY boat and from a source I don't control. I will call it a Wild current, using a dog on a "leash" analogy.

My suggestions were for a marina patrol device to Detect Wild currents. Using a pencil zinc is a great idea for one of the probes on my suggested Wild Current Direction Finder (WCDF).

I am attaching a Sample Ag/AgCl survey to show an example of the series of tests required to do a proper survey.
@woodster I reported the Wild current to harbor master and I very motivated to construct my WCDF (patient pending). I will let you know of the pitfalls. I am going to WestMarine Wed. to buy a pencil zinc.

Shooing off all the Wild dogs in my yard too...
Jim...

 

[Crazy Dave Condon]

Just a suggestion. Look at the other boats to see if the shore power chords in the water are worn out which could be the issue and I certainly would not want to be in the water with stray electrical currents. Also check the shore power as well to see if wired correctly with no issues.

 

[Philbert]

Woody, if you haven't purchased a new prop yet, perhaps an original two blade from my Bene 323 would work for you? You can email me at parcco@aol.com
Regards,
Phil

 

[woodster]

will do