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Old 04-30-2011, 08:40 PM   #1
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Electrolysis puzzler

OK - so we've got a Wesmar bow thruster on Gray Hawk and its bonded with #8 wire more or less directly to 2: 6 x 12 zincs mid hull.* It also has a "B" prop nut zinc directly on the thruster prop.*

When we pulled Gray Hawk for the survey the prop zinc was pretty well gone.* The PO didn't change it at that time but I hired a diver shortly after we closed and he replaced that zinc.* At the same time he added two eggs to the prop shafts which already had an egg apiece on them.*

Fast forward three months and we are changing marinas so I hired a diver to have a look-see and he reported a consumed prop zinc on the bow thruster, which we replaced again.* The rest of the zincs showed what seemed a more "normal" rate of consumption.* The plates are eroding but not abnormally as are the shaft zincs.* The oldest shaft zinc "eggs" were pretty well gone so we pulled them and put on new ones but the ones that we added in Seattle were still in good shape.

So what's the problem?**I don't know if that rapid erosion on the Wesmar zinc is "normal"?* I don't have any baseline so I look to the collective wisdom of this group for your input.*

I thought that the rapid erosion must indicate a bad bond between the bow thruster and the plate zincs but tonight I checked that out with a VOM capable of .001 ohm measurements and it shows 0.000 ohms between the bow thruster case and the plate zincs.*

It seems to me I have three possibilities:

- this is normal erosion of the bow thruster zinc which means I need to be checking it more often than every three months

- I've had some localized hot spot (dunno if that is even possible) in which case it will have a different erosion rate going forward

- my diver in Seattle neglected to change this zinc, despite the fact that he charged me for it and presented a consumed zinc

What am I missing?
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Old 05-01-2011, 01:44 AM   #2
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RE: Electrolysis puzzler

I would suspect a hot spot.
My experience is this:
In one place in our marina, my zincs lasted poorly until the marina rewired those docks, then the time between changes quadrupled.
In a different spot, my neighbours change frequently, as does the rate of zinc consumption, so I attribute a relationship to the way those boats are wired.
Shaft Eggs fall off quickly, as the amount of material is insufficient around the bolts, so I use donuts for 2 to 3 x the time between changes.
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Old 05-01-2011, 09:21 AM   #3
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RE: Electrolysis puzzler

I didn't know whether I could have a hotspot that was localized to the bow thruster.* I agree on the eggs - they didn't look very substantial so I am in the process of changing over to the donuts.* Right now I've got an egg and a donut on each shaft - eventually the eggs will be gone.* And I'm getting mama dive certified so we can keep an eye on the bow thruster.
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Old 05-01-2011, 06:49 PM   #4
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RE: Electrolysis puzzler

I know a hot spot can happen.

Hauled my old wood boat a number of years ago only to immediately see that the port side of the rudder was pinkish, and the shaft & propeller nut zincs were gone.* After getting in for a good close look, I reached up to grab one of the propellor blades, and about 3/8" of the tip of the blade crumbled off in my hand.

Anyway, to make this fairly short, my boat had been very stable*for many years thanks to a Electro-Guard cathodic protection system.* It turned out the my slip neighbor on the port side had a major wiring problem and put a lot of voltage into the water.* This happened over about 3-1/2 weeks.* We were glad that no one fell in the water.

Of course, one way you could test would be to end-for-end the boat, and hopefully it could tell you over a period of time if the zinc reaction has changed.* Unfortunately, these tests are*frequently inconclusive as the condition (hot spot) may no longer*exist.* In addition, chasing galvanic corrosion issuse seem to be*part science and part luck.

Is there someone in your marina who has experience chasing this issue?* At Edmonds, the port actually has a guy who has provided education, tested boats, and advised both boaters and the marina management.

Current boat goes 12-15 months with a prop nut zinc, shaft zinc, and 1/2" x 6" x 12" transom zinc.* Boats out on the end of our dock have to replace more often.
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Old 05-01-2011, 08:04 PM   #5
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RE: Electrolysis puzzler

Quote:
Jay N wrote:
Of course, one way you could test would be to end-for-end the boat, and hopefully it could tell you over a period of time if the zinc reaction has changed.* Unfortunately, these tests are*frequently inconclusive as the condition (hot spot) may no longer*exist.* In addition, chasing galvanic corrosion issuse seem to be*part science and part luck.

Is there someone in your marina who has experience chasing this issue?* At Edmonds, the port actually has a guy who has provided education, tested boats, and advised both boaters and the marina management.

Current boat goes 12-15 months with a prop nut zinc, shaft zinc, and 1/2" x 6" x 12" transom zinc.* Boats out on the end of our dock have to replace more often.
*The rapid erosion on the bow thruster zinc was in the previous marina so it's a whole new learning experience here.* So far I've met a long list of experts here but none who are likely to have an useful opinion on electrolysis (or very many other subjects for that matter) but I'll keep looking.
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Old 05-01-2011, 08:19 PM   #6
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RE: Electrolysis puzzler

Bob

What marinas?
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Old 05-02-2011, 11:08 AM   #7
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RE: Electrolysis puzzler

Elliott Bay (Seattle) briefly then roughly 3 months in Van Isle (Sidney, BC)

Now in Cowichan Bay.* If I was judging by the looks of the docks and the boats around me then Cow Bay would be of the most concern.* Time will tell.

*
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Old 05-03-2011, 08:07 AM   #8
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Electrolysis puzzler

My Side Power thruster zincs go pretty fast too. I wish that I knew why. The thruster is in the bonding system with a substantial wire.

In the last marina we were in they seemed to go faster than they do now with the boat behind the house.


-- Edited by Doc on Tuesday 3rd of May 2011 08:07:51 AM
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Old 05-03-2011, 09:54 AM   #9
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RE: Electrolysis puzzler

What are you missing?
*
Most bow thruster zincs are small and may be going fast because the way they are wired into the zinc protected loop and*protecting more than just the bow thruster.* Electric current will travel the least path of resistance, so if the bow thruster zincs has the least resistance that is where it going to go.* Also most bow thrusters have differnt metals in them ranging from aluminum, 7, to copper, 28.* *
*
When we moved to Everett on the salt*I*re balance the zinc load.* Instead of daisy chain loop, I ran separate wires to the main zincs.* The life of the bow thruster zincs last twice as long.* I also have a zinc anode connect to the bow thruster, and one to the engine room which has also prolonged the zincs and for extra protection.
*
The cold winter months is when stray current is the highest as most high amp items like heater, water heaters, and battery chargers produce stray electricity, and of course the more items on/used the higher the level. Battery chargers are know for stray electricity.* Our old battery charger is the highest source and 50% of stray electricity on the boat.
*
Most marinas have an acceptable norm of stray electricity, so you might want to ask the marina what is acceptable.* Large marinas if you ask them will check your dock, and surrounding boats which of course includes your boat also.* Tracing down stray electricity can be time consuming and expensive.* Two years ago our boat was tagged as the Everett marina does go around and check.*
*
So do some asking around and checking.* *
***
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Old 05-06-2011, 07:46 PM   #10
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Electrolysis puzzler

Time will tell ...

I just finished installing a home run of #8 AWG from the bow thruster to my diver's dream plates mid hull.* Taking a wine and almond break before I clean up the mess I left behind.

As far as getting our landlord to check for stray current I doubt he knows what it is much less how to check for it.


-- Edited by bobofthenorth on Friday 6th of May 2011 07:48:51 PM
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