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Old 11-07-2018, 05:15 PM   #1
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Zincs - can you have too many?

This may have already been covered but....

When I hauled out the only zincs were drivers dreams on the stern which re supposed to be bonded to everything. That is to be determined by a bonding check before we splash.

There were no zincs on the shafts and the remains of one zinc on one trim tab and the hole and circle of missing paint where there was one (at one time) on the other trim tab.

So I have replaced the trim tab zincs and the two transom plates. I am also adding zincs to the shafts and to the swim platform uprights where they are in the water.

My assumption is that the trim tabs were not part of the bonding system and probably no easy way to fix that.

Now can you theoretically have too many zincs? What is the downside of having more zincs? Is there a downside? How do you figure out how many zincs to use?
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Old 11-07-2018, 05:56 PM   #2
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Over protection can be as bad as under protection. Best is to measure voltage of each underwater fitting against a reference anode (silver chloride) and then add/remove zinc area as needed.
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Old 11-07-2018, 06:10 PM   #3
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Boatzincs.com sells the silver half cell to check your boat. About $120. You can check your boat and also your friends boats.
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Old 11-07-2018, 08:27 PM   #4
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Over protection can be as bad as under protection. Best is to measure voltage of each underwater fitting against a reference anode (silver chloride) and then add/remove zinc area as needed.
While I agree that a reference anode should be used, I don’t see how it will identify being over protected. A properly bonded and protected boat the voltage read between the half cell and the particular metal being tested will ideally be that between the anode and the half cell.

For example ....and I am going by memory for the exact voltage.... a boat with aluminum anodes being tested with a silver silver-chloride half cell, the protecting voltage read between the half cell and the tested metal should be about -0.8 volts. I cannot see how having more anodes will change that voltage BUT because adding more anodes creates a greater exposed anode surface area, there will be greater current flow to the protected metal from the anodes. A half cell will not measure that current
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Old 11-08-2018, 03:30 AM   #5
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Zincs on USS Oriskany CV-34 (upper right). And the other side. Late WWII built carrier. In Yokosuka drydock. Lost a prop and some shafting twice in 1972.
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Old 11-08-2018, 06:44 AM   #6
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Haven't heard of overzincing a fiberglass boat...but have seen some issues with bottom paint that I would have to refresh my memory what the exact cause and final issue was.

I think most trim tabs get zinced because the actual tab is electrically isolated from inside boat parts , so a hole and zinc are easiest.

On many glass boats you will see zincs on shafts, rudders, tabs and hull plates....so unless they are all oversized, I doubt you will have any issues.
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Old 11-08-2018, 10:20 AM   #7
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Over zinking a fiberglass boat is usually only an issue if you are using a 70% copper bottom paint. The issue shows up as a circle of missing bottom paint around each piece of metal (ie thru hulls, struts, rudder shaft log) on the bottom of the boat.

The solution is either to use a 50% copper bottom paint or add a cathodic protection device such as Electro-Guard.
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Old 11-08-2018, 10:30 AM   #8
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Each hull material has a recommended voltage for optimum protection. Measuring with a reference anode will determine if you are in the recommended range. Common problems with over protection are paint lifting, paint blisters, halos around zincs. On wooden boats it can actually destroy the wood around fasteners.
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Old 11-08-2018, 01:46 PM   #9
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Quote:
Originally Posted by Lepke View Post
Zincs on USS Oriskany CV-34 (upper right). And the other side. Late WWII built carrier. In Yokosuka drydock. Lost a prop and some shafting twice in 1972.
I was on the Oriskany when it went into dry dock in Long Beach in 1974, there were over 100 holes in the hull from electrolysis and about that many flooded compartments. They replaced the boilers to run Navy Distillate fuel, which actually made the ship slower than it had been running bunker fuel. It did run cleaner though, that nasty stink and chunks from blowing the tubes wasn't all over everything...
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Old 11-08-2018, 01:48 PM   #10
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My boat also had only two zincs mounted at the stern, no shaft or prop zincs. I added a zinc to the prop nut, and after the first year running found almost all of the wear was from the prop zinc and none from the two stern zincs. I say zinc, but all of mine have been switched to aluminum.
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Old 11-08-2018, 01:49 PM   #11
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Each hull material has a recommended voltage for optimum protection. Measuring with a reference anode will determine if you are in the recommended range...
Recommended range of cathodic protection for boats of different hull materials in saltwater from a BoatUS article.

Hull Material Millivolt Range
Fiberglass -550 to -1100
Wood -550 to -600
Aluminum -950 to -1100
Steel -850 to -1100
Non-metallic w/Aluminum drives -950 to -1100
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Old 11-08-2018, 02:32 PM   #12
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If one has the standard setup of stainless shaft and bronze prop, that immediately becomes a battery when you put it in salt water. The only way to protect *any* underwater metal is that metal must be in electrical contact with a protective anode. So in the case of a prop and propshaft, either shaft or propnut anodes or propshaft brushes grounded to the bonding system which should also be connected to any underwater anodes.



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Old 11-08-2018, 03:07 PM   #13
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reference anode (silver chloride)

Keep hearing about these reference anodes but the marine electricians i have used here do no more run a long wire from water to multimeter to fitting.

What magic does a reference anode actual do?
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Old 11-08-2018, 03:24 PM   #14
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Each hull material has a recommended voltage for optimum protection. Measuring with a reference anode will determine if you are in the recommended range. Common problems with over protection are paint lifting, paint blisters, halos around zincs. On wooden boats it can actually destroy the wood around fasteners.
Nonsense! There is no recommended half cell voltage recommended for hull material unless the hull is metal. There are 3 common anode materials, zinc, aluminum and magnesium used for boats. Magnesium will have higher voltages when tested with a half cell followed by aluminum and zinc and is recommended only for fresh water.

OH, check out Wooden Boat forum and see what they use for anodes....most use nothing unless the hull is enclosed.

Have you ever used a half cell?
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Old 11-08-2018, 03:26 PM   #15
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Recommended range of cathodic protection for boats of different hull materials in saltwater from a BoatUS article.

Hull Material Millivolt Range
Fiberglass -550 to -1100
Wood -550 to -600
Aluminum -950 to -1100
Steel -850 to -1100
Non-metallic w/Aluminum drives -950 to -1100

Since when has fiberglass been subjected to galvanic corrosion that require anodes for protection????
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Old 11-08-2018, 03:33 PM   #16
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Keep hearing about these reference anodes but the marine electricians i have used here do no more run a long wire from water to multimeter to fitting.

What magic does a reference anode actual do?
Essentially it forms a battery whereby one half of the battery’s cell is the the metal being tested and the other half is that of the reference anode. The tested voltage is the measurement made between the reference cell and the metal that one desires to be protected from galvanic corrosion.

NOTE—- there has to be a conductive solution between the two metals (reference cell and the tested metal)
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Old 11-08-2018, 04:11 PM   #17
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Nonsense! There is no recommended half cell voltage recommended for hull material unless the hull is metal...
ABYC has recommended range of cathodic protection for different hull types based on AG/AGCL reference cell in Publication E-2, Cathodic Protection.
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Old 11-08-2018, 04:13 PM   #18
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Since when has fiberglass been subjected to galvanic corrosion that require anodes for protection????
We’re talking hull types. See my previous post.

Edit: Here’s an article to help confuse the issue.

https://abycinc.org/blogpost/1678504...ion+Protection
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Old 11-08-2018, 06:01 PM   #19
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I was on the Oriskany when it went into dry dock in Long Beach in 1974, there were over 100 holes in the hull from electrolysis and about that many flooded compartments. They replaced the boilers to run Navy Distillate fuel, which actually made the ship slower than it had been running bunker fuel. It did run cleaner though, that nasty stink and chunks from blowing the tubes wasn't all over everything...
In 1968 I was on a WWII built destroyer in drydock with a sister ship. A snipe on the other ship actually fell through the hull while power chipping rust in the boiler room bilge. We were held up in drydock while the yard (Long Beach NS) replaced a band of about 50' of hull plating from side to side. Steam plants usually just drain all engine room and boiler room water and steam drips into the bilge and then pump overboard. So they're always wet. We replaced all the framing under the boilers during that overhaul. Both ships were about 24 years old and decommissioned 1-2 years later.
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Old 11-08-2018, 07:59 PM   #20
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Anything wrong with putting both a zinc and a magnesium anode on a stainless shaft in fresh water??

Down sides or up sides...
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