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Old 17-07-2011, 14:31   #1
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Sacrificial Zinc ?

Do we need a sacrificial zinc? We live aboard in a marina in fresh water...have a traditional zinc on the hull and one on the prop shaft. Get conflicting answers here on the dock...some say yes absolutely others say not needed?
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Old 17-07-2011, 14:34   #2
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Well...yes and no...since you have them, might as well keep them...there is really no such thing as a boat in pure water..there are always impurities enough to cause some galvanic corrosion....but do you need them..not really....
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Old 17-07-2011, 14:36   #3
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Re: Sacraficial zinc?

Anytime you have dissimilar metals under water, you need them.
Fresh water is not as active as salt, but it's still good policy to have zinc anodes.
Fresh water might need a different type though.
Some stern drives use magnesium anodes.
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Old 17-07-2011, 14:39   #4
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Re: Sacraficial zinc?

Zinc is for salt water and magnesium for fresh.
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Old 17-07-2011, 22:51   #5
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Re: Sacrificial Zinc?

And stray electricity will also eat the metals under water. Stray 110V at the dock is worse than normal galvanic action.

They are cheap so why not spend a couple of $$$. Aquick dive under the boat to change them..
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Old 18-07-2011, 05:48   #6
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Re: Sacrificial Zinc?

Yes, you need a sacrificial anode.

The chart below summarizes the anode choices, based on type of boat, and water type.
As you can see, the only anode type that is recommended for all water types is aluminum (or aluminum/zinc/indium alloy).
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Old 18-07-2011, 10:33   #7
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Re: Sacrificial Zinc?

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Originally Posted by GordMay View Post
Yes, you need a sacrificial anode.

The chart below summarizes the anode choices, based on type of boat, and water type.
As you can see, the only anode type that is recommended for all water types is aluminum (or aluminum/zinc/indium alloy).
So ... can I make an anode by cutting off a thick slab of aluminum and placing it where we normally have the zincs?

???

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Old 18-07-2011, 11:23   #8
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Re: Sacrificial Zinc ?

You may make your own anodes in a pinch, but they should be made of an alloy in which catalysts have been added to make sure they do their job (activated alloys), or at least very pure zinc or aluminium. Some alloys are corrosion resistant and you would not want those as they will not be sufficiently sacrificial.
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Old 18-07-2011, 13:37   #9
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Re: Sacrificial Zinc ?

Remember, the sacrificial anode needs to be electrically connected to the metal you're trying to protect -- you can't just hang it in the water without a connection.

If you're not keen on replacing / maintaining your anodes, you might look into something like the zinc "fish" that has a cable and alligator clip. You can clip it to a grounded backstay (or anything that's grounded) and hang it over the side. It will protect any nearby metal on your boat (as long as it's also grounded), and should keep any existing anodes from eroding as quickly as they otherwise might.

I've only seen the zinc fish, but there may be Al or Mg ones as well.

Also, it's possible to "over-zinc" a boat -- perhaps someone here can advise us on that problem?
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Old 18-07-2011, 13:44   #10
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I am in fresh water and have seen the zinc"fish" was considering getting one sounds as if magnesium or aluminum is better suited.
Any aluminum "fish"?
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Old 18-07-2011, 14:25   #11
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Re: Sacrificial Zinc ?

I have a boat in fresh water. It has a zinc sacrificial anode on the shaft. Never have to change it. In salt water a zinc usually lasts me about four months and that's with little time (a day or two at the most) in a marina.
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Old 18-07-2011, 14:31   #12
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Re: Sacrificial Zinc ?

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I have a boat in fresh water. It has a zinc sacrificial anode on the shaft. Never have to change it. In salt water a zinc usually lasts me about four months ...
A sacrificial anode, that never erodes, is doing nothing to protect your immersed metals.
Perhaps you don't need protection, but perhaps you do.
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Old 18-07-2011, 14:40   #13
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Re: Sacrificial Zinc ?

Remember; things can change in a hurry in a marina. If a improperly wired boat pulls in next to you and plugs in to shorepower you become a link in the battery chain. Not nearly as bad as salt water but "fresh" water is never really pure!
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Old 18-07-2011, 14:46   #14
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Re: Sacrificial Zinc ?

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A sacrificial anode, that never erodes, is doing nothing to protect your immersed metals.
Perhaps you don't need protection, but perhaps you do.
Props still there, Gord. Been seven years since the boats was in salt water, no pitting, nothing.
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Old 18-07-2011, 18:49   #15
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Re: Sacrificial Zinc ?

Go to Passagemaker.com. Steve D'Antonio just did piece on sacrificial anodes, zinc and other metals. He mentioned specifically fresh water issues. It was a pretty interesting piece.
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