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  #1  
Old 06-02-2008
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What does the zinc protect

We just replaced the pencil zinc in a Universal M25 Cat 30 that we just started working on.
The zinc was completly gone. No way of telling when it was replaced last. What damange should we be looking for?
The water injector fitting in the wet exhaust fell off. Is this collateral damage from the zinc? What else.
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Old 06-02-2008
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What does the zinc protect
Your wallet!!! Do NOT let them disapear!! Check and change often..
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Old 06-02-2008
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what does the zinc protect?......pretty much the metal to which it is attached. Zinc "sacrifices" itself in order to prevent corrosion on the metal that it is contact with, usually steel or stainless steel, so as Halekai said, check them often and change them when needed.

here is a good informational link
ElectroGuard | Manufacturing Top Quality Zinc Anodes Since 1940
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Old 06-02-2008
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Ugh. I am still waiting for a diver (or, barring that, short haul). I dread to think what can be there - the zinc is nearing a year old, all in the water.

Seriously though - whats the expected lifetime of underwater zinc on the propeller?
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Ugh. I am still waiting for a diver (or, barring that, short haul). I dread to think what can be there - the zinc is nearing a year old, all in the water.

Seriously though - whats the expected lifetime of underwater zinc on the propeller?
it totally depends of your situation. I am on a mooring and could get three seasons out of my prop zincs because I have almost zero stray current and I am not plugged into shore power.

Every marina is different and I've been in marinas where zincs last three weeks!! The diver was driving a Mercedes..

There is NOT one answer to how long a zinc will last as every situation is different!
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Old 06-02-2008
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The question as to how often zincs were replaced was asked at one of our Power Squadron meetings, and almost everyone said their zincs needed to changed at about 3 months. 3 months seemed to be how long it took to erode the zincs to 50% in the salt water in our marinas, and 50% is when they should be changed. In my case I have an isolator installed, and my zincs are at 50% at around 5 to 6 months.
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Do you keep the boat connected to shore power? I don't - I wonder if this helps with this. Well, anyway - I hope to find out next week, if the diver gets about. If not - in 2 weeks or so, have noone to help me get the boat out of the slip until then

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The question as to how often zincs were replaced was asked at one of our Power Squadron meetings, and almost everyone said their zincs needed to changed at about 3 months. 3 months seemed to be how long it took to erode the zincs to 50% in the salt water in our marinas, and 50% is when they should be changed. In my case I have an isolator installed, and my zincs are at 50% at around 5 to 6 months.
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electrolysis is a very tricky subject. A lot depends on how many metal thru-hull fittings were built into your boat, and how well your electrical system is maintained. You could be dumping energy into the water that is eating your own zincs. If you have many thru-hull fittings, make sure you are properly terminating all of them to where your zinc is located. Then, turn to understanding the boats around you. You likely have a boat or two next to your slip that is pumping energy into the water. Unless you (or a good harbor master) tests for leakage at every slip, you'll never know for sure.

In any case, if your zinc is completely gone, look for pitting on the prop and any other underwater metal surfaces. That's a clear sign your zincs are failing to protect the underwater metals.
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Chances are good that the pencil zinc was probably protecting the heat exchanger for the cooling system. If it was missing for long, it is a bad thing... check it often, and replace when it gets down to about half it's original size.

Quote:
Originally Posted by davidpm View Post
We just replaced the pencil zinc in a Universal M25 Cat 30 that we just started working on.
The zinc was completly gone. No way of telling when it was replaced last. What damange should we be looking for?
The water injector fitting in the wet exhaust fell off. Is this collateral damage from the zinc? What else.
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Old 06-03-2008
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Think of those "hang over the side" style zincs, often shaped like fish. They are hung over the gunwhale and dipped in the drink. The other end is coupled electrically to your prop shaft, or motor, or whatever, and when the zinc slowly fizzes off, it will tend to hold the protected component negative relative to the zinc. Being negative will radically slow the corrosion reaction... that thrives on being positive.
When the zinc fizzes off ("sacrificial anode") you get another one.
It does not protect when you are on the move, as you have to lift the fish out, but most of us are stopped, most of the time.

In fresh water, some outboards use magnesium sacrificial anodes, as zinc cannot quite get enough sacrificing going (it is not as reactive as magnesium) but I don't think that you need them. Fresh water is FAR more forgiving to metals. Often they don't need any protection at all when in fresh water.
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