Anodes disappearing quickly - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 10-11-2010
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Anodes disappearing quickly

To start off with, we purchased our 1982 34 Oday about 3 years ago and left it in the marina where we bought it....in Port Aransas TX (I love that place)
I left shore power hooked up with a batt charger on and a dehumidifier running.
I used 2 1" prop shaft zinc anodes and had to replace them every 6 months...I thought that was too frequent while using 2 anodes.
We moved the boat up to the Clear Lake TX area last June.
4 weeks ago, I put on 2 more 1" shaft anodes. This past weekend, I jumped under and found they are nearly 2/3rds gone...holy cow!
When I left the boat, I did not hook the shore power back up hoping this would buy some time before I can dive under and replace.

Whatís going on here and what can I do about it???
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Old 10-11-2010
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Could be a hot harbor. Bad wiring in the marina or a leaky (electrical wise) boat near you.

Be careful how you route your power cord. Heep it out of the water and try to avoid letting it touch metal parts on your boat. You can also use a "fish", it's a zinc in the shape of a fish with a wire on it that you connect to the ground on your boat. It will help your shaft zincs last longer.

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Would a “fish” zinc and a galvanic isolator both be recommended to help lengthen the shaft zinc life? If so, can some recommend a good isolator? I see them on Defender for $90.00 on up. How much $ do you have to spend for a 30 amp system to actually get good protection?
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Old 10-11-2010
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I had a similar issue on my boat. I put a fish on until I had time to install an isolator. After doing that the fish stopped corroding so I took it off. I use this model for my 30A shore power.

DEI Marine Fail Safe Plus 30 Amp Galvanic Isolator
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Last edited by nickmerc; 08-18-2011 at 06:46 AM.
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Old 10-12-2010
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A galvanic isolator will help the zincs last longer, at least it does for me.
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Old 10-14-2010
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Where exactly, do you attach the fish zinc to...the AC ground or the DC ground?
My boat has a bare bonding wire attached to the raw water intake seacock valve...should I clip it to that?
I would like to install a galvanic isolator. I assume you get what you pay for and the $90.00 from Defender may not be the best way to go...any thoughts on that???
I am having a hard time grasping the concept of the isolator. I understand it allows a regular flow of current but blocks smaller "stray" current from flowing thru the shore power ground. Does this mean you can still run AC equipment (dehumidifier) off of shore power without galvanic issues with the isolator installed?
Sometimes, electrical issues make my brain hurt as I will make them more difficult than they should be. Any simplified explanations will be greatly appreciated.
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Red face stray current

Just my two cents worth... I'm having a serious, similar problem with my slip, two months for a 6-8 months zinc.... but an isolator will not help you if the leak is coming from the water in the marina, only if it is coming from the shore power. An isolator stops the flow from the pedestal. You are likely getting a leak from the marina itself (electrical bleed from the pedestals into the water) , or more likely, a boat or boats around you coming thru your shaft or other grounded (non-zinc) metal. Have the marina check the pedestals near you for stray current, as well as the ground wire shore power plugs near you... say 6-8 boats either way.
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Old 10-14-2010
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You should attach the zinc fish to the bonding circuit (both AC and DC grounds will be part of this as well as all metal through hulls).

The isolator that I use, and posted earlier, is basically two ginormous diodes in series. They block current from entering from the shore side of the AC ground and they also block small stray currents from passing from the boat to the shore via the AC ground wire. But, it allows larger currents (ie lighting, ground faults in your electrical) to pass. It works because the diodes do have some resistance in the "open" direction and it is enough to stop the stray currents.

Yes, you can still run AC devices with the isolator installed.
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Last edited by nickmerc; 08-18-2011 at 06:48 AM.
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Old 10-14-2010
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If your anodes are disappearing that fast you are likely dealing with a stray current problem rather than a galvanic issue. I would not dive in that marina (or any other) under any circumstances. If there is enough current in the water to eat anodes that fast, there is enough to stop your heart.
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Old 10-14-2010
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It sound like you had a problem before the move too. The problem may be with you boats dc system. First, connect a wire to a zinc, hang it off the boat, and connect the other end to your dc ground. This will protect your boat until you get things fixed. Then test for a stray leak. Shut everything off, and disconnect your battery grounds. Use an ammeter to remake the ground to battery connection. The ammeter will measure stray currents. Check your circuits which are left on all the time. On my boat, that is just the bilge... I used this method to tract down a short in my instrument line.

As for diving in the water, don't worry about your heart stopping. The currents for galvanic corrosion are tiny. Measure it with your ammeter if you want.
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Last edited by dreuge; 10-14-2010 at 09:00 PM.
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