No AC hook-up why galvanic crrosion? - SailNet Community
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post #1 of 22 Old 02-11-2011 Thread Starter
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Unhappy No AC hook-up why galvanic crrosion?

Hey why and how do I stop the prop pitting on my boat that is not hooked up to the dock's AC current? The boat is only DC, there is a shaft zinc and strut zinc which I change often (every 6 months). The boat yard tells me it is my neighbors boat, but this has happened at different marinas for the past 10 years. I am becoming frustrated, any ideas?
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post #2 of 22 Old 02-11-2011
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Do you leave your battery switch on?
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post #3 of 22 Old 02-11-2011 Thread Starter
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Yes I do leave the power on? Is that the answer? I noticed a few of your posts and respect your profession and opinions.
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post #4 of 22 Old 02-11-2011
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Well, I'm no electrical expert, but 12 volts bleeding into a wet bilge can be a problem. You may have a bare wire somewhere in the 12-volt system. I'd start looking there.
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post #5 of 22 Old 02-11-2011
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you have a potential difference between your boat/systems and the marina, somewhere.

You can get a decent digital multimeter and test things, or have a marine electrician with one hour show you where things are leaking.
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post #6 of 22 Old 02-11-2011
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I wrote a post on testing for a DC side leak a while back. Here it is...

Quote:
If you want to test to see if you have a DC-based ground leak, the test for that is rather simple. The steps for seeing if you have a DC-ground leak are as follows:

First—the preliminary diagnosis test:


1) Turn off all equipment and disconnect any solar panels
2) Disconnect the positive side of the battery banks.
3) Leave the main battery isolation switch turned on for the bank in question
4) Set the meter to VDC mode, range appropriate for your battery bank
5) Connect the meter between the positive terminal and the disconnected cable

The meter should give no reading. If it reads XX volts for your XX VDC system, one of two things is happening.

1) You've left some equipment connected and turned on. This could be a bilge pump, a power feed to a stereo for the radio's memory and clock functions, or a hard-wired fume detector.

2) If you've disconnected all the "hard-wired" equipment and still get a reading, then you've most likely got a ground leak in your boat's DC system.

The Ground Leak Check:

1) Set the meter in Ohm mode and set it to the lowest range (x1).
2) Connect the leads of the Ohm-meter (or multimeter in Ohm mode) to the disconnected positive lead and the negative terminal of the battery.

The meter is now reading the resistance of any circuit to ground that exists in the boat's wiring. The reading on the Ohm meter display can help you identify the cause of the leak.

0-10 Ohms means it is most likely a piece of equipment left on
10-1k Ohms is a low-drain piece of equipment left on, or a serious ground leak
1k-10k Ohms is a minor leak
10k+ Ohms is an insignificant leak

How Big is The Leak?

The ammeter function of the multi-meter can tell you what the current leakage is. If your meter can read up to 10 Amps DC, then you can use it to measure amperage for leaks down to about 1.3 Ohms resistance on a 12 VDC system, or 2.6 Ohms for a 24 VDC system.

To see how big the leak is, put the probes on the positive battery post and the disconnected cable. The meter readings can be interpreted as shown:

<1mA — insignificant leakage
1–10mA — minor leakage
10mA–1A — major leak or some equipment left on
>1A — Usually some equipment left on.

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post #7 of 22 Old 02-11-2011
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What sailing dog said. The zincs act as a sacrifical anode making the rest of the boat more cathodic, (negative), but the voltage is low <1volt. a 12v battery wire in the bilge blows it right out. Since most electrical devices will be grounded to the boats ground, any could be leaking, (wet, corroded, worn insulation, etc...). Bilge pump wires are a good suspect.

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post #8 of 22 Old 02-11-2011
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Quote:
Originally Posted by CapnBilll View Post
Bilge pump wires are a good suspect.
Just a month or so with a bad bilge pump wire:


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Thanks sailingdog and fastbottom for the info. I will be looking into it this weekend. It was helpful for the range of leakage. And I don't want my prop ever looking like that one!
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post #10 of 22 Old 02-11-2011
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What type of water?

There are 3 different types of anodes: zinc, magnesium and aluminum.

Depending on the type of water you are in, you should be using a specific type of anode. Zinc is for salt water; magnesium is for brackish; and aluminum is for fresh water.

Also, there is a good article in this months Cruising World.

“Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.” -Mark Twain
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