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A Discussion of Zinc Anodes

By ACRA Electric, Inc. on February 3, 2014 in Latest News
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January 11, 2014

I decided to change the zincs on my 46’ Searay this weekend, so in doing so I thought it would be interesting to take some measurements and share them. As we all know, the job of zinc anodes is to protect underwater metals from galvanic corrosion. My Searay has stainless steel shafts, trim tabs and dinghy lift metal plus bronze props, struts, water intakes and rudders. Even though the boat is stored on a lift, it does spend up to a week at a time in the water when prep-ping for a trip or doing maintenance on the engines, so over the 9 years I’ve had the boat the zincs have definitely had a workout, and they were looking nasty. One of the shaft zincs was gone, I don’t remember when it left.

To test effectiveness of zinc anodes you need a silver chloride reference electrode and a good quality digital ohmmeter with a DC voltage scale. Connect the silver chloride electrode wire to the negative lead on the tester and touch the positive lead on the tester to any grounded metal part of the boat, in many cases this could be the fuel fill. With the silver chloride electrode in the water and the shore power cord unplugged, you will read a number from about -400mv to -800mv (millivolts, or thousandths of a volt).

Results

The readings I got on my boat are as follows:

With old zincs: -642mv
With NO zincs: -384mv
With new zincs: -843mv

Judging by my measurements, the old zincs were still effective, but obviously the new zincs made a difference. From what I’ve read, you need at least a 200mv difference to provide protection for your metal parts, so knowing what your boat reads without any zincs could be important to know. For boat owners that keep their boat in the water, the best time to take these measurements is on the day your diver shows up to replace the zincs. Do the same thing I did before changing, no zincs, and with new zincs. I’ve attached a good video demonstrating this test here.

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ACRA Electric, Inc. is a company based out of Cape Coral, Florida in Lee County. We service Cape Coral, Fort Myers, North Fort Myers, Naples, Estero, Bonita Springs, Marco Island, Port Charlotte, Lehigh Acres and more with electrical contracting services.