USAMARINE Web Group - Captains' Corner Boatbuilding Tips


Zincs, Bonding, & Galvanic Isolators

Basic Bonding Schematic

This tip comes about from a related thread on my bulletin board and is posted verbatum...

First off, buy only the American made zincs, the Canadian ones are cheaper, but don't last worth a squat. Ya' get what ya' pay for.

Over zinced simply means that you have more zinc than really needed. Kinda' like having too much gas in your tank to drive down to the Kwick Stop???

Stray AC current in the surrounding water is the main culprit to galvanic corrosion. The term electrolysis is incorrect and refers to hair removal that ladies, (and guys light in the loafers), spend big bucks to have done.

The stray AC currents may come from your boat when plugged to shore power, the guy's boat next to you, or the dock itself if there are bad & submerged wiring to the dock power box. Or, very likely all or any combination of the above.

Simply un-plugging your shore power cord may not eliminate the problem, certainly won't if the source is not your boat.

There are three things that need to be done to assure that you are fully protected and reduce the frequency of your zincs dissolving.

1) Check all of your bonding contacts, wires, and primary strips. The primary strip should be copper at least 26 gauge in thickness and 1" in width. The attachment studs should be bronze, (not brass, which is alloyed with copper and zinc), the nuts and flat washers also in bronze, and the connecting wire ends should have tinned terminal lugs. Secondary wires, running from the copper strip to the item being bonded should be at least 8 gauge marine grade tinned copper stranded wire. Don't use solid wire, aluminum wire, or whatever else the cat dragged in.

Don't run secondary wire more than a couple of feet to the primary strip. Secondary should be coiled for at least three inches to buffer any vibration, stretch, or accidental stepping upon. Rudder secondary wires should have the entire length coiled since there is several inches of travel that the wire must continually deal with. The coils can be made easily by taking a longer than needed section of wire and tightly wrapping it around a round rod.

The connections must be bare metal to bare metal, clean, and tight. After the connection is cleaned up and made, additional protection can be had by painting over the finished connection with clear lacquer or nail polish.

A light coat of TefGel is acceptable when making the connection.

Make sure that the copper primary strip is in good condition. Cuts, corrosion breaks, and too thin of a strip just make for more resistance from point A to point B. When all connections are good & the strip is the proper size, the ohm reading between any given two points should be zero or very close to it. .0001 ohms is good, .01 ohms is OK, .1 ohms don't cut it

2) Galvanic Isolators should be a standard item on every boat. Even if the boat spends most of it's rest time on a davit & out of the water.

Galvanic isolators are simply a diode bridge. An explanation of the physics of an isolator can be found by following the link below. The are available through just about all marine suppliers, and as the author of the linked article points, they can be built yourself for a few bucks as opposed to the $100+ tag at west Marine for a 30 amp unit.

Isolators are totally maintenance free. Once installed you don't need to do anything more, except check them with a multimeter every year or so to assure that it hasn't blown a diode and quit working properly. You'd notice a malfunction if all of a sudden your zincs started decaying at a more rapid rate than they should. Of course that would also be the case for a failed bonding system.

A properly bonded and isolated system should not require zinc changes more often than 6 to 8 months at a public marina. I've had zincs last as long as 1 1/2 years before needing replacement. That was on Mike Calvi's "Sea Hawk", which before adding the isolator and running down all the bonding issues, was eating up zincs on a 1 to 2 month basis...

3) BUY AMERICAN ZINCS, unless of course you're Canadian and want more to support your national industry and don't mind changing your zincs several times a year... Make sure that your running gear zincs are in a bare metal to bare metal situation. Transom zincs should have a 1/4" stand off between the zinc and the transom, and be tied directly to the copper strip primary, not pig tailed to the primary with a secondary wire. Add some more primary strips if necessary.


Installation/Operation Manual by Lewmar Power (30 amp Isolator)

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Capt Patrick McCrary

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