Boat Zincs – How Do They Work?


All of us listen in to talk on the waterfront on the subject of marine zincs, but what do they honestly do? How do they function?

As soon as you have 2 separate metals that are physically and electrically coupled and lowered into salt water, they set up an electrical flow. Some DC current moves between the two metals which can slowly corrode metals such as propeller shafts, underwater fittings and strainers in the engine space.

The accepted way to stop galvanic corrosion is to attach a piece of metal called a sacrificial anode, and most often it is a zinc alloy. In reality, most of us refer to sacrificial anodes only as zincs. By using zinc anodes on your craft you will protect it very drastically. The anode will be used to intentionally corrode away sooner than your other boat’s metal gear does.

At the top of the Galvanic Scale, Zinc is number 4, 1 being the most sacrificial to sea water, Aluminum is 12, Steel is 30, Brass is 51, 316 Stainless is 76 and pure Gold is 91. That is why zinc is used, we want for it to wear down and safeguard the other metal in contact with sea water.

Your boats zincs should be replaced when about one half of the anode has disappeared to corrosion. In a perfect world we want that to occur not more frequently than once a year but each boat if different. I inspect my trawler zincs regularly by diving under the boat and replacing those zincs that need it. When the boat is taken out of the water, a whole set is always installed. All zincs are not made in the same way. Insist on MIL spec zincs. There is a merchant on my website Links page that I get mine from. Premium zincs, at a very good price.

Props and Rudders

Propellers and prop shafts are more often than not protected by a zinc collar bolted jointly about the shaft. It is critical to ensure the shaft is clean and smooth before clamping the collar to it. You must have no bottom paint here at all.

Rudders and struts are also protected with zinc disks bolted to the metal. Be sure bottom paint does not cover them.

Hull PlatesHull plate zincs are fixed firmly on the exterior of the hull and are used to bond the boat metals inside the boat not openly exposed to salt water.

Bonding is the connecting together of zinc plates bolted to the hull to other metals by wires. All of the submerged running gear and the metal contained by the engine space is linked to these plates. Be positive you check the bonding between through-hulls and other metal gear by the use of a ohm-meter .

Hull plates are more often than not found underwater on the stern of the boat.

Tip: If you spot “green” bronze paraphernalia, the bonding has failed and corrosion is in development. Test and re-establish the bonding.

Cooling Raw Water

Engine heat exchangers and engine raw water systems are additionally at risk. Most heat exchangers are fixed with several zinc “pencils”. You will discover them under brass plugs. The pencil is unscrewed from the plug for replacement. In addition, your transmission and oil coolers will be fitted with pencil zincs as well.


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