Why You Need Zincs on a Motor Yacht and Other Boats

Published: 21st August 2009
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If you have owned your motor yacht long enough, you have had talks on the docks about the issue of underwater zincs for your boat, but what do they in all honesty do? How do they perform?

When you have two independent metals that are physically and electrically united and lowered into salt water, they set up an electrical (dc) direct current. Current moves between the two metal sections which can bit by bit corrode metals such as propeller shafts, underwater equipment and strainers in the engine room; anything that is in contact with the sea water.

The preferred way to prevent galvanic corrosion is to attach a bit of metal called a sacrificial anode, and it is usually a zinc alloy. In actuality, many of us refer to sacrificial anodes merely as zincs. By using zinc anodes on your motor yacht you will protect it considerably. The anode will be used to purposely corrode away quicker than your other boat's under water metal gear does.

At the top of the Galvanic Scale, Zinc is number 4, 1 being the most sacrificial to salt 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 would like for it to erode and safeguard the other metal in contact with sea water.

Your motor yachts zincs should be replaced when about 1/2 of the anode has disappeared to decomposition. In a ideal world we need that to happen not more regularly than once a year but each motor yacht is different. I inspect my motor yacht's zincs frequently by diving beneath the boat and replacing those zincs that need it. When the motor yacht is removed from the water, a full set is always installed. All zincs are not made in the same way. Insist on MIL spec zincs.

There is a business on my website Links page that I acquire mine from. First-rate zincs, at a very decent price.

Props and Rudders

Propellers and prop shafts are customarily protected by a zinc collar bolted around the shaft. It is essential to ensure the shaft is clean and smooth prior to clamping the collar to it. You must have no bottom paint here at all.

Rudders and struts are also protected with zinc disks bolted directly to the metal. Be sure anti-foulant paint does not cover them here either.

Hull Plates

Hull plate zincs are affixed securely on the exterior of the hull and are used to bond the motor yacht's metals inside the boat not openly out in the open to salt water.

Bonding is the linking together of zinc plates bolted to the hull to other metals by wires. All of the underwater running gear and the metal in the engine space are linked to these plates. Be certain you check the bonding between thru-hulls and other metal gear by the use of a electrical ohm-meter .

Hull plates are normally found underwater on the stern of the boat.

Tip: If you spot "green" bronze gear, the bonding has failed and corrosion is in progress. Test and re- institute the bonding system.

Raw Water Cooling

Engine heat exchangers and engine raw water cooling systems are also at risk. Most heat exchangers are provided with a number of zinc "pencils". You will notice them under brass plugs. The pencil is unscrewed from the plug for substitution. In addition, your transmission and oil coolers will be fitted with pencil zincs too.

Mike Dickens, the author, is a boat owner and owner/Broker of Paradise Yachts in Florida USA.

Paradise Yachts offers used quality yachts to customers worldwide.

Visit the Paradise Yachts website to view our selection of Used Trawlers, Used Motor Yachts, and Used Sailboats for Sale

National and international sales. We ship Used Yachts and Used Boats worldwide. Located in Florida, USA. 904/556-9431

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