Any time you have two different metals that are physically or electrically connected and immersed in seawater, they produce electrical current. Some DC current flows between the two metals which can corrode metals such as props, shafts, thru hulls and sea strainers in the engine room.

The way we stop galvanic corrosion is to add a piece of metal called a sacrificial anode, and most often it is zinc. In fact, most of us refer to sacrificial anodes simply as zincs. Using zinc anodes on your boat is very important. The anode will be used to corrode away before your boat equipment does.

On the Galvanic Scale, Zinc is number 4, 1 being the most sacrificial to seawater, Aluminum is 12, Steel is 30, Brass is 51, 316 Stainless is 76 and pure Gold is 91. This is why zinc is used, we want it to erode away and protect the other metal in contact with sea water.

Zincs should be replaced when about half of the anode has been lost to corrosion. Ideally we want that to occur not more frequently than annually. I check my yacht zincs routinely and replace zincs that need it. At haul out, a full set is always installed. All zincs are not made the same. Insist on MIL spec zincs. There is a supplier on my Links page that I get mine from. Good zincs, good price.

Yacht Props and Rudders

Propellers and shafts are normally protected by a zinc collar bolted together around the shaft. It is necessary to make sure the shaft is clean and polished before clamping the collar to it.

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

Yacht Hull Plates

Hull plates are used to bond the boat metals inside the boat not directly exposed to salt water.

Bonding is the connecting together of zinc plates bolted to the hull to other metals. All the underwater gear and the metal inside the engine room is connected to these plates. Be sure you check the bonding between thru-hulls and other metal gear by the use of a multi-meter set on the ohm settings.

Hull plates are usually found on the stern of the boat.

Tip: If you spot "green" bronze fittings, the bonding has failed and corrosion is in progress. Check and restore the bonding.

Boat Cooling Water

Heat exchangers and engine raw water systems are also at risk. Most heat exchangers are fitted with a zinc "pencil". You will find it under a brass plug. The pencil is unscrewed from the plug for replacement. In addition, your transmission and oil coolers will be fitted with pencil zincs as well.