How to Measure Marine Cathodic Protection
When boaters talk about cathodic protection, they're talking about how blocks of zinc are attached to the parts of a boat to prevent galvanic corrosion. Galvanic corrosion happens when the metal parts of your boat that are underwater act like a battery. An electrical current flows from the electrically weaker metals to the stronger metals. The blocks of zinc, called sacrificial anodes, are electrically weaker than your boat and step in to short circuit the current flow.
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Instructions
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Measure the size of all metal surfaces of your boat below the water line. Multiply length by width and the result by the depth or height of the metal. For example, an aluminum hull that's 10 feet long and 2 feet wide and has 6 inches of its hull below water --- multiply 10 by 2, the length by the width of the hull; multiply the result by .5, for a half foot. The result, 10 square feet, is how much aluminum requires protection.
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Measure all surfaces below the waterline, including those with two sides. If you have an outboard motor, measure the parts of the motor that are normally in the water as well. If a part, a propeller for example, has two surfaces --- a front and a back --- that are below the waterline, make a list of the measurements.
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Add up the sizes on the list of measurements, and multiply the result by 0.01. The result, 1 percent of the surface area of the submerged metal parts of the boat is the total size --- the number of square inches or square feet of zinc needed. For example, 10 square feet of metal requiring cathodic protection, multiply 10 by 0.01. The result --- 0.1 square feet, or 1.44 square inches --- is the minimum size zinc needed to protect those 10 square feet.
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Tips & Warnings
Boating experts, like Don Casey of the Boat Owners Association of the United States, say that "1 percent of the surface area of the protected metal" is useful "as a starting point" for determining effective cathodic protection. The size of the sacrificial anode "varies with the kind of metal being protected" and the waters in which you go boating: aluminum in the ocean's saltwater environment will require a larger anode than aluminum in a fresh water lake.
