YOu want to have the right amount of zinc---not too much, not too little:
Measure the length and width of the metal on your boat that is exposed to the water. This includes all metal surfaces on the exterior of your boat and on the interior as well: If you have a metal fuel pipe that's exposed enough to be splashed with water under any circumstance, measure it. Do not include your outboard or propeller in any of these measurements; they have their own zincs, prepared and sized by the manufacturer. These zincs should be used, in preference to any other, for those parts.
Use a calculator to multiply the length by the width of each individual metal element. For example, if the hull is aluminum, the exposed metal is equal to the length and height of both the inside and outside of each side, and the length and width of the inside and outside of the hull. If the length of your aluminum hull is 96 inches, its width is 40 inches and the sides of the hull are 96 inches long and 22 inches high, then 96 x 22 x 2 (by 2, because you have to include both the inside and outside surfaces of the hull) = 4,224 square inches inside and out for the sides, 96 x 40 x 2 = 3,840 square inches inside and out for the bottom; 4,224 + 3,840 = 8,064, the total number of square inches to be protected.
Multiply the result by 0.01: the size of a zinc anode should be 1 percent of the size of the metal it is protecting: 8064 x 0.01 = 80.64 square inches, or about 8.9 inches by 8.9 inches.
Read more: How to Calculate the Required Zinc Anodes | eHow.com http://www.ehow.com/how_6150344_calc...#ixzz1DUj36P4r