This is a bolt-on aluminum hull anode bar, CMZD78BA.Length: 12"Width: 3"Thickness: 1 1/2"Hole diameter: 9/23"Anode selection guide:Zinc anodes for saltwaterAluminum anodes for saltwater or brackish waterMagnesium anodes for freshwater onlyAdvantages of aluminum anodes:Used in saltwater, aluminum anodes last longer and protect better than zinc. Used in brackish water, aluminum anodes are superior to zinc which can passivate” and not work at all, or magnesium anodes, which corrode to rapidly. Aluminum anodes are also lighter than zinc and do not contain cadmium, a substance of environmental concern.