This bolt-on sacrificial anode is designed to protect outboards or outdrives in freshwater lakes and rivers. Its use in salt or brackish water may result in accelerated corrosion, leaving your engine with little or no protection. Magnesium anodes are your best choice for fighting corrosion problems caused by stray current or pollution in freshwater. Magnesium is very active in freshwater and will protect your engine parts much more effectively than zinc.3 1/2"L x 1 3/4"WThickness: 1 3/8"Hole diameter: 1/4"