1 7/8 in. Magnesium Rudder Anode | Canada Metals CMR01M Magnesium Martyr anode is designed to protect boat's rudder or trim tab from corrosion in fresh water lakes and rivers. Rudder and trim tab anode Designed to mount to rudders and trim tabs to protect from the corrosive effects of stray current 1 7/8 in. diameter Sold as 2 halves held together with a stainless steel bolt and nut Each half is 1/4 in. thick Magnesium anode is for use in fresh water applications Martyr anodes are made of a high purity specialized alloy based on the US Mil Spec