2-3/4 inch Zinc Streamlined Shaft Anode | Martyr CMX12 Traditional zinc Martyr anode protects inboard boat's prop shaft from corrosion in salt water. Streamlined inboard propeller shaft anode Designed to mount to 2 3/4 in. diameter prop shaft to protect from the corrosive effects of stray current Zinc anode is for use in salt water Martyr anodes are made of a high purity specialized alloy based on the US Mil Spec Anode secures to prop shaft with 2 interlocking halves and allen head bolts with bolt retaining washers 2 3/4 in. inside diameter 4 3/4 in. outside diameter 3 5/8 in. length