Tecnoseal R3 Rudder Anode - Zinc - 3-3/4" Diameter 

Military Grade Zinc Anode

Rudder Anode Material: Zinc Outer Diameter: 3.63" Height: 0.52" Weight: 1.56lbs Full size and weight Rudder Each includes Stainless Steel hex nut and socket cap screw The captive hex nut is a great feature for underwater installations

Zinc Is For Salt Water Applications.