5" OD Zinc Fitting/Rudder Tab Anode with Slotted Screw (CMR04S) by Martyr. Looking to protect your boat from the corrosion that can occur in brackish or fresh water? Then Martyr premium grade anodes are the way to go. Die-cast and made from a specialized alloy based on the US Mil Spec, they offer corrosion protection and longevity unmatched by the competition. Plus, these anodes are rigorously tested in the company's state-of-the-art laboratory, so you can be assured of superior quality.