Zincs should be replaced when there is 40% section remaining. This is why it is important to have a diver check the amount of section remaining with each monthly cleaning.
Zincs all corrode at different rates based on there mass. Current and water salinity can affect their rate of corrosion. Adding more zincs will not make them last longer; it simply improves their effectiveness by adding more surface area.
Zincs protect your running from galvanic corrosion. Galvanic corrosion occurs from dissimilar Alloys being in an electrolyte such as seawater. Zincs are a type of Anode, other types of Anodes include Aluminum and Magnesium. You might ask yourself which type of Anode should i be using on my boat! Zinc and Aluminum Anodes are used in saltwater while Magnesium Anodes are best suited for freshwater. We recommend Performance Metal Anodes.