With your experience installing zincs on boats, can you tell me if there is a general lifespan for zincs on prop shafts and pencil zincs in diesel motors? I moor the boat in the Pacific Ocean on Vancouver Island, BC, Canada. I just bought this boat in Spring, replaced the zincs then, and am wondering when I will likely have to do it again--annually? semi-annually? I know that this may vary depending on conditions, but is there a general rule of thumb?