Hi, the only zinc anode on the exterior of my boat is a 1" shaft anode on the propeller shaft, between the strut and where the shaft enters the hull. If there is a lot of galvanic activity in your waters, just put two on the shaft, back-to-back. The Yanmar has zincs but since it is converted to glycol cooling, you do not need to replace these. However there is a 'pencil' anode in the heat exchanger that should be changed every year or so. Location will vary depending on the heat exchanger that is fitted; it will probably be a 1/2" hex-head plug (looks like a drain plug) somewhere near a saltwater connection.