I see that your model Yanmar does not have zincs (the majority do). On my Westerbeke
engine the zinc is in the heat exchanger
. Shortly after I bought the boat the heat exchanger failed (corrosion ate a hole in the casing). The zinc was completely gone for who knows how long.
More on engine zincs:
Heat exchangers, because they are typically a copper alloy, are at risk of galvanic corrosion
. To combat this, many heat exchangers are fitted with a zinc "pencil" anode. You will find it (or not) under a brass plug
in the exchanger. The pencil is unscrewed from the plug
for replacement. Some engines have a similar zinc pencil inside the cooling-water jacket to protect dissimilar metals in the engine. Determine if your engine and heat exchanger are fitted with internal anodes, and if so, check them at least annually. If they are half depleted. . .well, you know."