to go through every hull fitting, valve, prop shaft, heat exchanger, rudder shaft, air conditioner, sea strainer and what-not to determine the potential (voltage) with it's relationship to the sea. The difference (in potential) between them, one to the other, will determine how long your zink will last. You should find that they all live in the 2 to 3 hundred Milliamp range (of current passing between them). If they are bonded together and are at the "same potential" no, or very little, current will flow talking your zink with it. Your underwater fittings suffer when the zink is gone or when you are over or under zinged. I use a Salty Dogg test kit once a year to make sure that all my fittings are staying in the correct range. Lock up Salty Dogg electrolysis tester on the internet.