The ocean is a battery. Salt water conducts. The various metal parts of the boat conduct.
And this means some parts are being galvanized. Other parts are giving up atoms to support the galvanization.
Since it's inevitable, and unstoppable, the solution is sacrificial anodes that can give up metal and get replaced when the rot away.
We have three.
A 5# zinc fish tied to the grounding system.
A zinc ball on the drive shaft to protect the bronze propellor.
A zinc pencil in the cooling system of the engine to protect the barrier between engine water and raw water.
The pencil looks like this:
The one on the right is a new pencil.
The three on the left are old ones.
"Why do you have old ones?" you ask.
There's no good reason.
There's a bad reason.
You can see that one of the old ones has a trace of threads. The other two have no visible threads.
That's because they sacrificed a bunch of material from around the threads. And then broke off. And wound up jammed inside the heat exchanger.
I couldn't get the new one in this year. Too much old zinc in the way.
I had to take the heat exchanger apart to pick out the zincy bits.
I think we're good, now. Hopefully it's all tight and won't dribble raw water when I start the engine.
Some day, I'm going to build a proper, permanent grounding stud for the zinc fish, to make it more-or-less permanent. The idea would be to clip off the silly alligator clip, crimp on a ring connector, and bolt it to the stud.