Originally Posted by sailvayu
You have to know the problem before you can fix it. tossing a zinc over the side only makes the owner feel like he has done something. Maybe he will get lucky and attach the wire to the problem metal, and maybe not.
This statement sums up why a silver chloride half cell electrode should be in the tool kit if one plans to have a lot of dissimilar metals underwater or plans to add additional zinc protection.
I see so many boats with zinc guppies randomly hung around and attached to a random assortment of boat bits. I'm sure almost none of them have any understanding of what they are doing or why, but patiently listen to them tell me how protected their boat is and how I should go right out and get one.
While I only use ours once or twice a year, taking measurements at defined places and keeping a log is invaluable for maintenance
and finding problem areas before they become $$$ areas.
And while it does a great job at telling you when your zinc needs replacement, that is not all too important because a quick look underwater will tell you that. What it is more useful for is telling you when your good-looking zinc has lost
contact with the part it is protecting. This happens more than most people realize, particularly those with saildrives where the zinc is only electrically connected by two small screws in a thin part of the anode.
With the s/sc half cell electrode, you can see exactly what you are protecting and how much protection you have. And too much protection is just as bad as not enough (it shifts the anode to the part you mean to protect). So even if one did know where to hang that guppy, one could be causing damage by doing so.