Jim, Several different things in your question that are worthy of mention. First, "over-zinc'd." That refers to the fundamental concept of what's happening here. "In the beginning..." there are two dissimilar metals (a galvanic couple) connected together electrically, either in direct contact, or via a wire, like the bonding system, and bathed in an electrolyte. Ex: propellor connects directly to shaft. Ex: transom zinc connects to other hull metals via bonding wires. In the basic galvanic couple, both metals are valuable (bronze & SS, for example), but one is anodic to the other, and whichever one is anodic will corrode/waste away. Enter now, the zinc (or aluminum, or magnesium) "sacrificial anode." What that anode does is, when added electrically to the original couple, it becomes the anode, and both of the original metals become cathodic to the zinc. The natural electropotential of the zinc makes it the overall anodic component, replacing the anode status of the metal of the couple. The zinc is the most anodic metal, so it becomes the metal that erodes, thus protecting both of the metals of the original couple. When that zinc is wasted, whatever other metal was the anodic metal will again begin to erode. "Over-zinc'd" refers to having too large a ratio of zinc mass to the mass of the metals being protected. That raises the eletcropotential to an unnecessarily large differential. It does not cause any issues on plastic hulls, but it is extremely bad on wooden hulls, because it causes the wood to deteriorate. It can affect bottom paints near thruhulls, where it will create a halo appearance. A galvanic isolator has a full wave diode bridge in it. Each diode will drop 0.6V, so the bridge will drop 1.2V. The AC safety ground is the electrical connection that connects all boats together. The GI blocks DC galvanic voltages from traveling down your boat's AC safety ground, so it stops your boat from protecting nearby boats on a dock. The shaft zincs protect the shaft and prop. Which one can vary with time. SS can exist in either an active or passive state. Which state it's in determines whether it's anodic or cathodic with respect to the propellor bronze. But, the football zinc on the shaft protects the couple, so it doesn't matter what state the SS is in at any given time. Coolers are DIFFERENT. There are two cases that occur with coolers. Coolers contain dissimilar metals, so one is anodic with respect to all of the others. And, the metals are connected together electrically, by solder or by mechanical interference fit (there is an exception here for coolers with removable core pacs). When sea water sits inside the cooler, it is an entire separate galvanic cell from other connected together by the bonding system. That galvanic cell is entirely self-contained. The zinc in a cooler protects the metals of the cooler. The external zincs in other parts of the boat have no role in protecting the coolers. Coolers with removable core pacs (like intercoolers) do not have an inherent electrical connection between the core pac and the shell. However, they do need regular maintenance, because they are affected bu the buildup of "salt bridges" that form across rubber seals, and that salt bridge can conduct. Furthermore, there is a phenomena called "single metal corrosion" that can happen inside a cooler that has electrical isolation, and that involves regions of an alloy that are slightly anodic with respect to nearby regions of the alloy that are slightly cathodic. If that happens, the anodic area will waste, and the surface will develop pits. In iron, we all call that "rust." In SS, we call it "surface pitting," "crevice corrosion," and "stress corrosion." In aluminum, we call it "poultice corrosion." And, it will happen in cupronickel in coolers, too, so the zinc anode protects the entire couple (the cooler and shell) from damage. Coolers do have particular susceptibilities. The metal mass ratio between the zinc and the noble metals is high, they operate at high temperatures, and they have high velocity water flow. All factors that affect the rate of zinc wasting. (Hydrogen gas forms on the surfaces of the cathodic metals, and slows the overall rate of the reduction reaction. Water velocity carries that gas off. It's called the "Hydrogen Evolution Reaction, HER.) The guppies do work. They are similar to transom zincs, connected to the system via a "bonding wire." Hope this helps, Jim Peg and Jim Healy, living aboard Sanctuary http://gilwellbear.wordpress.com <http://gilwellbear.wordpress.com/> Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436