Black Box Techniques
Most outboard motors have a small zinc anode fastened to the motor shaft. It is very important that this be present and that the motor be bonded to the hull or other metals on the boat.
Chapter VIII. The Chemistry of The Electric Charge on Your Boat
Electrolysis or galvanic action as it is sometimes called is the generation of electricity through a chemical reaction. Any time dissimilar metals are present in an electrolytic solution a battery will be created and electricity will flow. The amount of electricity generated depends on the metals involved and the nature of the electrolytic solution. Sulfuric acid is a strong electrolytic solution. It is used in your car battery to generate enough electricity to start the car and power your electric systems. Water containing minerals is a weak electrolytic solution but it will still generate electricity.
There is a table ranking all metals by what is called nobility. When a metal of high nobility is coupled with a metal of low nobility, more electricity will be generated. Zinc is very low in nobility and will react strongly with almost all other metals. This is why zinc anodes are placed on boats. Downrigger wire, which is stainless steel, is relatively high on the metal nobility table. The reaction between stainless steel wire and zinc will normally generate about .8 volts of electricity in saltwater.
If any two of these metals are connected in an electrolyte, the one standing higher on the table will be anodic, lower one cathodic. The further apart they are in the series, the greater will be the galvanic (electrical) potential between them. For example, if the stainless steel reading of .24 (downrigger wire) is subtracted from the zinc reading of 1.04 (boat anode) the result shows a voltage potential of .80 volts being generated. This is what we would expect to read if we tested the boat's natural voltage.
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Black Box Electronic Fishing Technology