A Brief & Free Electrolysis Education

The theory of electrolysis states that whenever there are two dissimilar in an electrical solution, i.e., saltwater, a battery is created. For our purpose, we will call it “underwater rust” …even though the term is technically incorrect. The idea of rust brings to mind something falling apart and this is what happens to your boat’s metalwork underwater. If something is not done to protect underwater metalwork it will fall apart. To prevent this, another metal called a “sacrificial anode” is used to stop the corrosion of the valuable metal. The most commonly used sacrificial anode is zinc, which intentionally corrodes in order to protect the expensive metals of your boat. This significantly lengthens the lifespan of props, struts, rudders, thru hulls, etc.

There are several ways in which zinc is connected to a boat:

  1. In the DIRECT method, the zinc is attached directly to underwater metal, i.e., shaft collar zincs, strut and rudder zincs, trim tab zinc, etc.
  2. In the BONDED system the metalwork is wired to each other inside the boat and all the wires go to a centralized zinc. Zincs measuring 4×6 inches and 6×12 inches are used for this system.
  3. In the CONTROLLED system a device controls the corrosion of the zinc so that it takes longer to decompose.

Sometimes a combination of more than one system is used. Zincs should be replaced on a regular basis. The general “rule of thumb” recommends replacement when the zincs are reduced in size by 50%. There are two types of zincs, MIL SPEC and NON MIL SPEC. MIL SPEC (military specification) zinc will slough and slowly becomes smaller. NON MIL SPEC creates a hard shell around itself, which needs to be knocked off to expose the good zinc underneath. I personally prefer using MIL SPEC zinc because it is easier to tell when it needs to be replaced.


Another problem which can quickly destroy your boat’s metalwork is a STRAY CURRENT. This happens when a direct current from the generator, dock cords, batteries, ect., somehow comes in contact with the metalwork. The resulting damage is similar to putting the metal into an acid bath. I once saw a thru-hull dissolve in a week! If this happens to your boat you need to have it checked out as soon as possible.


This is simply meant to be an introduction to the subject of electrolysis. For more details and specific information check with an electrolysis expert.