Propeller and Shaft Anodes
If you are new to galvanic corrosion, welcome to one of the most insidious forms of metal corrosion found on boats. What is it you ask? Galvanic corrosion. Often erroneously called "electrolysis", this is an electro-chemical reaction that can damage or destroy underwater metal parts of boats. This reaction can occur when dissimilar metals touch each other and are immersed in an electrically conductive fluid, like saltwater. One of the metals (the “least noble”, called the anode) will corrode faster the other (the “most noble”, or the cathode) will dissolve more slowly.
Example of Galvanic Corrosion
A typical example of damage due to galvanic corrosion can be seen where a bronze propeller (the anode) is installed on a stainless steel shaft (the cathode). Since bronze is less noble than stainless steel, the propeller becomes anodic to the shaft and "sacrifices" itself in a galvanic reaction. Evidence of this damage can be seen in bronze propellers that have turned red, or in later stages become eaten away.
How to Prevent Galvanic Corrosion
Going back to our example of the bronze propeller installed on a stainless steel shaft, the best way to prevent galvanic corrosion from damaging the prop is to install sacrificial anodes on the shaft, on the prop or on both. In this case, zinc, aluminum or magnesium anodes, being the "least noble" compared to the stainless steel shaft or bronze prop will take the place of the prop as the anode and sacrifice themselves in the reaction, thus protecting the prop.
Shaft Anodes
West Marine offers a variety of shaft and prop anodes. Shaft anodes, also called "collar" anodes are offered in a range of diameters to fit shafts from 3/4" up to 4" in diameter. Shaft anode shapes include streamlined, egg-shaped collar anodes and donut-shaped, low-clearance collar anodes.
Prop Anodes
West Marine offers several types of prop anodes. These include generic propnut anodes, which are designed to fit the propellers on most boats. Generic prop anodes are offered with or without a prop nut, which is a conical bronze nut that screws on to the threaded end of the shaft. The propnut anode installs over the propnut to which it fastens with a single propnut screw. Propnut anodes are offered "complete" with the propnut or as a replacement anode alone.
Specialty Prop Anodes
Feathering propellers, such as those by Max-Prop and others found on Beneteau sailboats require special propeller anodes. Max-Prop anodes are designed with a bolt hole pattern designed to fit these types of props. Beneteau prop anodes can be purchased with, or without Beneteau's unique propeller nut.
What anode material works best?
In the past, most sacrificial anodes were made of zinc, which is why they (regardless of their material) are still often referred to as "zincs". However today, boaters have a choice of three different anode materials: Zinc for saltwater, aluminum for salt- or brackish water and magnesium for freshwater. It is important to install anodes made of the correct material for the type of water where the boat is berthed or used. Not doing so can result in little or no protection against galvanic corrosion.