The Ocean Eats Propellers
BELFAST, ME — Here’s a side of the Sunbeam rarely seen: the underside. Notice the gray square blocks attached to the boat in strategic places? These zinc “Sacrificial Anodes” are very important and useful. They keep the salt water from eating brass propellers.
These photos show the zincs in the stern near the Sunbeam prop, and also, on the bow near the prop in the bow thruster. As of this writing, the zincs are all submerged. Sunbeam Engineer Storey King says zincs have a lifespan of about three years.
Whenever different metals are placed in a conductive liquid, such as salt water, you create a battery. If you connect these pieces of metal together, current will flow. This current, trying to equalize the conductivity of the metals, will be removing metal from one of the metal pieces. This removal is called “electrolysis”. If the piece being removed is…one of the pieces is your propeller — it is bad.
When you pull your boat to do the bottom you may wonder what those pitted, ashen-white pieces of metal are on your shaft, rudder or possibly on the transom. These are called zincs and, as luck would have it, are made of zinc. The zincs you use on a boat are called “Sacrificial Anodes.” Zinc is used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller.