Make the right choice

You can not judge the quality of an anode according to its appearance. On the market, one finds the worst and the best. To ensure maximum protection, the zinc must be composed of a minimum purity of 99.996% and have an iron content of less than 0.0014%. If zinc is the most commonly used, there are also specific aluminum anodes for outboard engines or in Hydral (an alloy of aluminum and magnesium) for aluminum boats.

Different Anodes for different surfaces

On polyester boats equipped with an inboard motor, the points to protect are the shaft or motor base, the propeller and bracket and eventually the chair fittings. On a stainless steel hull, one places the anodes close to the propeller (shaft and bracket) and rudder. On a 10 metre steel boat, we require 4 to 5 kg of anode to protect the entire submerged surface of the boat. For a lightweight alloy hull (aluminium,)there are specific anodes and mooring anodes. The mooring anodes are mobile and one places tham around the boat as a barrier to corrosion(2 on a boat up to 12 metres, 4 between 12 and 14 metres.)
The motor bases regardless of type (Z-drive, sail drive ...) and the bow thrusters, are almost always made of aluminum alloy.They have their own anodes that are found in detached pieces. For outboard engines, manufacturers have provided sites for the anodes. It is imperative to respect them and take the anodes corresponding to the engine.
For inboard motors, certain have anodes. These are usually placed on the water system. It is necessary to check them annually and change them if needed (do this at the same time as wintering.)

The anodes should never be painted. To be most effective, they must be brushed regularly especially mooring anodes. The connection between these and the hull must be perfect. It is common that in contact with air, there is oxidation and that the continuity between the anode and the hull is no longer assured.

Anode Wear

Electrochemical corrosion (galvanic corrosion) is a natural phenomenon. It occurs when two different metals are in contact with or immersed in a conductive liquid. One of the two metal acts as anode and one the cathode. The anode corrodes while the cathode remains intact.In practice, it is in ports that ships are most exposed and where protection is more difficult to control. There is, to begin with, the presence of boats, constructed of varying materials ranging from wood, copper passing through steel, stainless steel, aluminium…water pollution and detritus found there are also involved in the process of electrolysis. It is normal that anode wear occurs there and better than if it were instead part of the boat that was damaged (valves, propeller, shaft…)
It is difficult to quantify normal wear. It depends on the boat, its environment and the port. It can be estimated on a boat that is continually afloat that the anodes may be sufficiently challenged (60-70%) to be changed annually. Rapid wear (three months or less) is of concern, ensure that there is no electrical leakage and monitor the neighbouring boats.