A few photos showing the importance of properly selected and installed sacrificial anodes
These are proper shaft anodes after one season of use (about 4 months). They can be compared to a new anode.
Here's a photo of an improperly installed anode. This rudder anode was installed over a painted surface and the rudder box (as well as the strut and shaft log) was not bonded to the common ground. Compare the new anode to the single season anode. There is little consumption. It is no wonder the shaft anodes pictured above show more consumption!