Greg,

You have a serious stray current problem if your anodes are disappearing that quickly. This is very important to fix. Once your anodes are destroyed, the next thing to go will be other underwater metals. If there is enough current flow, very serious damage can occur very quickly (in weeks, not months!). Ordinary galvanic corrosion is a slow process.  Stray currents accelerate this by many orders of magnitude.

Somewhere on the boat you have an electrical connection between the battery and the bonding system that should not be there. Amel installed the "MAS" light to help find this problem.

The anode on the bowthruster that was not destroyed is not properly connected to the bonding system, but should be.  

Bill Kinney
SM160, Harmonie
Simpson Bay, Sint Maarten