They cost about the same. An interesting disadvantage from someone who said they didn't think they last as long as zinc. That may be because when the zinc gets into fresh or brackish water it developes some coating which makes them ineffective, so they might last forever. So would a piece of plastic, but it may not protect the boat. I can't see any reason to have zinc anodes at all. (Supposedly the ones in the engine and genes are in a different "lake," bonding wise, and don't need to be changed at the same time as the others.