I'm signed up for a dive service that checks my zincs and replaces them as needed. I have them leave the spent zincs on the boat. The boat is in the water year round here.

A couple years ago I added a hull zinc and when at the dock hung a zinc "grouper" in addition. The goal was to decrease the prop zinc replacement costs. That seemed to work. I was needing a prop zinc replacement about twice a year.

Then last fall I needed a new zinc after only 4 months and the next one only 3 months later. First rule of troubleshooting: what changed?
There's a steel boat next to me now. Hmmm.
The last two spent prop zincs look like pumice, not the surface of the moon. Hmmm...

I talked with the diver. Oh yes, he's using aluminum anodes now because they are better for the environment. Zinc anodes also typically have some cadmium. Both elements are bad for marine life. Indeed, aluminum anodes do seem to be a good choice all around (see the West Advisor article among others). Aluminum anodes are good, but the recommendation is that if you go to aluminum you should make sure that all your anodes are aluminum. In my case, the more galvanically active aluminum prop anode assure that the hull zinc and zinc grouper effectively did nothing!

Since I still had a zinc grouper and a zinc hull anode, I got, the diver to revert to zinc for the prop anode.

Then I got a BFO (brilliant flash of the obvious)... Why not use an aluminum grouper? That would take over pretty much all the load. It would protect the stainless prop shaft, brass stern tube, bronze propeller, and various rudder bits as well. Oh, and it would protect the hull zinc and the prop zinc!

So far so good. Current flow through the zinc grouper had been 8 milliamps. The flow through the aluminum grouper is 35 ma. That suggests that it is taking most of the protective load.

So far I haven't found any reason to think that what I have done is a bad idea but I'm always ready to learn from other people in preference to learning the hard way from experience!

By the way, the "aluminum" galvanic protection anodes are an alloy of mostly aluminum, about 5% zinc and a small amount of Indium. Pure aluminum isn't workable because it tends to oxidize itself such that you can't maintain a good electrical connection. (Remember the aluminum house wiring fiasco?)

The move to aluminum anodes is a happening thing. Imtra now sells only aluminum anodes for their bow thrusters. Isotherm doesn't yet have aluminum anodes for their sink drain heat exchanger units. Martyr makes a large selection of aluminum anodes that meet mil specs.