It is a good question. Did some quick reading and from what I found the aluminum anodes are more active. If placed on the same circuit as zinc they will protect all the metals including the zinc. It makes sense that if the aluminum goes first zincs will kick in later. If the metals are not connected (bonded) to one another no harm is done they will protect whatever they are attached to.
When I do bonding checks propeller shafts usually have the highest resistance connection to bonding systems. Keeping an anode(s) on the prop would seem to be good insurance. If you put an aluminum there and zinc everywhere else it would be depleted more rapidly negating your efforts.
Aluminum has not caught on in the area I am located (brackish) possibly due to lack of local inventory. I find a lot of sleepy zinc on hulls. Most are caked with a layer of general corrosion.
Originally Posted by jhance
Is there any harm in mixing the two? From what I am reading, it's OK to do. My boat is in salt water year-round (San Juan Islands, WA) and I have anodes on the transom, skeg, prop shaft, swim step struts, and bow thruster.