I don't quite understand why but these alloy anodes by Martyr and some others are more active than zinc and can even protect an aluminum alloy sail drive. I don't know why this should be. You can see in the link I provided.

You can also check out West Marine's "West Advisor" and choose "Preventing Galvanic Corrosion"

With all of this to back up the claims for aluminum anodes, I fee reasonably safe.

Not sure about my Yanmar 4JH3TBE but assumed it was probably an iron block. I think that's a side issue but wouldn't mind knowing the answer. I'll try a magnet on it next time I'm there!