I'm no expert, but from a chemistry point of view, zinc is even further from brass or bronze than aluminum is. More dissimilar = more likely to degrade easily? More sacrificial and hence more protective? Many of the sites offering aluminum anodes say they last longer than zinc - is that a good thing? Don't you *want* them able to degrade easily?
article will shed some light?
One advice that has been consistent - all your sacrificials should be made of the same material (otherwise they sacrifice for each other, rather than your prop/shaft!!!)
Hope this helps at least a bit.
Thanks eryka! OK, after reading the article in your link, and also re-reading some Nigel Calder as well as some West Marine catalog, here's what I figured out.
You are correct that aluminum is more "noble" than zinc. The order would be aluminum, zinc, then magnesium. So this confused me--why would they recommended zinc anodes for salt water, aluminum for brackish water, and magnesium for fresh? That seems out of order.
OK, so according to the West Marine catalog, in brackish water, zinc can become less active, and become covered in zinc oxide film which totally shuts it down, leaving other metals at risk. Aluminum apparently doesn't do that.
This is all new to me, so if I'm wrong, please set me straight. Anyone? Anyone? Bueller?
Of course, even if this is all accurate, if teh vast majority of boat-owners in my area are successfully using zinc, then I have no reason to do anything different. Less complicated is a good thing.
Eryka, do you use zincs on your boat? Our boat won't be very far away.