Yes the zinc won't work at all in freshwater, as the aluminum outdrive will become the sacrificial anode. Something to keep in mind that if you do boat in salt water, keep in mind that once the boat comes out of the water a barrier coat will form over the zinc and cause it not to work until its burnished off prior to putting it back in the water. This is why manufactures normally use aluminum anodes (an alloy that is different from the outdrive) as they work in both fresh, salt and brackish as well.
If I had a trailer boat that went back and forth, I would use the Performance Metals Navalloy anodes.
http://www.performancemetals.com/anodes ... ucts.shtml I haven't used them personally, but have heard good things about them.
If I was going to be in fresh water only I would and did use Magnesium from boat zincs. Just make sure that you never put a boat in with Mg into salt. Its just a few hours and you will start to loose the paint is all it takes.
