Interesting read here: http://www.thehulltruth.com/boating-forum/22884-stainless-steel-prop-vs-aluminum.html
Just remember that the ally prop is the weakest link when you hit something. If it isn't anymore, where will that force go? Gearbox?
Also you will need to drop pitch etc.
Some of the raider guys had joy with 4 blade ally props, but, you may already have that, doesn't help if you clonk it mind!
R