Ebb,
From what I have gathered in my reading of rudder hydrodynamics, the reason for the doing away with feathering and keeping a flat blade would be to prevent the blade from stalling when turning. A feathered blade shape when turned at an angle would induce the fluid to create a vortex on the low pressure side of the blade, thus causing the blade to stall and the boat won't turn. If the boat were always moving in a straight line the ideal blade shape would taper off in an infinitely small edge to have the least amount of drag. I'm sure this is a very oversimplified understanding as there are a lot of variables involved, but makes sense to me.
Here is the spec sheet of the Silicon Bronze I got a quote for at Alaskan Metals it is a PDF, hope that comes across. I also included the spec sheet for Aluminum-Nickel Bronze which seems like another possible choice due to its strength and lack of zinc, but I have not seen much discussed here.