All that is needed is one zinc on the shaft. I locate mine in front of the strut with enough room to remove the shaft coupling from the shaft at the transmission while the boat is in the water. This is usualy about three inches from the zinc to the strut.
Thanks, Dave. Do you have any idea of the propshaft Diameter? Our boat is in MA, and I wanted to order the zinc ahead on time, and I cannot find the info anywhere. I think I saw 1 1/8" somewhere, and I 1 1/4 in another place on the Net. I put a tape measure on it last summer, but I should have gotten a more accurate measurement.