I have a 18.5' 1989 Ski Deck made by Anchor Ind. It is powered by a Ford 302 (5.0L) with a OMC drive (M/N 985687). I currently have a 4 blade 14.75 x 15 that came on the boat when I bought it and a 4 blade 14.5 x 18 a friend gave me. I also have a 3 blade 15 X 17 that was in the boat as a...