Is that 16' sanger a flatbottom?? If so good luck finding a 21" pitch prop for it. That is way, way too much pitch. It will give your boat so much lift at the tail it will not even be funny. 17" pitch props are just about the limit as far as pitch goes and still having a good handling boat. I ran an 11 X 16 on my flatbottoms. You are only running a 14% gear so you should step that up big time. What prop are you running now? It might be fine with the correct gear.