Hey, I have a alumacraft 1436 flat bottom with a 13hp beaver dam mud runner. I am usally running a two man rig with battery, trolling motor, and 2 gal gas tank, and fishing gear. The most I got out of it was 15mph with the 8" prop. I have a 8.5" prop and was wondering if it might help gain a few more mph. Any help would be appreciated.

IMO the speeds your getting now are about as good as your going to get with load your pushing. 1/2" larger pitch or diameter won't make to much difference in speed, but may pull motors WOT rpm down. Really need to know what RPM your pulling with prop your running now.