my 9/15 started life as a 12/18. the 9/15 will let the boat spin 200-250 more rpm. In a blaster with right setup it will turn 7200. It will out accelerate the 12/18 but might be 1 mph slower up top.Not exactly the same but I am running a Solas 12/18 big hub in my 96 B1 with BPipe etc, awesome low end in the surf but can still get decent speed up on top. I went with the big hub thinking of future upgrade to mag pump, not done it yet but still love this prop!
LB