Actually... get the ESC rated for 10% to 50% more current than the motor. The size prop will be dependant on the motor power... and with a 9 v rated motor its probably going to be small. (probably too small to move the Sunfish) I'd expect to need a 1500 watt minimum (appx 2 hp) and probably want 2500 to 3000 watts to be able to "motor out" at a reasonable speed. Its not going to spin a standard trolling motor prop. Its going to spin an RC boat prop. Much smaller and much faster RPM and very inefficiently.