what about water balast ?
That is a little bit of a tough question because not all 9.9 hp engines are the same. Prop size and type, length of the shaft, gearing as well as 2 or 4 stroke comes into play. In a general sense 9.9 hp should be more than enough in flat water. Any outboard is next to useless in short chop and unfortunately, the Gulfstream and the Bahamas can dish out some of the worst short chop there is.