Is yours this small rib?
If so, having owned a 9.8 Tohatsu and used it with a 3.1m old Zodiac SIB that weighed the same, it should easily plane with one adult and one child. Mine would plane the Zodiac at 16mph with an adult and two children or two adults. From a standing start it would only take a few seconds to rise over the hump onto the plane.
To be honest the prop, assuming it's the standard one, isn't so critical to stop you planing. Another guy here had just the same speed problem recently with a 15hp only to find after loads of discussion that his outboard was only firing properly on one cylinder.
BTW your standard prop should be marked something like F8.5 for an 8.5" pitch.