Took my new rig out this evening for a quick ride to see how it performed. First off my mudbuddy is kind of a weird combo it's an 04 hyperdrive with a Honda gx620 on it rated at 18.1 hp.
With 2 guys around 210 per, two group 27 batts and 6 gals of fuel the best we could do was 19.6 on the gps. The motor was turning approx 3300-3450. This combo wll be good for a lil while, I wanna upgrade motors as soon as I can swing a new one. I'd like to get somewhere in the high 20's with this rig.

change the gears, I am running a 24 Honda with 1.47:1 gears. I turn a godevilsd prop 12x12(ones used for the 35) at 3800 rpms @ 25 mph. So try gear reduction and pro, i bet you can get low 20's out of that combo. FYI i run an old beat up 1542 hull, guessin i would get more out of a smooth bottom hull.