Top rpm is 3600, and hull speed is 6.8 kn. Cruising speed is something you want around 80% of max rpm. Before doing anything, I would try the boat on a calm day for a run with the current and same distance against the current at top speed. Then average both runs to see what you real top speed is. In your case, 2880 rpm would be about 80%. So your 3000 rpm is close enough. And therefore 6 knots sounds about right ! As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). .... Do the maths with your LWL and compare with the results of the test mentioned above. As an example, my 34 with a 3GM30F engine has a top rpm of 3400. Hull speed tested at max was 7.1 knots. Coincidentally, the calculation with the LWL also give me 7.1 ! 80% of 3400 is 2720. I cruise at roughly 2650 and get 6.3 to 6.5 knots depending on conditions. I run a 3 blade 15 X 11 RH. I used to run a 15 X 15 2-blade. Went to the 3-blade to get more punch in heavy seas and increased maneuverability. Speed remained the same. Don't forget it is a displacement hull and you can play all you want with the prop, physics will not change and hull speed will remain pretty much in the same range. I did notice a small drop in speed under sail in light air. Now if you really want higher speed, do find a way to increase your water length...