Displacement hull theory is also working against you. The thrust curve is exponential. As you approach planing speed, the thrust required to go a little faster is huge. The difference in speed between the original 46 pound thrust MG and my new 82 pound thrust MG is less than 1 mph on my 17.5 foot boat. The top speed with the big TM is a little over 3.7 mph. I could go about 3 mph with the smaller TM.

The length of your boat is also working against you. Longer boats can go faster on the same power when not on plane. It has to do with the wake interaction from the front and back of the boat.