Drag characteristics of RIB

The "RIB" part of the question is a red herring. You are talking about the characteristics of a small planing boat, which will vary widely with weight, beam, deadrise angle, strakes etc. 20 knots is pretty slow as the top end of your range of interest, a loaded up boat might not even be planing cleanly at 20 knots. The answer to the question is "it depends on a few things".

 

I agree with a lot of that. My main point was that the high hump resistance meant that the OP's problem was a difficult one. In reality, we will happily take the problematic characterstics of the RIB because it has so many good points.

 

This happened to us. We tested a small boat, catamaran split hull, about 26 feet with outboard. As it reaches its top speed at 26 knots, the bow rises and starts to hobby horse. We cure this by increasing the motor thrust angle. The bow goes down, we reduce throttle, get the same speed and the boat planes beautifully.

We get the same behavior with a bow rider monohull of a slightly longer length. The bow would go up as we reach max speed so we transfer the crew to the bow and it behaves beautifully. No problem at low speed.