Hull Speed vs RPM
The rule of thumb that I've used is that you have 18 hp-hrs in a gallon of diesel. For a 2GM20 turning 2800 rpm with a 14dx12p prop cruising @ 6.3 knots, I burn just over a gallon in 2 hrs. (about 9hp)
If I run at 3200 rpm, I burn about an additional 1/2 gallon without much improvement in hull speed. (about 14hp).
See attached graph....
Cal 9.2 #19 SilverSwan