Nocomplaints:

While on plane, you are generally correct that fuel consumption is linear
with rpm. Once you fall off of plane there is a peak as the boat plows a big
bow wave. It then drops exponentially towards idling rpm.

Gas engine manufacturers rarely give you prop horsepower and fuel
consumption vs rpm curves. To see the curves for a diesel go to the Yanmar
or Cummins marine engine sites.

David