You are trying to stay close to the ideal "pump curve" of the RPM, Horsepower consumed. (load/work/whatever).

A fixed prop pitch isn't optimum at all RPM's -- it is just supposed to be balanced (close to optimum) in the range of load, vs. RPM, vs. capacity of the engine, etc.

A lot of the diesel "dust" which is put-out at higher RPM's results from UNBURNED (or partially burned) fuel that's just wasted.

You will inherently burn more fuel per mile at higher ranges in a displacement hull than lower or more moderate speeds.