It appears to be along the nominal propeller curve. Note that the propeller curve on those drawings is a nominal curve based on a simple exponential expression and the requirement that the propeller curve and the engine output curve intersect at maximum rated engine rpms. That propeller curve can differ greatly from the actual curve for a boat depending on the details of the hull form, the transmission gear ratio and the installed propeller (diameter, pitch, blade form and count). It will give you an "approximate" fuel burn. The real propeller curve is easy, if a bit complicated, to calculate if you have some information about the boat (hull lines, center of buoyancy, center of gravity, displacement, surface roughness, etc) the drive train and the propeller.