OK, you asked for it ;-):
Theoretically a propeller
absorbs horsepower in an exponential relationship to its rpm. That exponent varies from 2.5 to 3.0 for various propellers and boats. So lets use 2.7.
The formula is hp = f *rpm^2.7 where f is a factor based on a lot of stuff- transmission
ratio, diameter, pitch
, blade area, etc. But since we know the hp of the engine
is 425 at an assumed rpm of 5,000 and all of that power is absorbed by the prop (if propped properly) we can back calculate the f factor.
So for your engine the factor is .0000000438. Plugging that into the formula for various rpms gives the following and dividing the hp by 12 hp per gph (a good rule
of thumb for FI gassers) gives the gph burned at each rpm:
Now all of this is theoretical and based on the 12 hp per gph rule
of thumb which is no good at low rpms. It takes a certain amount of hp just to turn over the engine machinery irrespective of the load on its crankshaft. Also the thermodynamics of a gasser engine get poorer at low rpms and low power
loadings. So don't pay any attention to the 1 and 2,000 rpm values.
But the table definitely shows the benefits of slowing down on fuel consumption!!!! Your best fuel consumption
will probably be the point where the boat is just up on a comfortable plane. Any more just burns more fuel.