Originally Posted by billknny
Fuel consumption on a diesel engine
is a function of the RPM AND the load. Under no, or light load, the fuel usage can be 10% of what it might be at the same rpm under maximum load.
Sure, revving up in neutral is a different issue - but if you are turning your prop through the water
, load and RPM are directly related.
The load (and hence fuel consumption) when running at various boat speeds varies a lot depending on windage and sea state.
That's because your RPM will vary according to that load.
Talking about gph versus boat speed is pointless.
Consumption versus RPM is the critical factor and will be fairly constant.
It may take 2800 RPM to make 5 knots in heavy weather
and only 2000 RPM in good conditions or conversely 2000 RPM may give you 5 knots in good conditions but only 3 knots in adverse conditions - but gph at 2000 RPM will be much the same in both sets of conditions, as will gph at 2800 RPM.
If you know how much fuel you have, you can tell how many hours you will be able to motor
at any given RPM. How far you will travel in that time though depends on your SOG at that RPM - which obviously depends on wind
, sea state and currents.