I think something needs to be cleared up here. We seem to be mixing up four different measures of efficiency.
The term used to describe an engines efficiency is called its brake specific fuel consumption (BSFC). The definition is the amount of fuel burned per amount of horsepower generated. An engines specific fuel consumption is determined by the engines design, and nothing else.
engines specific fuel consumption will varie with its RPM. For a diesel
, the best specific fuel consumptions are found in the diesels higher RPM ranges. What this amounts to is that if you want to get the most horsepower for the amount of fuel burned, then run your diesel in its higher RPM ranges.
The second measure of efficiency is the amount of fuel burned per amount of speed generated. This measure of efficiency is determined by the engine, the drivetrain, the hull
, the hull
speed and environmental factors such as wind
and chop. To double the amount of speed a boat goes through the water, you pretty much must cube the horsepower. This means that you can save an enormous amount of fuel by backing down a few knots.
The third measure of efficiency is the amount of fuel burned per unit of distance. Slowing down the boat maximizes this efficiency by a tremendous amount.
Pitching the boat correctly really has nothing to do with any of these efficiencies. To maximize this fourth type of efficiency, you must pitch
the boat so that it can reach its maximum governed RPM. Overpitching as well as under pitching will cause less efficiency. Think of this curve as the X-axis being the pitch and the Y-axis as the fuel burned per mile. The curve is a parabola and you want to be at the very peak of the parabola. It works out that you will be at the top of the parabola if you pitch the propeller so you can reach the maximum governed speed, and no more.
What all this amounts to is that with a properly pitched boat, slowing down will minimize your fuel consumption per mile even though your diesel engine is not running at its most efficient RPM on the brake specific fuel consumption curve. The reason is that it takes a lot less energy to push a hull through the water at lower speeds. This difference is a much greater difference than the difference between the BSFC at different engine RPM's.
For practical reasons relating to engine longevity, its a good thing to run your diesel at maximum governed RPM at full load periodically to burn the carbon out of the combustion chamber. Cummins recommends 1/2 hour for 10% of the engines operating time.
I hope this helps to paint
a clearer picture of the four different types of efficiencies that are being discussed.