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Old 09-25-2020, 09:42 AM   #7
MYTraveler
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Fuel consumption of a diesel engine is very largely a function of how much power it is producing. Those curves typically show how much power a particular engine can produce at various rpms, and how much fuel is consumed at that power. Any given fixed propeller needs disproportionately more power as rpms increase. If a prop is pitched such that the power required to turn it when the engine is at max rpms is very close to the max power its engine can produce at those rpms (any more pitch would create an "over propped" condition), it will not need nearly the power that the engine can produce at lower rpms (there are exceptions, for example the discontinuity at the speed where the boat is coming on plane), and will consume correspondingly less fuel than the engine would consume if max available power were demanded of it at those lower rpms.
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