MapisM
Well-Known Member
Just to throw in another example which - being rather extreme - hopefully gives a good idea of what affects fuel burn:
My 53'/35T full displacement boat has two 350hp engines.
At 8.5 kts, the engines spin at 1600rpm (max rated rpm being 2800).
According to the prop demand curves (exponent=3), at 1600rpm each engine makes 65hp, with a fuel burn of 13.4 lph.
Actually, the boat "only" burns 20/22 lph in total when cruising at that speed/rpm, which means that the hull is a bit more efficient than the prop demand curves would suggest.
But that difference is neither here nor there, in this context.
What can make a HUGE difference is that according to the max power curves, at 1600rpm each engine can deliver up to 153hp, if required.
Now, if you imagine the same engine installed on a planing boat, whose cruising speed is probably achieved somewhere between 2000 and 2400rpm, you can easily guess that at 1600rpm the hull is right in the middle of the transition between displacement and planing, where the props load is at its peak.
Therefore, it might well be that on such boat, at 1600rpm, almost all of those 153+153=306 ponies are demanded to the engines.
Which in turn means a fuel burn of 63 lph - vs. the 20/22 on my boat at the same rpm, go figure.
My 53'/35T full displacement boat has two 350hp engines.
At 8.5 kts, the engines spin at 1600rpm (max rated rpm being 2800).
According to the prop demand curves (exponent=3), at 1600rpm each engine makes 65hp, with a fuel burn of 13.4 lph.
Actually, the boat "only" burns 20/22 lph in total when cruising at that speed/rpm, which means that the hull is a bit more efficient than the prop demand curves would suggest.
But that difference is neither here nor there, in this context.
What can make a HUGE difference is that according to the max power curves, at 1600rpm each engine can deliver up to 153hp, if required.
Now, if you imagine the same engine installed on a planing boat, whose cruising speed is probably achieved somewhere between 2000 and 2400rpm, you can easily guess that at 1600rpm the hull is right in the middle of the transition between displacement and planing, where the props load is at its peak.
Therefore, it might well be that on such boat, at 1600rpm, almost all of those 153+153=306 ponies are demanded to the engines.
Which in turn means a fuel burn of 63 lph - vs. the 20/22 on my boat at the same rpm, go figure.
