Planty
New member
I hope I can do this justice and you understand it all, Mathemetician, I aint!!
But as requested earlier last week here are the calcs I found in a brilliant book that have literally saved us £100's, by cruising most economically. It takes some reading and understanding (eventually) but stick with it, IT WORKS, here Goes.
Do sea trial on calm day, set engines at lowest revs, allow to settle at a speed, note speed against revs, set revs up 100revs, allow to settle note speed, up another 100, note speed, etc. etc. etc. to flat out!! Now if you plot a graph of Boat Speed divided by engine rpm/1000 (Yaxis) against Engine RPM/1000 (Xaxis) you will get a graph that shows a distinct S curve showing least efficient and most efficient RPM.
To work out fuel consumptions FOR YOUR BOAT at given RPM's. (I just did it for our most efficient RPM) you will need the manufacturers techy sheet for your engines including torque curves, mpg curves etc. Then note the following for a given RPM:
N1 = Engine Maximum RPM
N2 = Engine RPM consumption figures Required for.
N3 = Engine max output (horsepower at Flywheel)
T1 = Engine Max Torque
T2 = Engine Torque at N2 RPM
Z = Fuel Consumption in grams/hp hour at Max RPM.
(N1, N3, T1 & Z are taken from engine Tech sheet)
Now comes the clever bit!! The calc, to obtain LITRES PER HOUR is :
N3 x N2 x T2 x Z divided by N1 x T1 x 830, (why 830? No idea!!) REMEMBER THIS IS PER ENGINE!!
Obviously once you've done all this you can ascertain the litres per mile too, litres per hour divided by miles per hour = litres per mile.
Thus range can also be calculated, by Tank(s) capacity divided by litres per mile.
I hope this all helps and it makes at least a little sense, if you need any help, PM me and I'll send you the ISBN of the book I originally saw it in. Does work though I assure you. On our F43, 2100 revs 23-24 knots is most economic and I really can't believe how much we have saved this season by cruising most places at this speed. Can't resist the odd blast up to 33 though!! Paul
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But as requested earlier last week here are the calcs I found in a brilliant book that have literally saved us £100's, by cruising most economically. It takes some reading and understanding (eventually) but stick with it, IT WORKS, here Goes.
Do sea trial on calm day, set engines at lowest revs, allow to settle at a speed, note speed against revs, set revs up 100revs, allow to settle note speed, up another 100, note speed, etc. etc. etc. to flat out!! Now if you plot a graph of Boat Speed divided by engine rpm/1000 (Yaxis) against Engine RPM/1000 (Xaxis) you will get a graph that shows a distinct S curve showing least efficient and most efficient RPM.
To work out fuel consumptions FOR YOUR BOAT at given RPM's. (I just did it for our most efficient RPM) you will need the manufacturers techy sheet for your engines including torque curves, mpg curves etc. Then note the following for a given RPM:
N1 = Engine Maximum RPM
N2 = Engine RPM consumption figures Required for.
N3 = Engine max output (horsepower at Flywheel)
T1 = Engine Max Torque
T2 = Engine Torque at N2 RPM
Z = Fuel Consumption in grams/hp hour at Max RPM.
(N1, N3, T1 & Z are taken from engine Tech sheet)
Now comes the clever bit!! The calc, to obtain LITRES PER HOUR is :
N3 x N2 x T2 x Z divided by N1 x T1 x 830, (why 830? No idea!!) REMEMBER THIS IS PER ENGINE!!
Obviously once you've done all this you can ascertain the litres per mile too, litres per hour divided by miles per hour = litres per mile.
Thus range can also be calculated, by Tank(s) capacity divided by litres per mile.
I hope this all helps and it makes at least a little sense, if you need any help, PM me and I'll send you the ISBN of the book I originally saw it in. Does work though I assure you. On our F43, 2100 revs 23-24 knots is most economic and I really can't believe how much we have saved this season by cruising most places at this speed. Can't resist the odd blast up to 33 though!! Paul
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