You can can build a spreadsheet in Excel and graph engine horsepower
and ideal prop horsepower vs. rpm. Once you have the prop curve, you
can also calculate the ideal fuel consumption vs rpm.
I have done this for my last boat and it was very close to actual fuel
consumption. Remember, calculating fuel consumption between fillups is
only an average value and includes both high and low speed operation
where fuel consumption varies widely. The only way to get an accurate
reading other than a Flowscan, is to operate for a long period of time
at a sustained rpm, i.e. 10 hrs @ 1800 rpm, or draw from a measured
container, i.e. 1 gal @ 1800 rpm for 'X' minutes.
bhp @ rpm [prop bhp] =
(bhp/rpm^2.8) [where prop curve meets hp curve] * (rpm^2.8)
i.e., for 160 bhp @ 2600 rpm engine [assume prop curve meets here]
bhp @ rpm [engine bhp] = 110 bph @ 1800 rpm [from manuf. curves]
bhp @ rpm [prop bhp] = 160/(2600^2.8) * (1800^2.8)
bhp @ rpm [prop bhp] = 57 bhp
NOTE: the "rpm^2.8" means "rpm raised to the 2.8 power".
It appears that Cummins uses 2.7 instead of 2.8. This exponent is an
estimate of prop efficiency.
Monte R. Smith
(Between Boats)
Palm Beach Gardens, FL
Do You Yahoo!?
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You can can build a spreadsheet in Excel and graph engine horsepower
and ideal prop horsepower vs. rpm. Once you have the prop curve, you
can also calculate the ideal fuel consumption vs rpm.
I have done this for my last boat and it was very close to actual fuel
consumption. Remember, calculating fuel consumption between fillups is
only an average value and includes both high and low speed operation
where fuel consumption varies widely. The only way to get an accurate
reading other than a Flowscan, is to operate for a long period of time
at a sustained rpm, i.e. 10 hrs @ 1800 rpm, or draw from a measured
container, i.e. 1 gal @ 1800 rpm for 'X' minutes.
bhp @ rpm [prop bhp] =
(bhp/rpm^2.8) [where prop curve meets hp curve] * (rpm^2.8)
i.e., for 160 bhp @ 2600 rpm engine [assume prop curve meets here]
bhp @ rpm [engine bhp] = 110 bph @ 1800 rpm [from manuf. curves]
bhp @ rpm [prop bhp] = 160/(2600^2.8) * (1800^2.8)
bhp @ rpm [prop bhp] = 57 bhp
NOTE: the "rpm^2.8" means "rpm raised to the 2.8 power".
It appears that Cummins uses 2.7 instead of 2.8. This exponent is an
estimate of prop efficiency.
===
Monte R. Smith
(Between Boats)
Palm Beach Gardens, FL
_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com
