I'm a little confused about something.
With a 50 gallon tank, my calculated range is about 200 miles @ 2300 rpm (~7 mph)
If I reduce to 1800 rpm (5 knots), my range per tank drops to 180 miles.
Why does the range drop if I use lower rpm (lower fuel consumption rate)? I expected greater range per tank with lower rpm. I know it would take longer to get somewhere at lower rpm (lower speed), but why can't I go farther @ lower rpm?
What I do not know if my actual fuel consumption at a given rpm. I have never measured it. These values are from a power/fuel consumption curve.
Engine is a Universal 5444.
Formula I used: (tank cap. divided by gal/hour rate ) x (speed)
Example @2300 rpm: (50/1.75)x7mph = 200 miles/tank
Below are my input data:
gal/hr @2300rpm: 1.75
gal/hr @1800 rpm : 1.4
gal/hr @2000 rpm: 1.55
Total tank capacity is 50 gallons.
Speed at those rpm:
2300 rpm: 7 miles per hour
1800 rpm: 5 miles per hour
2000 rpm: 6 miles per hour
You may be reading the engine graph incorrectly. My engine curve is for the Westerbeke 35DThree so it is different than your engine curve. My readings are as follows:
1400rpm = 0.25gph = 4.45mph = 16mpg = 800 miles
1800rpm = 0,50gph = 6.00mph = 12mph = 600 miles
2000rpm = 0.63gph = 6.50mph = 10mph = 500 miles
My calculations are in miles since we predominately cruise in the ICWW.
I use my 4-107's actual burn rate of .75 gph @ 2300rpm (my cruise rpm). That gives me approx 70hrs of running time.
I then use 5kts to calculate average range (worst case depending on conditions) and get 350nm.
Since we also carry 4 x 6 gallon jugs, that gives us about 7 hours each or 35nm. So in my mind, our range is about 500nm.
It's a rough estimate since actual conditions vary so much (beating into the trade winds, we have been as slow as 2.5kts), that it works out pretty close.
Also. our boat is always fully loaded for cruising at over 20,000lbs which probably makes us much slower than yours however.
Bottom line...just measure your ACTUAL burn rate and go from there.
We get 0.6 gals per hours at ~1800 rpm at max endurance cruise. That sends us out at 5.3 knots when there is no wind. This is based on over 4000 miles of cruising to date.
This is going to vary widely for other people depending on what prop they have, how clean and fair their hull and other factors.
One thing for sure is that the Beke just isn't big enough to propel our boats at hull speed in most all conditions without using a ton of fuel.
Wayne
Propelling a displacement hull at hull speed uses a ton of fuel, regardless of the horsepower of the engine.
Seems to me the only way to calculate what your fuel consumption should be, would to somehow know what HP you are putting to the propeller shaft. You can't assume you are using the rated HP of your motor.
So if I work backwards from Wayne's number of .6 Gallons per Hour at 1800 RPM. The Westerbeke graph says that is 14 HP on the propeller curve of the 107-4 data sheet.
My Westerbeke engine which is a marinized Mazda (same as a Perkins 154) rates the fuel consumption in Lbs./ Horsepower Hour. An archaic unit of measurement mostly used by the Railroad industry. But looking at my graph for fuel consumption it is actually lowest at 2300 RPM. It goes up below that and way up above that speed.
But converting .38 Lbs/ Hp Hr to gallons gives .053 Gal/hp hour. And if I can guess that I'm putting 18 hp to the prop, ( I have a bigger prop) and if this is accurate, my consumption will be .95 gallons per hour. Not bad because I think I can do 7 knots.
I will have to see. But based on empirically measured fuel consumption I can calculate the load on the motor in HP. Pete