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Old 03-09-2019, 12:59 AM   #1
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Default Fuel Consumption at Cruising speed in 46 ft SCB

We have a Maxum 46 SCB with 2 x Cummins 450HP Diesel engines. Cruising at 1800 rpm's has anyone worked out estimated fuel consumption and speed?
Would really appreciate any input.
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Old 03-09-2019, 05:33 AM   #2
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Default 8.3 Cummins

If you are able to find out about the mileage, please let us know. Thanks
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Old 03-10-2019, 08:28 PM   #3
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I was getting about 25g/hr combined at 19-20 knots. About .8 mpg
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Old 03-11-2019, 02:10 AM   #4
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Thanks
Very close to what I had figured.
Ron
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Old 03-11-2019, 03:49 PM   #5
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Because speed (SOG) can vary for the same RPM due to sea state, tide, current and wind, fuel consumption over distance is not an accurate measurement. Only consumption at RPM over time is accurate.

Therefore MPG is not a relevant number when calculating fuel consumption. GPH based on a variable RPM is what is used.
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Old 03-11-2019, 04:27 PM   #6
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You sound like you know it all. GPH doesn't tell you how much fuel you will need to get somewhere. And since time is spent in and out of 5mph zones at varied rpms GPH doesn't tell you squat. I know I need about 125 gallons to travel 100 miles. Of course that depends on tides and sea conditions. That is obvious.
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Old 03-11-2019, 07:08 PM   #7
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Itís all good. Condition of boat and props will figure in the too. Iím averaging a $100/hr on step, 20 hrs...ish. $60/hr @12kts
Thanks
Ron
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Old 03-13-2019, 12:45 PM   #8
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Quote:
Originally Posted by dbriceno View Post
You sound like you know it all.
My apologies, that wasn't my intention. Do you have examples of consumption specs that rate based MPG? I'd like to make sure I'm clear on my oversight.
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Old 03-13-2019, 03:14 PM   #9
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Incidentally, here is an example of my rationale which I strongly encourage anyone to correct my mistakes or oversights.

Distance: 30 Nautical Miles (NM)
RPM: 2400
Fuel Consumption/hr @ 2400: 2 Gallons
Tide/Current: 2kts
Speed @ 2400: 8 kts

Speed into Head Sea: 6 kts
Speed with Following Sea: 10 kts

(30 / 10) * 2 = 6 gallons (0.2 kmpg)
(30 / 6) * 2 = 10 gallons (0.33 kmpg)

This a 66% increase in fuel consumption. Obviously a planing boat would see a less dramatic impact, however, the difference is there. In this case, calculating kmpg will potentially lead to a significant miscalculation.
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Old 03-14-2019, 12:56 PM   #10
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Quote:
Originally Posted by shrew View Post
In this case, calculating kmpg will potentially lead to a significant miscalculation.
Correction 'Calculating Nautical mile per gallon'.
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