Fuel Consumption at Cruising speed in 46 ft SCB

[Thomtyler]

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.

[Scb46]

If you are able to find out about the mileage, please let us know. Thanks

[dbriceno]

I was getting about 25g/hr combined at 19-20 knots. About .8 mpg

[Scb46]

Thanks
Very close to what I had figured.
Ron

[shrew]

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.

[dbriceno]

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.

[Scb46]

It’s all good. Condition of boat and props will figure in the too. I’m averaging a $100/hr on step, 20 hrs...ish. $60/hr @12kts
Thanks
Ron

[shrew]

Originally Posted by dbriceno

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.

[shrew]

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.

[shrew]

Originally Posted by shrew

In this case, calculating kmpg will potentially lead to a significant miscalculation.

Correction 'Calculating Nautical mile per gallon'.

[BakesFam]

Were you ever able to track your miles MPG with the 4600 SCB. I know this is an old post and I know MPG can be a topic of debate but I am just looking for an estimated average if you have one? Thanks

[Davestic]

Originally Posted by shrew

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.

Hi Shrew,

Can you help me with some data on your 4600?

I am looking to purchase and can't seem to find this info online. The boat I am looking at has the 450 Cummins. What I am looking for is what the fuel burn/speed curve is at different RPM's.

I don't know if you have this info - or if someone else does. It would be greatly appreciated!

Best,

David

[shrew]

I dont have such a breakdown.