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Old 01-03-2020, 07:56   #16
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Re: Fuel Burn - Net or Gross?

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On a slightly tangential issue, I have Volvo engines (D2-40) and I have been trying to get fuel consumption figures at various revs but all I can find in the Volvo docs is a chart that is indicative but not precise. Does anyone know a good source for numerical fuel consumption information?
There is no way to get the kind of PRECISE data you are looking for in a printed graph.

Fuel consumption on a diesel engine is a function of the RPM AND the load. Under no, or light load, the fuel usage can be 10% of what it might be at the same rpm under maximum load.

The load (and hence fuel consumption) when running at various boat speeds varies a lot depending on windage and sea state.
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Old 01-03-2020, 14:34   #17
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Re: Fuel Burn - Net or Gross?

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Fuel consumption on a diesel engine is a function of the RPM AND the load. Under no, or light load, the fuel usage can be 10% of what it might be at the same rpm under maximum load.
Sure, revving up in neutral is a different issue - but if you are turning your prop through the water, load and RPM are directly related.



Quote:
The load (and hence fuel consumption) when running at various boat speeds varies a lot depending on windage and sea state.
That's because your RPM will vary according to that load.


Talking about gph versus boat speed is pointless.


Consumption versus RPM is the critical factor and will be fairly constant.



It may take 2800 RPM to make 5 knots in heavy weather and only 2000 RPM in good conditions or conversely 2000 RPM may give you 5 knots in good conditions but only 3 knots in adverse conditions - but gph at 2000 RPM will be much the same in both sets of conditions, as will gph at 2800 RPM.



If you know how much fuel you have, you can tell how many hours you will be able to motor at any given RPM. How far you will travel in that time though depends on your SOG at that RPM - which obviously depends on wind, sea state and currents.
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Old 01-03-2020, 19:38   #18
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Re: Fuel Burn - Net or Gross?

We have had this discussion a few times. If we do not have an electronic load indicator, the best approximation is to measure fuel consumption at specific rpms for your propeller in average sea conditions, then program the curve into an electronic rpm meter that gives you direct fuel consumption.

One idea, how to do this is have a graduated day tank with the fuel and the return line fed to it, then you do different runs at constant rpms until you burn say 1 fl oz of fuel.

I have noticed that wind will not affect speed that much but waves will. But then you can adjust for that mentally.

The simpler way is to say that most of the time you run the diesel at close to optimum rpms, around 80-85% of max where the vibration/noise balance seems to be optimal. Just use that.
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Old 02-03-2020, 00:35   #19
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Re: Fuel Burn - Net or Gross?

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We have had this discussion a few times. If we do not have an electronic load indicator, the best approximation is to measure fuel consumption at specific rpms for your propeller in average sea conditions,

Assuming that you are suitably propped and just reach published max RPM at WOT that fuel consumption will be very close to the manufacturer's published curve.
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Old 02-03-2020, 07:48   #20
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Re: Fuel Burn - Net or Gross?

Stu, I agree with you. In my case (2GM20), I went from a fixed prop to a slightly larger folding prop, so now my max rpms went down by 200-300 (interestingly the speed went up). Do you think I should just shift the curve down a bit and use that to program my totalizer?
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Old 02-03-2020, 15:20   #21
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Re: Fuel Burn - Net or Gross?

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Stu, I agree with you. In my case (2GM20), I went from a fixed prop to a slightly larger folding prop, so now my max rpms went down by 200-300 (interestingly the speed went up). Do you think I should just shift the curve down a bit and use that to program my totalizer?

I'd figure on shifting the values on the X-Axis of the factory fuel and power curves to the right so that the same fuel rate and HP are against a lower RP.
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