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Old 05-11-2021, 07:08 PM   #1
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IG 36 cruise speed vs fuel burn

I have a IG 36 with twin cummins 270hp, anyone have similar? Curious about fuel burn at different cruising speeds. Any info is greatly appreciated!
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Old 05-11-2021, 08:19 PM   #2
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Wow2 - thats a bit of grunt you have down in the engine room - we have two good old boys - Ford Lehman 135s- I calculate about 16Litres per hour total @1700rpm so you may be able to extrapolate from that
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Old 05-11-2021, 08:27 PM   #3
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I will say I feel that my power is a bit excessive which is why I'm curious if anyone else has similar. Thank you for the reply!
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Old 05-11-2021, 09:44 PM   #4
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Jono, is that a repower?
I had an IG 36 with more than adequate Lehman 120s which used 12-16L p/hour total at around hull speed.I know of 200hp Volvos factory fitted to 36s.

My IG36 based Integrity 386 has Cummins 210s(yea,no aftercoolers!) and seems quite economical. The Qld. Integrity 426 I didn`t proceed with had C 270s and hit 17 knots on sea trial.
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Old 05-12-2021, 06:07 AM   #5
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You can easily prepare a fuel burn yourself with no instruments. Print out the attached data sheet for your engine and particularly note the third graph- engine rpm vs fuel consumption.

Then take your speed at various rpms and make another graph by plotting speed vs fuel consumption from the rpm graph times two. It should be within 10-15%.

Another way is to join boatdiesel and use their prop calculator. But that is totally theoretical whereas the above method at least uses your real world speed vs rpm data.

You can obtain fuel burn reports from fellow TF members, but in my experience they are highly variable and don't take into account generator use, currents and are often just guesses.

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File Type: pdf Cummins 6BTA 270 hp datasheet.pdf (224.6 KB, 6 views)
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Old 05-12-2021, 06:30 AM   #6
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Quote:
Originally Posted by Jono57766 View Post
I will say I feel that my power is a bit excessive which is why I'm curious if anyone else has similar. Thank you for the reply!
Jono, create your chart. RPM, SOG reading from you very own meter, GPH, then you can figure max. distance/RPM.

Anything someone else provided may not be accurate for your boat due to weight, sea state etc.
To negate the current etc, 2 mile out, 2 miles back. You can refine the chart over time.

Do this w/o the generator running then figure in about 1.5gph.
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Old 05-14-2021, 01:13 AM   #7
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I can't add too much to this conversation as I have the same FL120 configuration as Bruce's old boat. Except to say that I did a graph as discussed above and noted that that the fuel consumption on our IG 36 increased 90% when measured against our cruising speed at 1600rpm to WOT (2600rpm). All for a gain of 1.8 knots!
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Old 05-14-2021, 04:59 PM   #8
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With that much HP on a 36’ boat you might be able to go 20kts at a gallon a mile. You will probably use one two two tenths more gallons per hour over the guys with the Lehman’s if you match their speed.
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Old 05-14-2021, 07:55 PM   #9
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With that much HP on a 36’ boat you might be able to go 20kts at a gallon a mile. You will probably use one two two tenths more gallons per hour over the guys with the Lehman’s if you match their speed.
Why more fuel for the Cummins? The Cummins has the same displacement as the Lehman, so the mechanical parasitic losses should be the same. The turbo on the Cummins might actually improve efficiency a bit, even at low rpms as well as more modern injectors.

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Old 05-14-2021, 08:06 PM   #10
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Why more fuel for the Cummins? The Cummins has the same displacement as the Lehman, so the mechanical parasitic losses should be the same. The turbo on the Cummins might actually improve efficiency a bit, even at low rpms as well as more modern injectors.

David
. I have not done the figures but my impression, watching the sight tubes etc, is that the turbo Cummins 210s in a boat with hull very similar/derived from the IG36, look to be more economical than the L120s even at slightly faster speed.
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Old 05-15-2021, 12:40 AM   #11
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Why more fuel for the Cummins? The Cummins has the same displacement as the Lehman, so the mechanical parasitic losses should be the same. The turbo on the Cummins might actually improve efficiency a bit, even at low rpms as well as more modern injectors.

David
That is very possible. I thought the Cummins was a bigger displacement engine. If you are correct then the tenth might go the other way. My main point was no significant difference in fuel consumption.
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Old 05-15-2021, 07:16 AM   #12
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Our IG 36 has twin Cummins 210hp. With light boat--less than half fuel and water we can do 14knots. So you should easily do 15 in any load conditions. We cruise 1150 rpm at 7.5 knots burning 3 gallons per hour total--no genny running. At this relaxed pace the boat runs very quietly--sound level is so low it is amazing. Is running this slow good for the engines? They are 25years old and not causing any issues so I am not overly concerned. With my 410 fuel cap I have close to 1ooo mile range. I have two wing tanks and a 90 gallon center tank. When the wing tanks are approx one quarter fuel I have transfer system to drain the tanks thru a filter into the center tank as a day tank. This way I can use all available fuel if needed.
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