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Old 05-14-2020, 02:14 PM   #1
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Help Interpreting Fuel Burn

Hello

I was reviewing my manuals for my Detroit Diesel 8.2 engines to help calculate fuel burn and it's a bit fuzzy to me. On the attached image I'm not sure which line to use to or if it's a range for fuel burn. At 1200 RPM's I can't imagine I'm burning 6GPH per engine and at 2500 RPM I can't imagine burning only 5.5GPH per engine.

I typically cruise at about 1750 RPMs and sometimes kick it up to 2500 if I need to run from weather or get somewhere quickly.

Thanks in advance
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Old 05-14-2020, 02:23 PM   #2
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Look at the prop load graph line. At 1750rpms it looks to be a about 15gph.
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Old 05-14-2020, 02:31 PM   #3
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At 1750 the prop curve says 2.5 gph each. You use the bottom curve and the scale is on the right side.
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Old 05-14-2020, 10:25 PM   #4
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The bottom prop curve is the one to use.

THe others are projections from engine builder for the maximum the engine SHOULD be allowed to burn at those revs. Otherwise you drive the engine into overload and potential damage.

Note those are projections. They are not fact for all boats. THe actual hull, its shape, the speed it is run at, the actual real weight all play into the REAL fuel burn.

You can take the same engine and install it in two different boats with changes to the above conditions and the engine will burn different amounts of fuel even though run at the same rpms.

This is also why props must be matched to the boat.
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Old 05-14-2020, 11:02 PM   #5
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Yep, lowest of the lower two curves, right hand scale. Still an approximation as posted above. But usually in the ballpark for a non-planing hull.
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Old 05-15-2020, 07:40 AM   #6
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Thanks everyone, I can use that as an estimate and start monitoring fuel levels to get a better idea.
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Old 05-17-2020, 06:00 AM   #7
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I have been looking at a boat with the DD 8.2s installed and have discovered they are fine , but can not be run hard .

The engine hassle , the current owner blew one, and the first "rebuild" was unsuccessful (no parts to rebuild or rebuilders) J&T wont touch them .

So go easy on them , do not overheat them or operate at full throttle .

They seem to be fine long term with modest power levels .Many folks are happy with them.

Your 2500 RPM would be a good redline

Volvo D 4 with trannys are $61,000 a pair , not installed.
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Old 05-17-2020, 10:49 AM   #8
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Those are great fuel burn numbers.
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Old 05-18-2020, 06:35 AM   #9
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Thanks for the info FF. Yep, I run them easy most of the time and never go above 2500rpm when cruising fast. I've had a couple DD mechanics look at them and they've both said they look great and should be fine as long as I don't over heat them. I keep an eye on the temp gauges when running.

So far they've been great engines for us, been very reliable
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