Port and Starboard Fuel Consumption Difference

[Tman]

I have a 2005 390 Dancer, twin Cummins QSB5.9, 21x23 props, 540 hours on the engines. I notice a fuel consumption difference between the engines at cruise (~2540 RPM's on each engine) of about 1GPH with the port engine drinking a bit more. The % load on the port engine is also running about 5% pts more than the starboard engine which would indicate to me it is working harder than the stbd engine to achieve the same RPM. I've had the boat for about a year, it was surveyed by a credible firm, and overall, she's in great condition and maintained the same. I also recently had the props reconditioned hoping that might level off the fuel consumption...but it didn't. Overall, my fuel consumption on the boat is on spec for the 390DA with these engines(~29GPH at cruise). Any ideas as to what is causing the difference? I've heard (but never validated) that since the props are counter rotating, one engine will consume a bit more fuel than the other due to fundamental gearing differences between the transmissions. Not sure I totally buy that. I've ruled out fuel consumption by the genny as the culprit. My last question is; what is excessive....is 1GPH "a bit" or "a lot"? Over a long haul, its a fair amount of fuel, although on refills, I'm adding very similar amounts to each tank. The fuel consumption does level off as RPM drops....at idle or trolling speed, they are exactly the same.

 

[bmac]

That's a very good question and one I've been contemplating qite a bit lately. Although I've got a different boat with different engines and transmissions (straight drives vs. your v-drives) I see something very similar. My port engine consistently burns 1 gph more than the starboard does and my port engine loading is typically 4% higher than the starboard. I watched this day in and day out on the way from Fl to NY this spring. Coincidence? I don't know but hope someone with more diesel knowledge than I weighs in....Frank W?

 

[Gofirstclass]

Have you possibly overlooked something......does your genset draw fuel from the port tank?

I would consider 1GPH a minor amount. I've never heard the bit about counter rotating props consuming more fuel. Not saying it's not true, just that I've never heard that. I don't pay that close attention to what my engines burn. Since fuel is such a tiny part of the overall expense of boating I just put in the fuel and the Cats drink it. I'm not their mother or their father so I don't keep track of how much they drink. I have other things on the boat to spend my time worrying about.

Happy boater GFC

 

[Tman]

Definitely not the genny as I get the same differential when the genny is off. I'm not at all concerned about fuel cost, but rather if there might be an underlying drivetrain problem brewing...perhaps a cutlas bearing or some other problem on the port side. Maybe nothing at all to be concerned with as it could be a series of things driving the load up slightly.


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[cod]

Were the props done by Prop Scan and to what class? I'm running within 1-2% right now but have seen identical readings (upon reconditioning)in load which equates to identical fuel burn. It usually is the props or at least that's been my experience.

 

[mistercomputerman]

Mine are off by 3 gph. Starboard burns more.

 

[Tman]

Not sure I know what analysis was used other than "HydroComp Prop Expert software and 3–D MRI Propeller Mapping to determine precision propeller geometry for pitch, rake, track, blade spacing, thickness, and camber so that your repaired or reconditioned prop delivers synchronized blade loading." That is from the propeller recon facility. Post prop conditioning, each engine is slightly more efficient, but I still get the same absolute difference between them when it comes to engine load and fuel consumption.

I've talked with other 390DA boat owners w/ same configuration and its a mix of results ranging from exact same % loadings to 10% pts difference. My gut is pointing me to the cutlass bearing or slight shaft misalignment??


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[Tman]

Had my props recently done to Class 1, shafts pulled, checked and trued, and my cutlass bearings replaced.

Results of the above is no more than 1.0% point difference between port and starboard engine load and subsequently fuel consumption. Basically the engines are spot on to each other. Big difference from previous condition.


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[Blown284]

Ever figure this out? I have a similar issue with port burning approx 1gph than starboard ... new props, no vibrations, very clean bottom (just had it hauled to clean and put new props on). Thoughts?