Engine Load Calculation

[Jerome Billett]

I have been trying to determine the fuel burn/loading on my Cummins 6CTA 450s and finally made a trip long enough at plane speed to get some data and make some calculations.

I spent 9 hours at 2300 rpm which gave me 17 knots of speed over 143 nautical miles. At the end I refilled tanks with 244 gallons of diesel fuel.

The Cummins Fuel Consumption- Prop Curve for my CPL 8089 engines suggests the fuel burn at 2300 rpm is 15.4 GPH per engine or 30.8 total. Over 9 hours that would be 277.2 gallons of expected fuel burn.

I’d like some help to interpretation to these findings and calculations. During the cruise my boost pressures were about 18.5 psi and EGTs 790 degrees F. Bottom is brand new paint. Is it possible to do some calculations of engine load with these numbers?
Since I burned less fuel than the prop curve would estimate, this seems like a good reading. However, my fuel economy works out to only about 0.57 nautical mpg which was kind of disappointing.

My boat is a 2002 Sea Ray 420 Aft Cabin weighing in at 38,500 lb. with 22X21 props. I’ve taken 3” off the original prop specs and done all the usual engine maintenance and repairs to get to 2670 rpm at WOT fully loaded.
Would someone be willing to figure out the meaning of these numbers in the context of how my engines are performing and if they are working too hard? Any comments or suggestions are welcome.

Topic

[Jerome Billett]

Correction re Props

My props are actually Michigan Hy Torq MY-T4, not the Dyna Quad I mentioned in the previous post. I’m not sure what difference that makes but they were recommended for a Sea Ray.

[Jerome Billett]

Thanks Tony!

I very much appreciate the response Tony. I was hoping to discover that I am close to what is expected. My props are Michigan Wheel Dyna Quad Nibral 22×21 with Light Cup (which Michigan said Sea Ray uses). I bought them new after bending my old props from 22X24 to 22X22 and still only getting 2650 rpm at WOT. I expected another 50-60 RPM going to 22X21 (to get over the magical 2700 number) but lo and behold, no change. Is it the cup that prevents what I expected?
The boat does seem to pop up on and hold plane nicely though and if all the parameters look good I will stick with them.
My boat is dry weight 30,000 but when on the travel lift it clocks in at 38,000. Man that “stuff” adds up!
The .57 is Nautical MPG so I will be content with that.
Any further comments are welcome.

[Tony Athens]

I’ll give you a very simple answer, even though it appears you have gone the extra mile to do it all 100% right..

1) You have all that can be done as to making your engine load properly and are well within what I would consider “excellent prop loading” in this application.

2) Your boat is asking all that can really be asked of these engines if you want maximum life in this application. Your prop loading is 100% right..

3) Your vessel MPG (KPG?) ——–All I can tell you is this.. If you want to travel at 17 Kts in a 38000 Lb + vessel, getting .57 MPG is about at good at it gets.. ( Curious–where did you get that actual “operating weight” ??)…….. Sure there some that will tell you differently, but from what I have seen driving dozens of different vessels at this weight, your are right where most are.. Some a tad better, some a tad worse..Travel at 8-9Ks and you should see 2-3 MPG..

4) And one comment–What is the “quality” of your props? There are 22×21’s and there are 22×21’s……….There can be differences as to the HP they absorb vs. the speed you vessel moves forward………………………………… Think of the prop design/type & quality as a $ 400 per tire on your touring $80K+ MBZ on the Autobahn at 80-100 MPH vs. the same scenario us ing Pep Boys $100 each tires..

All is good..

Tony

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