QSM11 RPM/Fuel Burn Test

[Curtis J McNamee]

Been looking at a 2002 Bayliner 5788 with twin QSM11 Cummins with 3900 hrs on them. Was able to get the boat out yesterday and did the RPM/Fuel Burn Test.

In a previous post, I was concerned with 7 & 10 ppm of Chromium in the 250 hr oil in these engines and with no other negative engine symptoms….seems like most people are not concerned. Just changed the oil and filters in both engines, ran the boat at various RPM’s for 1.5 hrs and are now retesting the oil. Cummins NW said to run the engines for about an hour and then retest.

Starboard Engine

600 rpm- 1.1 gph
960- 2.8
1100- 4.7
1300- 8.1
1500- 11.5
1700- 17.3
1900- 18.5
2000- 22.6
2100- 26.7
2260- 33 WOT

I found out that the owner mostly ran the boat at 1150 rpm and since new, the total gallons of diesel through this engine was 19,858

In a previous test, we did hit WOT of 2300 with no wind, this time we were heading into a pretty strong wind and also I do not know how clean the bottom is, it has been a year since it’s been out of the water.

The owner says that the props are original, the boat had almost full full and water when tested.

Thoughts ??

Thanks,

Topic

[westbrook_tate]

Additional re-prop questions for QSM11

I have read your superb articles on the zen of QSM-11 engines, and based on the “Propping the Cummins Marine QSM11 to Prevent Exhaust Issues” article, am about to haul my new (to me) boat to have it re-propped to prevent dry exhaust/turbo issues as you advise.

I just ran the boat yesterday to collect the data and build my fuel burn curve. The file is attached with the line graph and table of the data I collected. (The table figures may be more enlightening due to the limitations of the Powerpoint graphing function I used to make the graph). Bottom line: I seem to be well above your recommended burn rate of burning under 20GPH at 2000RPM.

Test conducted in calm seas, clean bottom, 36,000 pound 42-foot Legacy sedan flybridge; lightly loaded (1/4 tank of fuel; water and sewage tanks empty, one person onboard), trim tabs pulled up to minimize other variables. Engine achieved WOT of 2300 RPM. Engine is a 2005 year with 1769 hours on it; shows no early signs of exhaust leaks as warned in the article.

Questions:
1. Based on my table, I am just under 20GPH (19.3) at 1900, but well over (22.1) at 2000 RPM. As I read the guidance listed in bold in the article “Reduce the load at CRUISE RPM (1900-2000 RPM) to under 20GPH (400hp). Period,” I want to be sure I am not overreacting and about to spend a lot to haul the boat and have a prop shop flatten my prop pitch a bit. I am at 19.3 GPH at 1900, but 22.1 at 2000. Can I get away with just cruising at 1900, or does my overall fuel curve, especially the 2000 RPM burn rate, still require the prop work. See attachment for the full range of RPM/Fuel Burn.
2. Assuming I am about to make this prop change, how much do I have the guys at the prop shop remove? This single QSM11 (660hp) is turning a 27” 5-bladed prop with a 28” pitch with a cup. Is the reduction that I need linear? So, to cut my 2000 RPM burn rate from 22.1 GPH to 20 GPH, a reduction of 9.5% (I will round up to 10%), should I tell the guys at the prop shop to reduce 10% from the pitch? For my prop that would be a reduction of ~2.8” from the 28” pitch?
3. My QSM-11 has been modified by the previous owner from 635hp to 660hp (I presume this is a software mod in the ECM). Can I fix the high fuel burn vs. RPM symptom by merely reverting the software to 635hp?

Many thanks!

Tate

[Larry Backman]

Try it again/calm conditions

Your burn at 1900 isn’t too far off the curve, but it jumps way over at 2000 and above.

I run a lot of hours at 1875 RpM on long trips and my momentary fuel burn can fluctuate from 17.5 GPH to 20GPH based on where the boat is on a wave.

[Curtis J McNamee]

Yes, that is what it looked like to me….I will see what the bottom looks like when we pull her out.

[Rob Schepis]

Wow, she is overloaded in a big big way. Scroll down in this link to the bottom of this page: https://www.sbmar.com/articles/propping-cummins-qsm11-to-prevent-exhaust-leaks/

[Author]

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