Proper Loading of a QSM11-715

[Steve Lewis]

Hello forum members. I am new to the forum but have been reading Tony’s Tips for a couple of years now. I sent him an email yesterday directly and he replied and wrote that instead of emailing him I should post in the forum for all to see. So here is the post and it appears I have been learning…….

My wife and I are looking at cruising the great loop in the future and the Back Cove 41 looks like a nice candidate for what my wife and I would want for this purpose. Based on my reading of your extensive writing in regards to prop loading, it looks like the BC41 comes from the factory over propped. No surprise I guess. My question is to validate that I am looking at these numbers correctly??

I have attached the factory provided performance info. When I compare the Gallons per hour numbers in their test runs with the Cummins Provided prop curve, this boat looks like it is 28% over propped in the 1000 – 1200 rpm range and then 21% – 10% over propped in the 1800 – 2400 rpm range. I would target most of the operation of this boat on the loop at displacement speed running some where in the 1000-1200 rpm range and then every now and again get up on plane, 2000 – 2200 rpm, to quickly cover some of the distance passages on the Gulf and Great Lakes etc…..

My Conclusion/Question

This boat needs a “different propeller” is my conclusion. I am thinking that it needs ~1.5-2 inches less pitch. It ships with a 28×34.5 prop and the “realistic full load” that is reported in the details of the report make me think that they loaded the boat in a manner that would be similar to how we would have it loaded on the loop. we would have fewer people and probably not need to run with full tanks all of the time but we would definitely have more “Cruising Stuff” on board. We would weigh in the same or heavier would be my guess.

The long and short of it is I am wondering if I am thinking about this correctly? Validation that I am understanding what you have written and applying that knowledge appropriately.

Tony’s response, minus the recommendation to post in the forums………..

Remember this if you remember nothing else.. It is 100% completely impossible to underprop this engine. You can only over prop it

Remove a solid 2 to 2.5 Inches of pitch based on the info you have posted.. You’ll thank me later..

It looks like I was on the right track but needed some real world knowledge of how much pitch to take out when acquiring the new propeller. My wife and I have a few more years of “earning” to do before we start our adventures around the loop and the Great Lakes. Happy Boating everyone!

Topic

[Rob Schepis]

Corey Schmidt wrote:

It still amazes me that boat builders pay NO ATTENTION to the prop demand curves for these engines when choosing and installing them…

And don’t forget about safe exhaust systems, they fall into the same black hole with most builders..

[Corey Schmidt]

Overloaded…

It still amazes me that boat builders pay NO ATTENTION to the prop demand curves for these engines when choosing and installing them… boats only get heavier over time, not lighter. Both of those seatrial graphs prove that the engines were grossly overloaded to begin with. Absolutely amazes me…

If I had a choice, knowing what I know… personally, I would choose the QSC engines burning roughly the same amount of fuel at higher RPM… less potential exhaust issues and more oil/coolant flowing around the engine. Granted that personal assessment is holding a lot of external factors at control. However, both engine models, when setup CORRECTLY, will prove to be work horses and provide years of service. It’s usually not the engines fault when failure occurs, its the “nut behind the wheel”.

Bottom line = pitch down to where your M11s burn less than 19-20 GPH at cruise and you will never have to think about the “greener” grass with another engine such as the QSC ever again.

[Steve Lewis]

I think I might have had one of those “A HA!” moments. I am looking at both performance reports for the Back Cove 41. See attached. This boat can be equipped with either the QSM715 or QSC600. Upon close inspection of the performance data I see something interesting. Both engines are overloaded in the cruise rpm range so they will both need less pitch. It also appears that they both provide similar cruise propulsion for this boat. If you look at the reported “cruise speed” of the QSM equipped boat at 2000 RPM it is showing 20.7Kts. When you look at the QSC equipped boat at 2600 RPM it is 20.8. When applying “recommended fuel curves” you would prop the “Full Load Boat” to burn 20GPH or less at 2000 RPM on the QSM OR 20GPH or less at 2600 on the QSC. In my mind both of these setups would net similar cruise speeds on plane and at displacement speeds for my intended use. Cruising the Loop.

Am I making a bit of leap in thinking that either boat would burn similar amounts of fuel and be moving at a very similar speed just at different RPM’s? Those RPM’s would be less than the max cruise rpm on both engines as published by Cummins.

I went 1 step further on this exercise. I used the decreased pitch that Tony indicated for the QSM(28×32) and then went with a 28×28 (removing 3 inches from the factory supplied prop) on the QSC version and got inches/min numbers that are very close. Translation – It appears I would get similar real world performance from either engine even when we decrease the pitch of the props to get the fuel burn under spec.

All of that leads me to ask the following questions……..

Which engine would be the better choice given that both engines push this boat at similar speeds with similar fuel burn rates, just at different RPM’s? Is one more reliable than the other? Is one easier to maintain than the other? Is one less expensive to maintain than the other? Is one more tolerant of overloading than the other so that when I go to look on the used market I might pref one engine over the other?

Or did I stray off the reservation?

Calculations: (RPM/Trans ratio)*Prop Pitch = movement through space without slippage
QSM (2000/2.037)*32 = 31418 inches/min
QSC (2600/2.3)*28 = 31652. inches/min

[Rob Schepis]

This thread I link below is on topic as to why you are doing your homework now and what you are trying to avoid down the road…..

https://www.sbmar.com/community/topic/qsm11-prop-size/

[Steve Lewis]

Corey,

I have 1 follow up. After reading both of these articles and then thinking back on the rest that I have read THE most important point is Fuel Burn at any given RPM. Meaning really disregard the Load % and just prop the boat so that the Fuel Burn is under the Prop Curve in the Cummins published graphs. And mainly in the midrange cruise portion of the RPM band. Load is an interesting statistic but not really the main thing. Load is a by product of Fuel burn so Fuel burn is the stat to prop to and give myself some “Breathing Room” to allow for the inevitable changing conditions. This approach will help the Cummins Diesel engine(s) in my dream boat last longer than I will cruising the Great Loop and other..

Cheers!

[Corey Schmidt]

Based on your numbers… I agree with Tony on this one that AT LEAST two inches need to be removed. More is ALWAYS better. These engines do not like to be lugged down, they like to be able to spin up freely with LESS load. You can NEVER under-prop an engine

This QSM 11 article was created for this engine specifically… the QSM11 535 curve is and always has been the best curve.

Propping the Cummins Marine QSM 11 to Prevent Exhaust Issues

Another good read:

Propping Electronic Engines

[Author]

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