Cummins QSM11 losing hours

[rkurrle]

2021 Back Cove 41 with Cummins QSM11 715hp, serial number 35366228.  The engine data is displayed on a Raymarine Axiom display at the helm pictured below and in the engine room on the Cummins display panel.  

I log each trip including engine hours, distance, fuel, etc, from the display at the end of the trip .  My last trip the hours were 119.1 when I shut down.  Today when powering up the engine, the hours red 118.1 both on the Raymarine and the Cummins engine room display.  It was displaying one less hour then when I shut it down.  This has happened before.  So I have more hours on the engine than the display indicates.

Any thoughts on where the problem is.  During any trip, I have verified that it is counting the engine time correctly.  When it is shut down, the additional hours seem correct.  Next trip on turning on ignition, the display shows less hours then when last shutdown.  Does not seem to occur every trip, that’s why the hours are increasing, even though many hours are missing. 

Look forward to any ideas.

 

thank you – Bob

Topic

[Tony Athens]

‘I would guess that Steve’s strange “negative” result was due to a significant change in boat resistance due to a different state of being on plane’ 

Change the weight in any planing boat or bottom condition,  and slip goes up or down accordingly………….

For out poster– removing 2″ of pitch would make those manifold turbo last noticeably longer.

 

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rkurrle

[Gene Fuller]

Bob,

 

Don’t worry about prop slip. You cannot control it directly.

 

It is real, and it can be a useful tool to do a detailed analysis. But you should consider it a result, not an input. As Steve showed you can end up with surprising results if the boat speed changes. I would guess that Steve’s strange “negative” result was due to a significant change in boat resistance due to a different state of being on plane.

 

Gene

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Tony Athens

[rkurrle]

re-pitch

Steve

Again, thanks for the reply.  Yes, I have the ZF trans with the 2.037 reduction.  I have read your referenced articles.  I run at 1800 to 2000 rpm to get 18 to 20 knots.  At those numbers, using your technique, looks like about 1.5 inch reduction..  I calculated that at 1800 – my burn is 17.1 gph.  Using the Cummins performance curve to achieve 17 gph, the engine would be turning about 2000 rpm.  150 rpm per inch would be 1.33 inch reduction, so I rounded up to 1.5 inch reduction.  We agree.  I don’t understand the prop slip, but you are saying add another inch reduction for prop slip.

So, to achieve my 18 knots, what rpm would I be turning?  How about 20 knots?

I would appreciate if Tony would comment.

thanks again for all your help – Bob

[Steve Lewis]

RK,

When on plane, at what RPM do you cruised your boat? 2000?

My prop tuning document goes into detail on how to figure out pitch reduction.  Summary is that you can generally assume that 1 inch of pitch will move your numbers up the RPM range by about 150rpm.  That is not hard and fast but it is a decent base assumption.  The other thing that needs to be understood is that will only work if your prop slip is reasonable.  I am assuming that you are setup like the factory report with the ZF trans with the 2.037 gear ratio.  With your chart that you posted I am calculating the following:

28% slip at 1800rpm

25% slip at 2000

22% slip at 2200

With those slip numbers, my “guess” is that the first inch of pitch that you remove will reduce prop slip and get it down under 20% at 2000 rpm.  That is a guess but one that is informed by my own experience in this regard.  You can read my posts about my continuing effort to dial in our boat. I wanted to move our fuel burn/speed at 2200 to 2300 so I had 1 inch removed.  The result, the fuel burn/speed that was at 2200 was now at 2140.  Wrong Way!  Prop Slip.  Slip went from 28% down to 23%

So my recommendation is in the 2-3 inches range.  Maybe Tony can narrow it down a bit.  My reasoning is that right now your fuel burn at 1900rpm is ~20gph.  You would need to move that up to 2100rpm.  That is 200rpm so about 1.5 inches.  But you have about the same slip that I had on mine.  Granted I have twins with smaller props.  Your boat is a single with a much larger prop so the slip characteristics might be a bit different.

so 1 inch to handle the slip + 1.5 inches to move the fuel burn = 2.5 inches so my guess is that you want to end up with a 28×32 prop.

@Tony am I in the right ballpark?

As for the diver vs. haul out, I don’t know what to tell you there.  Since I am in Ohio we have winter layup for things like this.  I am hauled out by definition so I will defer to others on that question.

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Tony Athens

[rkurrle]

repitch

Thanks for the response Steve

I have the 34.5 inch pitch.  After reading your reference and Tony’s articles, I agree with your suggestion.

How do I determine how much to decrease the pitch?  I only want to do it once.

Should I haul the boat out or let a diver remove the propeller?

thanks – Bob

 

[Steve Lewis]

Kinda what I thought

RK,

If you are interested in understanding how something “Should work properly” then I recommend reading up on the QSM exhaust manifold topic.  Back Cove prop’d your boat to impress from a cruise speed perspective and not for long term QSM exhaust health.  The fuel burn target is 20gph @2100 rpm.  Using linear interpolation you are ~24gph @2100rpm.  If your prop came from the factory with a 34.5 inch pitch you should seriously consider having 1.5 inches of pitch(or more) removed so that you do not toast your manifold and possibly the turbo longer term.

Look at Tony’s Tips for Propping the QSM for manifold health. 

EGT measurement is also something that you could use in conjunction with the pitch removal to manage your operation to healthy exhaust temps.   You are early on in the life of your QSM so  you can make some mods that will work in your favor for the long term health of your entire propulsion system.

I have attached a couple of documents.  The Back Cove factory performance test and then a doc that I wrote around how to tune your prop for long life. 

Let us know if you have questions…..

[rkurrle]

Thanks for the reply Tony.  You are right, in the big picture it probably doesn’t matter.  I guess it is the principle of thing, I just like things to work as they are suppose to.

My numbers for fuel consumption:

    1800 rpm    17.1 gph   18 kts

    2000 rpm   22.2 gph    21 kts

    2200 rpm   26.9 gph    24 kts

thanks – Bob

 

 

[Tony Athens]

This is a tough one as it seems you started out with w/ the new overly complex Cummins Command electronic instrument system.. The good news is you are under full warranty (warranty start date June 2021) –Hit up your local Cummins dealer / distributor–Let’s see how “customer service orientated” they are.. In S. Calif, these days,  I’d give them a “C+” at best.. You paid dearly for that warranty, so use it.. Also, are you sure this really matters in the big scheme of things ( a 1% error or less??) ..

 

On a more important issue, at least in my mind.. What is you average GPH at 1900, 2000 and 2100 RPM?        

[Author]

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