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Old 05-09-2011, 08:41   #1
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Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

We all enjoy an impressive forum of like minded people with a liking for keeping our boats in pristine working conditions!

Too often I read about diverting opinions regarding the correct use of diesel engines. While our Grand Banks 47 cruises well at 18.4 kt @ 2300 rpm at a consumption of 7.8 liter per Nm, our favorite speed is 6.6-7.5 kt @ 1000-1100 rpm at a consumption of only 1.7-2.1 liter (both engines) per Nm. We have TRAC STAR stabilizers installed that makes the displacement ride extremely convenient in all weather conditions up to 20Ms.

As we prefer to cruise at cruising speed only the very few cases were we are too short of time a late Sunday or similar, do I in actual fact risk any damage to engines from long haul cruising at not mor than 1100 rpm at which speed I am having a very long range.

Would be really appreciating your advices and sharing of technical insights.

Kind regards from Singapore

See ALMA at www.askaer.com
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Old 05-09-2011, 08:52   #2
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Re: Cummins Diesel Twin 500HP QSC8.3 Permanent Slow Speed

Diesels periodically need the carbon blown out of the combustion chambers. This is done by periodically running them at the engines maximum governed speed for a given period of time. Your Cummins owners manual will describe the intervals for doing this.

Otherwise, you should be fine at running your engines at not much above an idle for most of the time. The downside is engine longevity. Diesels like to be run fast and under a near full load. The plus side to not running them like this is much better fuel consumption.

You might also consider running one engine at a time periodically. This puts a little more load on the engine being used, which for a Diesel is a good thing.

I have a pair of B-Series Cummins that require blowing the carbon out.
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Old 05-09-2011, 09:10   #3
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

I am new to this forum and large boats; however, I have a lot of experience with diesel engines. The condition that causes carbon build very quicly is called "lugging". Lugging is putting a heavy load on your engine and running it at low RPMs, but if you engine is comfortably moving your craft at these speeds, which is seems like it is based on fuel flow, you are not lugging, but just running at a very light load which can build up carbon as well, but as mentioned previoulsy it can be cleared by running the engines under a heavy load (80% capacity or better). Lugging also increases EGTs in some cases which increases combustion temperatures which transer to your exhaust system and turbos.

If you are Lugging this can cause carbon buildup on injectors nozzles and compressesion rings. If it gets bad enough the carbon deposits can lead to plugged injector holes, damaged injector nozzles, or keeping the compression rings from properly rocking in their ring lands.

The good news is that modern diesel engines (like yours) are controlled by ECUs rather than mechanical fucel controls which allow them to adjust injector pulse width (amount of fuel delivered) based on varying engine and environmental conditions rather than the older mechnical techology that had rigid fuel mapping which is why we see a lot less black smoke out of modern diesel engines.
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Old 05-09-2011, 09:34   #4
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I am grateful for the very useful and informative sharing of expert advice. I have run the engines for 250 hours and never seen any kind of smoke neither during start, idle, slow speed or at 80% load. The owners manual is not too specific on the matter and while several boaters tend to recommend full load every one hour I just wonder if it in fact applies to modern electronic engines. Any bad experiences out there from similar engines being driven at low rpm for longer durations. I am obviously not interested in damaging my engines. Perhaps other Cummins owners have something to add. Thanks
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Old 05-09-2011, 16:21   #5
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

Happy to help. I do not want to alarm you at all. As long as you follow what the owners manual tells you to do religously you will most likely get very long life out of the engines.
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Old 05-09-2011, 21:15   #6
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

i have the same motors and i have the same experience -- a sweet spot of 1150 rpm
you will be okay but you should run it harder once in a while, to get the turbo running. if not the turbo can gum up and it will not work.
also if you do not run it hard once in a while you will not spot the problems beginning which do not show so much at lower rpms
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Old 05-09-2011, 21:24   #7
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

Quote:
Originally Posted by ASKAER View Post
I am grateful for the very useful and informative sharing of expert advice. I have run the engines for 250 hours and never seen any kind of smoke neither during start, idle, slow speed or at 80% load. The owners manual is not too specific on the matter and while several boaters tend to recommend full load every one hour I just wonder if it in fact applies to modern electronic engines. Any bad experiences out there from similar engines being driven at low rpm for longer durations. I am obviously not interested in damaging my engines. Perhaps other Cummins owners have something to add. Thanks
Running the engines up periodically to maximum governed speed does apply to newer engines. Mine are Tier 2 engines that are computer controlled that are three years old now. Cummins requires that for 10 percent of the time that I run the engines for a half hour at full speed...not 80% speed. This mean once every 6 hours for half an hour at a time. I would call your Cummins distributor or dealer and find out what the specifics are for your engines with regards to burning out the carbon.

For a boat, lugging a diesel is what occurs when your propellers are over pitched. If your engine cannot reach their maximum governed speed because they are over pitched then you are lugging them, which will shorten their useful life.

The Cummins warranty requires that the engine be able to reach its maximum governed RPM.
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Old 06-09-2011, 05:42   #8
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

Askaer:

This question has been debated on every cruising forum on the net. Any diesel engine needs to be run with enough load on it to bring the cooling and oil system up to operating temperatures. If you have an IR gun, shoot the water pump and the oil filter at your low speed cruising load. Water temp needs to be 170F or greater and lube oil about 10-15 deg F higher.

A 5-10 minute run at high power at the end of the day of low speed cruising will burn/blow off any carbon on the injector tips and rings and is good insurance.

The foregoing advice is a compilation of recommendations from the gurus who hang out at boatdiesel.com, particularly Tony Athens, a Cummins dealer, and Ski, an independent diesel mechanic. To paraphrase Tony: "I have never seen a propulsion diesel engine damaged by low speed operation."

David
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Old 06-09-2011, 15:12   #9
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

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Originally Posted by djmarchand View Post
Askaer:

This question has been debated on every cruising forum on the net. Any diesel engine needs to be run with enough load on it to bring the cooling and oil system up to operating temperatures. If you have an IR gun, shoot the water pump and the oil filter at your low speed cruising load. Water temp needs to be 170F or greater and lube oil about 10-15 deg F higher.

A 5-10 minute run at high power at the end of the day of low speed cruising will burn/blow off any carbon on the injector tips and rings and is good insurance.

The foregoing advice is a compilation of recommendations from the gurus who hang out at boatdiesel.com, particularly Tony Athens, a Cummins dealer, and Ski, an independent diesel mechanic. To paraphrase Tony: "I have never seen a propulsion diesel engine damaged by low speed operation."

David
Hello Heine,

I think the first point above is key - be sure your engines are making it up to full operating temp. The engine spec sheet states the min coolant temp at 160 F (71C). I find mine run right around there as long as I keep them above 800-900 RPM, and run up to the low 170's at cruise RPM to wide open throttle.

The QSC 500 (I've got the same boat and engines) are rated by Cummins as "High Output", with operations restrictions as follows quoted directly from the application manual:

"This power rating is intended for infrequent use in variable load applications, where full power is limited to one hour out of every eight hours of operation. Also, reduced power operation must be at or below cruise speed (rpm). Cruise speed (rpm) is dependent on the engine rated speed (rpm):"

Note the limit on full power operation, as well as the requirement to otherwise stay below "Cruise speed". On the QSC 500, full speed RPM is 2600, and Cruise Speed RPM is 2400. Both of these restrictions limit how hard you can push the engines. Notably absent is any limit on how lightly you can load the engines.

Last, but not least, the QSC operators manual says nothing about limits on low power operation. It does say to be sure they reach full operating temperature, but the only operating restrictions are on high power operation, not low power.

My conclusion from all the Cummins literature, plus Tony Athens comment quoted above, is that as long as you get the engines up to normal operating temp, there is no harm to be done by extended low speed operation. I do, however, like the suggestion of at least occasionally running them hard enough to exercise the turbos. In contrast, there is extensive damage that can be done by overloading the engines, either through extended operation above cruise speed, or by over-propping as indicated by fuel burn in excess of Cummins specs at given RPM.

You are getting almost exactly the same speed and fuel burn at 2300 RPM as I am, and I've concluded that I'm slightly over-propped. Not too bad, but I plan to correct it over the winter haul-out. In the mean time, I compensate by keeping my cruise RPM below 2200-2300 to avoid overloading the engines.

One last comment. All this is strictly in the context of the Cummins QSC engines which are common rail, electronically controlled, high output engines. Mechanically injected engines, 2-cycle detroits, and engines operating a lower load ratings have different considerations, and I believe are the context for a lot of the other recommendations that you have and will get.
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Old 07-09-2011, 09:47   #10
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Dear all:
My Cummins are very stable temperature wise at 160-170F (71-73C) irrespective of rpm. I have run them occasionally at full throttle (100%) showing a load at both engines of 98% that to me indicates that the props are right? I have gone through all manuals, searched the Cummins home pages and never been able to find any proper advice on slow speeds or strong advice against permanent slow rpm. Yet there seem to be a strong view among diesel experts that it is better to load the engines 80% at times - even a few who recommends 80% as operating "standard". Talking to Cummins engineers here in South East Asia I get different views while the user guide is very specific about not to overload as clearly confirmed by Twistedtree above. I still wonder if we are all burning unnecessary fuel out of past experiences with less electronically controlled engines. Sorry to keep this matter still open- but truly few technical matters could be of higher importance to motor boaters!
Kind regards from the Tropics.
Heine
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Old 07-09-2011, 10:08   #11
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

With the turbos, you will need to run the engines up over 2000 rpm occasionally to keep them working. How often and how long is a matter of debate, but I would say a minimum of 5 minutes once a month. As mentioned before, if you briefly firewall them, you can use the RPM and speed attained to check for operating problems.
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Old 07-09-2011, 10:49   #12
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

The "run at 80% of max rpm" rule is part based on fact, but mostly myth.

The fact part is that for most diesels 80% is where the BSFC -brake (a way to measure horsepower) specific (per unit of power) fuel consumption, peaks. It is also the point that most diesels can be run continuously and get decent engine life. So truckers particularly set up and ran their diesels at that rpm.

But since absolute horsepower and therefore fuel consumption per mile for boats goes up at the 2-3 exponent, that is not THE most efficient speed.

For old Lehmans, Perkins, DDs as well as modern common rail Cummins, Yanmars and Volvos the most efficient practical speed is the rpm where the engine has just enough load on it to stay at operating temperatures. That is very generally at 50% of max rpm.

If you have the time and inclination, run your boat for 5-10 minutes each at 1000, 1100, 1200, 1300, etc, etc rpm. The temperature on the gauge (or by IR gun) will slowly climb and then stablize- usually at 160-190 deg F. When it first stabilizes is when the thermostat begins to open. That rpm is probably a bit too low for best life because the oil temperature isn't yet up to its best temperature. A little more rpm and power will make sure that the oil is also up to temperature.

For my Mainship 34T with a Yanmar 370 the thermostat opening rpm was about 1400. I cruised at 1,700 rpm to give me some margin which is also 50% of max rpm. That coincidently gave me hull speed. I probably could have cruised at a couple of hundred rpm less and been ok and saved some fuel.

David
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Old 09-09-2011, 06:10   #13
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

Engines are typically the most efficient at the peak torque rpm.

So you want to select the gearbox and propellor so the engine runs at the peak torque rpm at the cruising speed that suits your hull. As per the post above the peak torque rpm is generally around 50% of of peak rpm
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Old 09-09-2011, 06:49   #14
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Re: Cummins Diesel Twin 500hp QSC8.3 Permanent Slow Speed

Engine operating temp. is both oil and water. Make sure your oil temp. is getting up to operating level (as mentioned above).
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Old 16-09-2011, 20:25   #15
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Dear Diesel Friends:
Let me use this early and pleasant Saturday morning at the Marina@Keppel Bay in Singapore to thank each of you for very valuable sharing of experiences in running our engines consciously for a sustainable living both for the engines and the environment. I am very concerned about both! I have concluded from above that it is safe to operate my twin Cummins 500hp at 1100 rpm (hull speed) for longer durations providing us an excellent economy and great cruising pleasure. We will anyway on and off "race" once in a while that for no other reasons seems to be good for the blood circulation and the heart; that for the engines are the turbos!
We have just had to change our two ZF couplings under warranty as the rubber dampers were broken and started to disintegrate. A bit messy but was well done by ZF Singapore and at their cost.
Happy cruising and once again thanks for your advices.
Heine
GB47EU - hull #88 - ALMA - www.askaer.com
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