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Old 01-30-2019, 01:18 AM   #41
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Percentage load is the ratio of power actually being produced to power available from that engine at that rpm. On a boat you cannot control it directly, it is a result of the engine, prop, and rpm. If you consider an example of the standard engine/prop curves it may become more apparent:



In the chart, the top curve is the power the engine is capable of producing at each rpm setting (though commonly called "throttle", there is no throttle on a diesel, only an rpm lever). That curve is designed into the engine at the factory. The lower curve is the power that the propellor can absorb at each rpm. It cannot be changed without changing the prop. For a particular engine and prop, a particular rpm setting will result in the the engine running at the percentage load represented by dividing the propellor absorbed power by the available engine power. This is fixed for each rpm point by the engine/prop setup. The only freedom the operator has is to select the rpm operating point. If you do not like the percentage load at your selected rpm, then you must change engines or props (or gear ratios, but let's not make it more complicated). If you are determined to run a certain percentage load, you must advance the rpm until you reach a point in the curves that achieves that load. On a traditionally set up boat, the prop is selected so that the rising propellor absorption load meets the engine power available load at the manufacturers maximum spec'd rpm.

If you had an on-command variable pitch prop, then you could choose what load to run regardless of rpm, within limits. Increasing the pitch would raise the propellor absorption curve, changing the ratio of absorption to available power.
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Old 01-30-2019, 02:11 AM   #42
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Horses are clearly not created equally.

Our 14 litre 855 Cummins generates around 90 horses @ 1150 rpm and pushes us at around 7.5knots

According to the pic above we could put one of those smaller Cummins in, run at 1250rpm for around 90 horses burning half the fuel BUT I doubt those horses could spin the fan we have and get us to 7.5 knots.
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Old 01-30-2019, 03:27 AM   #43
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Originally Posted by Simi 60 View Post
Horses are clearly not created equally.

Our 14 litre 855 Cummins generates around 90 horses @ 1150 rpm and pushes us at around 7.5knots

According to the pic above we could put one of those smaller Cummins in, run at 1250rpm for around 90 horses burning half the fuel BUT I doubt those horses could spin the fan we have and get us to 7.5 knots.
Hi, if you know your CM torque Nm-curve, they can compare qsb 5.9 to 380hp which gives 1200RMP / 639 Nm.


Qsb 5.9 max torque 2000RMP / 1218Nm and CM855 400hp is 1500RMP /1559 Nm.


Max torque difference 341 Nm big block good, is not much?


NBs
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Old 01-30-2019, 05:05 AM   #44
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NTA855 @ 350hp
Not CM 855 @ 400hp.
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Old 01-30-2019, 07:48 AM   #45
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"According to the pic above we could put one of those smaller Cummins in, run at 1250rpm for around 90 horses burning half the fuel BUT I doubt those horses could spin the fan we have and get us to 7.5 knots."

If the published figures are honest all it would take is a higher reduction ratio in the tranny to have the smaller engine operating at the same shaft speed.

You might save more than half the fuel because the smaller engine should be in a better BMEP range.
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Old 01-30-2019, 08:03 AM   #46
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Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?
That's a good question and you got a number of different answers. I'll chip in for what it's worth. My engines are JD engines and the owner's manual defines the term "Load Factor" on page 20-3 as "the actual fuel burned over a period of time divided by the full-power fuel consumption for the same period of time". They use load factor to set operating parameters.

Note - you can take a snapshot of load at a point in time or you can look at average load over a stretch in time (an hour, a day, any unit of time you capture your total fuel burned over the full power fuel consumption for the same time).

There are other legitimate ways to describe "load" - so it's fair to start a conversation by asking how that individual is defining load. And when you do, you will get all sorts of answers that might all be legitimate engineering definitions of "load".

Under John Deere's definition, on my engines: 2300 RPM is 100% load and burning 8.8 gph.

80% load is around 7.0 GPH and I reach that at a hair over 2100 RPM.
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Old 01-30-2019, 08:27 AM   #47
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Originally Posted by Simi 60 View Post
Horses are clearly not created equally.

Our 14 litre 855 Cummins generates around 90 horses @ 1150 rpm and pushes us at around 7.5knots

According to the pic above we could put one of those smaller Cummins in, run at 1250rpm for around 90 horses burning half the fuel BUT I doubt those horses could spin the fan we have and get us to 7.5 knots.
You are reading the chart wrong. Yes the little motor can make 90hp at 1250, but that is maximum. Desired loading points are on the lower curve, or at rpms above (really to the right of the curve, a bit misleading there). So propped right, if you want 90hp you want rpm to be around 1800.

Your 855 is the right engine for your boat. Big, quiet, and working easy. Yes, the little engine could make the boat do 7.5kts, but it would be noisy and not likely live as long.
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Old 01-30-2019, 09:42 AM   #48
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.....Sunchaser explained in the original post that the Passagemaker author defined percent of load in terms of fuel burn. I pointed out early in the string that the Passage maker article is on their website (free). The concept of prop curves and fuel burn as a measure of % load is explained in the writeup. Here it is (thirty seconds to find it):

https://www.passagemaker.com/technic...my-engine-last
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Old 01-30-2019, 09:53 AM   #49
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Does anyone have an engine manual which recommends running the engine at 80% or any other loading to preserve the life of the engine?.

Gordon
Anyone know of an engine that wore out at 80% load?

My theory is run-em light and fast. Relatively light load and relatively high rpm.
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Old 01-30-2019, 10:20 AM   #50
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Anyone know of an engine that wore out at 80% load?

My theory is run-em light and fast. Relatively light load and relatively high rpm.
The cause of very few engine failures is 100% known. With most, it's this happened, likely caused by that or that. We speculate it was poor maintenance or how it was run or something like that.

It's a bit like human deaths. Most death certificates have a primary cause and secondary and more. Someone dies of cardiac arrest, but what caused their heart problems. Then you say they had xyz heart condition, but what caused that. Oh might be their drinking or smoking or diet or who knows but if there was one of those known it gets listed.

Well, engines are the same. As someone else mentioned, if you could run a scientific study with a thousand engines you'd learn something, but even then you wouldn't be duplicating real life conditions.
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Old 01-30-2019, 10:23 AM   #51
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Anyone know of an engine that wore out at 80% load?

My theory is run-em light and fast. Relatively light load and relatively high rpm.
Depends what you mean by light. Lots of genset engines have reportedly been ruined by very light loading.
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Old 01-30-2019, 11:06 AM   #52
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Under John Deere's definition, on my engines: 2300 RPM is 100% load and burning 8.8 gph.

80% load is around 7.0 GPH and I reach that at a hair over 2100 RPM.
Good point BK. Interestingly, Steve Z's PM article states 80% load is around 90% rated RPM. He must have been peeking at a JD sheet. On most marine engines, 35% load is 60 - 70% rated RPM.

Regarding pre-failure analysis (or better said repair it before it breaks) it is commonly done for tens of thousands of variable use engines under extended warranty programs. Especially during the last few decades as on engine diagnostics are downloaded during routine maintenance and oil sampling.
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Old 01-30-2019, 11:34 AM   #53
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Good point BK. Interestingly, Steve Z's PM article states 80% load is around 90% rated RPM. He must have been peeking at a JD sheet. On most marine engines, 35% load is 60 - 70% rated RPM.
Or he looked at some prop charts, which are shaped pretty much the same for a given hull configuration.
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Old 01-30-2019, 11:50 AM   #54
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Or he looked at some prop charts, which are shaped pretty much the same for a given hull configuration.
Very true. Detailed tank testing for optimizing some hull designs tends to run contrary to generic hull configurations vs prop curves. Specifically Dashew FPBs, Zurn's work product, MJM, and Sam Devlin. There are others.
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Old 01-30-2019, 11:56 AM   #55
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The act of greatest wear on our engines is when the crankpin is just a bit above mid-point and the fuel injected is for max power. This is where the sideways force is greatest piston to cyl wall.

It’s load that wears out engines.
Bad operating, poor maint, and neglect that “kills” engines. Very very few engines in pleasure trawlers get the chance to get worn out.
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Old 01-30-2019, 12:07 PM   #56
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Percentage load is the ratio of power actually being produced to power available from that engine at that rpm. On a boat you cannot control it directly, it is a result of the engine, prop, and rpm. If you consider an example of the standard engine/prop curves it may become more apparent:



In the chart, the top curve is the power the engine is capable of producing at each rpm setting (though commonly called "throttle", there is no throttle on a diesel, only an rpm lever). That curve is designed into the engine at the factory. The lower curve is the power that the propellor can absorb at each rpm. It cannot be changed without changing the prop. For a particular engine and prop, a particular rpm setting will result in the the engine running at the percentage load represented by dividing the propellor absorbed power by the available engine power. This is fixed for each rpm point by the engine/prop setup. The only freedom the operator has is to select the rpm operating point. If you do not like the percentage load at your selected rpm, then you must change engines or props (or gear ratios, but let's not make it more complicated). If you are determined to run a certain percentage load, you must advance the rpm until you reach a point in the curves that achieves that load. On a traditionally set up boat, the prop is selected so that the rising propellor absorption load meets the engine power available load at the manufacturers maximum spec'd rpm.

If you had an on-command variable pitch prop, then you could choose what load to run regardless of rpm, within limits. Increasing the pitch would raise the propellor absorption curve, changing the ratio of absorption to available power.

A muchmuch simpler way to express it is to say % of load is the % of max fuel burn. My engines max is two gph. I burn 1 gph so I run at 50% load.
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Old 01-30-2019, 12:12 PM   #57
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Originally Posted by DDW View Post
Percentage load is the ratio of power actually being produced to power available from that engine at that rpm. On a boat you cannot control it directly, it is a result of the engine, prop, and rpm. If you consider an example of the standard engine/prop curves it may become more apparent:



In the chart, the top curve is the power the engine is capable of producing at each rpm setting (though commonly called "throttle", there is no throttle on a diesel, only an rpm lever). That curve is designed into the engine at the factory. The lower curve is the power that the propellor can absorb at each rpm. It cannot be changed without changing the prop. For a particular engine and prop, a particular rpm setting will result in the the engine running at the percentage load represented by dividing the propellor absorbed power by the available engine power. This is fixed for each rpm point by the engine/prop setup. The only freedom the operator has is to select the rpm operating point. If you do not like the percentage load at your selected rpm, then you must change engines or props (or gear ratios, but let's not make it more complicated). If you are determined to run a certain percentage load, you must advance the rpm until you reach a point in the curves that achieves that load. On a traditionally set up boat, the prop is selected so that the rising propellor absorption load meets the engine power available load at the manufacturers maximum spec'd rpm.

If you had an on-command variable pitch prop, then you could choose what load to run regardless of rpm, within limits. Increasing the pitch would raise the propellor absorption curve, changing the ratio of absorption to available power.
Excellent explanation.
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Old 01-30-2019, 12:30 PM   #58
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Because fuel burn/power produced is relatively constant, you can use fuel burn as a proxy for load, but only if you are comparing fuel burn to max fuel burn at that rpm. For example in the QSB chart, the maximum possible burn at 3000 rpm and 100% load, is about 20 gal/hr. If I'm operating a 1200 rpm, the actual fuel burn will be about 6 gal/hr. at 100% load. If you divide actual burn by the 20 gal/hr maximum at max rpm, you might think the load is 30% and that is not correct - it is 100%.

On an electronic engine the computer will tell you the load, and it does so by estimating fuel burn - but compares it to the 100% load fuel burn at that rpm.

Note that in the Cummins chart, the fuel burn listed is the fuel needed to spin the prop at that rpm, not the maximum fuel that can be burnt at that rpm if the engine was loaded to 100%.
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Old 01-30-2019, 03:12 PM   #59
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DDW -Thanks for clarifying the need to include the prop in the picture.

I tend to look at engine speed at a wholistic level.
I don't use % load or % rpm to decide best cruising speed. I just listen and feel. This method takes into account not only the engine, but the gearbox, damper plate, shaft, prop, engine mounts, alignment, and cavitation at prop as well. These all should be set up perfectly, but in real life nothing is perfect. All of these things, either individually or as a combination, could have an effect on engine longevity.

For me, my usual cruising rpm is about 1850 rpm on my 3000 rpm rated engine. This happens to be the point of maximum torque as well. At this speed the vibration and harmonics seem to be at their minimum. My boat speed is a bit less than hull speed, noise level is ok, fuel burn rate is good and the world is wonderful. It seems like a happy place for my engine as well.
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Old 01-30-2019, 03:27 PM   #60
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This is the table that I came up with some years ago for my single 6BT5.9 engine in WESTERLY.

The key to the table is having accurate fuel oil consumption at varying rpm's based on actual propeller load. The Cummins data sheets are then used to determine HP/gal, and finally a calculation of load %.

It's a simple table, and verifies Eric's observation about "% of load is the % of max fuel burn".
Attached Files
File Type: pdf FUEL CONSUMPTION TABLE.pdf (7.1 KB, 62 views)
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