Engine Longevity

I think there are too many nuances to live with many of the dumbed down rules of thumb. On my Volvo’s I was able to observe that increasing the cruise rpm on a heavy boat loaded with dive gear actually saw an almost immediate drop in exhaust gas temps. We would tend to say that the engine was run “harder” because more throttle was applied, but empirically we see that the engine was actually run easier. Rpm by itself is not a bad thing, it’s all about load.

Think about pulling a weight up a hill with a rope directly or with a block and tackle. You will do the same work. But with the block and tackle, you will pull the rope that much further. If you use an engine and increase the rpm with the block and tackle, you will do the same work over the same time, but with much less load per revolution. Clearly rpm is not the whole story.

In my case, my engine would lug more at lower rpm, than higher, just as when you put your car in 2nd gear it lugs more than just before you shift into third.

So when we prop a boat, we often like to claim that getting more rpm out of the higher rpms is better. Because this is setting a very low load at high rpm, ensuring the load at the lower rpm we actually run at is done without lugging. At least, as far as it goes.

Surely these nuances presented themselves are incomplete. Even so, trying to make a blanket statement with just these few is pretty challenging. What if we throw turbocharging into the mix and achieve Intake pressures capable of a much more efficient fuel burn and overall engine efficiency?

Lots of the advice, even the dumbed down variety, carries some valuable insights. But it only goes so far and when people try to turn it into religious style dogma, much of the value vanishes. There is a lot more going on than simply running at 30% or 80% of throttle, sometimes, maybe, probably.

Gordon J said:

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

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I have one that recommends running at 80% but does not say that is to preserve the life of the engine. It's simply recommending a cruise speed and considering the boat and engines aimed more toward not running at WOT than anything.

What I've seen too is most recommendations of how to run engines are the recommendations for break in. We choose to follow those long term. Typically, not run at WOT over X minutes at a time, vary speeds, run at 80% load or less. While some might consider following break in procedures babying engines, I've done it for 35 years and never had an engine problem so not likely to change now.

My old 3208 manual recommends running at 400 RPM below WOT, which would be 2400RPM on a 2800RPM motor. That is 86% of WOT and just about 60% load for a FD hull propped appropriately where load as I now understand is calculated as fuel consumption at 2400/fuel consumption at WOT of 2800 (7GPH/11.9GPH).

If someone would just fund the purchase of 100 engines and dynos, we could do a test to determine the longest life. First we would need to define what that is: Longest number of hours run? Most rotations before failure? Most power produced over the life? Most miles done in a particular application? These are all likely different answers, and the latter highly dependent on the specific installation. The engine manufacturers probably have this data, but hold it closely.

An additional problem in a boat (unless you are designing it from scratch) is that load is not an independent variable. Most owners do not set the throttle by load without regard for fuel consumption, sea state, noise, current, wind, handling, etc. If you knew the answer to the above question you might not act on it. On my own trawler, running at 80% load is simply impractical. Perhaps the builder fitted too large an engine, but I'm not going to change it out just so I can run 80%.

Taken to extremes, I'm quite sure that Tony's advice is correct: If you burn no fuel at all in an engine, it will last indefinitely.

When I said above that I was skeptical of the "run it hard or it'll die" mythology, it is because I am aware that a mountain of empirical evidence exists against that claim, and that is truck engines which are run easy, and last a long time. Would they last 5% longer or shorter if run at 80%? We know that all of the manufacturers derate the engines substantially in the commercial versions of light truck diesels compared to the consumer versions, which suggests lower power = longer lasting. But 5% isn't going to make a difference to most people one way or another.

The prop on your boat will be the determining factor on when and how much load is on your engine. Exhaust Gas Temp will tell the story. Throttle speed is pretty much irrelevant.

“Should have stated the prop combined with gear ratio will determine load”

BandB said:

I have one that recommends running at 80% but does not say that is to preserve the life of the engine. It's simply recommending a cruise speed and considering the boat and engines aimed more toward not running at WOT than anything.

What I've seen too is most recommendations of how to run engines are the recommendations for break in. We choose to follow those long term. Typically, not run at WOT over X minutes at a time, vary speeds, run at 80% load or less. While some might consider following break in procedures babying engines, I've done it for 35 years and never had an engine problem so not likely to change now.

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BB
Questions:
- what engines, model and HP are on your larger vessel
- what is the measured max RPM under load
- have you spent time with Steve Z talking engines

sunchaser said:

BB
Questions:
- what engines, model and HP are on your larger vessel
- what is the measured max RPM under load
- have you spent time with Steve Z talking engines

Click to expand...

We have various so I'll just give two examples:

44' Riva with twin 800 hp MAN's, i6-800's. Max RPM is 2300 and it runs 42 knots at that. Fast cruise for us is 1950 RPM, 36 knots, 75% load. 80% load would be just a little over 2000 RPM and about 37.2 knots or so.

85' Pacific Mariner with twin 1500 MTU's, 10V2000's. Max RPM is 2450 and 27 knots. Normally we cruise at 1800 to 2000 RPM, 20-23 knots, 50-65% load. 80% load is 2170 RPM and speed is just over 24 knots. Very little gained between 65% load and 80%.

These engines do have electronics reporting load which comes very close to calculations based on fuel consumption.

No, I've never spoken to Steve Z. Most of what little I gather about engines comes from talking to Captains and Engineers we employ and people I've talked to over decades as well as manufacturers recommendations.

Steve Z's 35% load on the above two boats would be about 1275 RPM and 17 knots on the Riva and 1580 RPM and 16 knots on the Pacific Mariner. Interestingly, on the Riva 35% gives about 40% of the speed of WOT, whereas on the PM it gives 59%. We do sometimes run the PM at 35% but never the Riva.

Where it becomes really interesting is looking at jet RIB's, both gas and diesel. I might have to do that later, but back to a meeting for now.

DDW: "I have long questioned the "run it hard" mythology."

I agree and this topic has been batted around on boatdiesel.com many times over the years, with many opinions questioning the need for hard running to achieve engine health. With the thousands of work and fish boats that spend most of their lives at idle and with only small PTO hydraulic loads and that go 20,000 hours+ BMOH, it tells the tale in my opinion. I do goose mine up to 80% power once in a while which seems to reduce smoke at startup, but that's it. Otherwise I'm at about 50% load or trolling. No problems.

Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?

I understand "% of RPM" as a concept - it's easy to know that 80% of WOT RPM is "x" and set the throttle at that.
I understand a general "% of HP", as one can read the generic manufacturer prop curves to determine the RPM as a % of the horsepower/kilowatt.

But what is "% of load"?!

If I'm in neutral and idling at 850rpm, is there "0% of load"? Well I'm in neutral so I assume so!
If I'm at 1000rpm and in forward, and have a WOT of 2400rpm, what "% of load" am I at? Surely I'm more at a "% of RPM" rather than "load"?

mcarthur said:

Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?

I understand "% of RPM" as a concept - it's easy to know that 80% of WOT RPM is "x" and set the throttle at that.
I understand a general "% of HP", as one can read the generic manufacturer prop curves to determine the RPM as a % of the horsepower/kilowatt.

But what is "% of load"?!

If I'm in neutral and idling at 850rpm, is there "0% of load"? Well I'm in neutral so I assume so!
If I'm at 1000rpm and in forward, and have a WOT of 2400rpm, what "% of load" am I at? Surely I'm more at a "% of RPM" rather than "load"?

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There is equipment that will actually tell you. There are various formulas considering peak torque and air flow or fuel flow and even taking barometric pressure into consideration. However, a very good approximation is Fuel consumption divided by fuel consumption at WOT. So, if a boat consumes 20 gph at WOT then 60% load would be that speed at which it consumes 12 gph. Most of the time load % will be less than the % RPM. Often 50% load will be at around 70% RPM.

You're trying to determine who percentage of the engine's capability are you using.

Some more mechanically minded might be able to actually explain some of the more complex calculations used by electronics and technically more accurate than this rough method.

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.

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.

Simi 60 said:

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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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

NTA855 @ 350hp
Not CM 855 @ 400hp.

"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.

mcarthur said:

Can someone define "% under load" please since the main (only?) control we have on a diesel is throttle/RPM?

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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.

Simi 60 said:

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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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.

.....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/technical/how-long-will-my-engine-last

Gordon J said:

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

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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.

Nomad Willy said:

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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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.

Nomad Willy said:

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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Depends what you mean by light. Lots of genset engines have reportedly been ruined by very light loading.

Bkay said:

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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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.

sunchaser said:

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.

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Or he looked at some prop charts, which are shaped pretty much the same for a given hull configuration.

Rufus said:

Or he looked at some prop charts, which are shaped pretty much the same for a given hull configuration.

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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.

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.

DDW said:

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.

Click to expand...

A muchmuch simpler way to express it is to say % of load is the % of max fuel burn. My engine’s max is two gph. I burn 1 gph so I run at 50% load.

DDW said:

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.

Click to expand...

Excellent explanation.

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%.

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.

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".