Great article on running engines at proper loads from Passagemaker

[Steve DAntonio]

Why 15m per 4 h? What’s the rationale for those numbers? Also been told beyond those short runs periodically a long run of hours with substantial loading is worthwhile. Is that true? For NA? For CR?

There is no science behind that recommendation, just my experience for elevating oil and combustion gas temps and the effect that has on internal engine components, much of which is based on my review of oil analysis reports. But again, it's anecdotal.

A long run of hours at higher load is almost certainly more beneficial, it's just most cruisers avoid that because of the cost, so the periodic short runs is a more palatable alternative.

 

[rslifkin]

I would expect that the longer it's been since the last run-up, the longer a run-up you'd want to get moisture, etc. adequately cooked out of the oil. Of course, how rapidly the oil gets junked up under light load will vary between engines (depending on oil temperature, amount of blowby, and other factors).

 

[BandB]

There is no science behind that recommendation, just my experience for elevating oil and combustion gas temps and the effect that has on internal engine components, much of which is based on my review of oil analysis reports. But again, it's anecdotal.

A long run of hours at higher load is almost certainly more beneficial, it's just most cruisers avoid that because of the cost, so the periodic short runs is a more palatable alternative.

Ours is a combination of anecdotal and experience by professionals with manufacturers' recommendations.

Steve reemphasized proper break in. Well, in my opinion. what you're told for break in also remains good to consider beyond. I believe in varying speed. Also understand we're running planing and semi-planing and not engines designed for slow speeds and constant running. I don't believe in long sustained runs at WOT. We limit WOT typically to 15 minutes and no more than 30. Many manufacturers recommend brief runs at full cruising speeds of up to 80% load. I've heard experienced captains and engineers say 15 minutes every four hours and 30 minutes for six hours and 15 for six and various other combinations. No one knows the perfect numbers. Nor is 80% some perfectly defined number. Actually, it's my opinion they often chose 80% to mean "not above" as in not WOT. We go with ranges based on what we've determined to be our ideal high cruising speeds. And where do those speeds come from? Well, looking at our performance curves and seeing where fuel usage changes significantly. I look at various boats and here are examples.

Boat 1, between 65% and 85% load, uses 33% more fuel for 8% more speed so cruise at around 65-70%.

Boat 2, between 60% and 85% load, uses 37% more fuel for 14% more speed so cruise at 60-65%.

Boat 3, between 30% and 75% load, the nm/gal stays constant. Between 75% and WOT uses 30% more fuel for 13% more speed so cruise around 75% load.

Boat 4, between 67% and 84% load, uses 25% more fuel for 11% more speed, so cruise at 65-70%.

Boat 5, between 50% and 70% load, uses 40% more fuel for 18% more speed. Between 70% and WOT, uses 42% more fuel for 20% more speed. 50% is clearly the sweet spot, but because we like a bit faster and are still well under the 80% number the engine manufacturer recommends as a max cruising, we generally run between 50% and 70%. Now, that boat is high performance and 50% is 38 knots while 70% is 45 knots. WOT is 54 knots.

I would say that good performance curves and understanding your boat are important. I feel like our engines tell us, provide us feedback on good fast cruises just as they do on good slow cruises.

For instance, I look at one chart and I notice a 32% drop off in efficiency (nm/g) between 7% and 13% load and then another 25% drop off in efficiency between 13% and 19% load and then another 32% drop off between 19% and 36% load. So overall from 7% load to 36% load you increase speed 89% and fuel usage 440%. This is boat 2 from above. Why such an odd curve? Very heavy semi-planing boat. From 7% to 36% load you're just pushing a displacement boat faster through the water. Speeds go from 9 knots to 17 knots, but at 17 knots you plane and from that point upward then you remain on plane and don't lose efficiency as rapidly. We run at planing speeds, but there are owners who have crews relocate the boats at the slowest speeds, below displacement speed. We run at 17 to 22 knots, others run at 9 to 12 knots.

Now, I do not believe anyone here is going to destroy their engines running them at low loads. However, I do believe that by occasionally opening up and achieving temperatures and reaching 60-80% loads, you will improve and maintain the performance under load. If you never intend to run at even moderate load, may be completely unimportant to you. If you do intend to run at greater speeds, then I think the 15 or 30 minutes every 4 or 6 hours is beneficial. Our goal is to maintain peak performance. If you always run at low load, then that's likely not your goal.

 

[Steve DAntonio]

But wish to understand if a longer run periodically is required to burn off carbon and clear sludge entirely. Also whether using full synthetic or doing more frequent oil changes when low loads are more common is helpful.

Oil analysis will help answer this question, you should be doing it in any event.
Hard to justify syn oil for recreational use and less than 450 hrs a year. Most engine manufacturers recommended shorter change intervals for lightly loaded or idling applications.

 

[mvweebles]

Broadly speaking, there are only a few buckets of engine failure causes:

1. Lax maintenance leading to premature failure of a component (failed heat exchanger due to old age for example)

2. Bad luck leading to premature failure of a component (broken pump shaft for example)

3. Poor install leading to premature failure, commonly exhaust related

4. Engine wears out due to thousands of hours of use.

5. Improper operation - running too hard to too light.

For our class of boats that average well under 200 hours per year, #4 is rare. But so is #5. Combined these two probably accounts for <0.1% of engine failures. Yet the amount of air time they get in forums is huge in comparison.

Just sayin'

Peter

 

[DDW]

I'm trying to reconcile the marine mythology "you got to run it hard or it won't last" with the many millions of diesels in light trucks (several orders of magnitude more numbers than in boats), almost none of which are run anywhere near 80% for the vast majority of their lives. More like 30% load. My Ford for example runs at 38% load, with the camper on the back, at freeway speeds, at less than half rated rpm. They do not seem to die from coking, carboning, or any of the stated marine problems.

It is very hard to see how running hard for a few minutes every few hours could have any affect on engine life (other than perhaps negative).

Every explanation of this mythology I've seen is anecdotal. Has anyone published objective tests?

 

[BandB]

I'm trying to reconcile the marine mythology "you got to run it hard or it won't last" with the many millions of diesels in light trucks (several orders of magnitude more numbers than in boats), almost none of which are run anywhere near 80% for the vast majority of their lives. More like 30% load. My Ford for example runs at 38% load, with the camper on the back, at freeway speeds, at less than half rated rpm. They do not seem to die from coking, carboning, or any of the stated marine problems.

It is very hard to see how running hard for a few minutes every few hours could have any affect on engine life (other than perhaps negative).

Every explanation of this mythology I've seen is anecdotal. Has anyone published objective tests?

There are no objective tests as that would require two large control groups and we don't have them. However, don't dismiss anecdotal. Experienced engineers observe. Just like experienced doctors on medical.

The comparison of truck to marine just isn't valid. Here are some of the differences. Much different ancillary equipment. Much different environment. Different usage pattern as trucks are run much more regularly. Trucks also regularly run hard through acceleration as stop and go and passing enter in. That's versus a marine diesel starting at low load and never increasing it. Your load is far more than 38% as you accelerate from 0 to 70. Your load is higher when you pass other vehicles. Truck load isn't constant.

Now, I've never argued if you don't do it, your engine will die. Neither have others with knowledge, that's old dock talk. What I do argue is that it won't perform at peak. I'll also argue, the little old lady who drives her truck only at 25 mph, won't get maximum performance either if she decides to open it up.

Engines run very little and run at low load aren't going to die. However, you decide to run them faster and they won't perform at peak either. That's fine if you never have that desire.

I outlined what we do to get peak performance our of planing and semi-planing boats. If I only ran at 20% load and 100 hours a year, I wouldn't worry.

 

[rslifkin]

I'm trying to reconcile the marine mythology "you got to run it hard or it won't last" with the many millions of diesels in light trucks (several orders of magnitude more numbers than in boats), almost none of which are run anywhere near 80% for the vast majority of their lives. More like 30% load. My Ford for example runs at 38% load, with the camper on the back, at freeway speeds, at less than half rated rpm. They do not seem to die from coking, carboning, or any of the stated marine problems.

It is very hard to see how running hard for a few minutes every few hours could have any affect on engine life (other than perhaps negative).

Every explanation of this mythology I've seen is anecdotal. Has anyone published objective tests?

Automotive engines tend to run hotter under light load. They often have no oil cooler or an oil/coolant heat exchanger. Coolant thermostats are typically hotter as well. This is all related to automotive engines not seeing the same sustained heavy load marine engines can, but instead just see relatively short bursts of heavy load. That duty cycle is also why the same engine in a heavier duty truck will typically have a bigger cooling system than in a lighter truck, as it's expected to be worked harder.



In general, if you can get a marine engine to run warm enough under light load without cooking it to death under heavy load, light load operation will carry less concerns (and have less negative impact on oil lifespan).



With my gas engines, in a truck, the same block would use a 180 or 195* thermostat and often have no oil cooler. In the boat, they've got 160* thermostats and seawater oil coolers with no thermostats. So they run much colder. They run about 180* on plane anyway, so I'm planning to swap to 170* thermostats (should have very little effect on temps under heavy load, but will bring the light load temps up a bit more). And I've been looking at oil cooler thermostats to keep the oil temps up to at least the 180 - 190 range under light load. I've seen them as low as 155 - 160 cruising at 1300 rpm in 50* water (coolant temps were steady at 165).

 

[Hippocampus]

So B you discuss your use pattern and thank you fo that. Think most users have their own which often reflects the hulls behavior. Although our experience is limited it’s fairly clear our NT42 likes to go at around 10kts. At that speed things just settle down and it’s quiet comfortable. Ride is excellent even with SeaKeeper off and a chop running. Noise and vibration trivial both inside and out. All is right with the world.
At 10.2k rpm 1890 temp 165f load 62% oil pressure 64 turbo boost 13.6 and fuel consumption 11.6. So it seems the 540hp Cummins C likes this speed through the water as well.
This was fine behavior when fuel was cheaper if you were doing only rare short runs. However, we will be chasing the sun. Instead of New England to leewards will be doing new England to Bahamas with the seasons. Now forced to look at the sharp bend in the mpg curve. From sea trial this occurs right at the beginning at 6.8 kts. and 1030rpm. Coolant temp is still 161f but load only 22% with virtually no boost but fuel use falls to 2.3gph. 81% load occurs at just 12kts. with coolant at 174f but gph at 13.6.
Clearly for this hull/engine combination 6.8kts is a more efficient cruising speed when long legs are involved.
So wonder if folks have changed their thermostat set points to be in concert to usage? Is this really worthwhile? In terms of the concerns noted above by Steve and other knowledgeable people is there a difference to the engine and auxiliaries between running a common rail at low load for a few hours once or twice a week and running at low load constantly for 2-3 days with fair frequency?
One would think some evaporation is occurring at 161f and with a tier 3 or 4 engine little unburnt fuel leaves the chamber. What’s the reality? What am I missing? Should my 15m be at 12kts which is so inefficient? Can I stretch the time between the higher speed intervals when running 24/7?

Typed this as R was posting. Thanks R

 

[rslifkin]

So wonder if folks have changed their thermostat set points to be in concert to usage? Is this really worthwhile? In terms of the concerns noted above by Steve and other knowledgeable people is there a difference to the engine and auxiliaries between running a common rail at low load for a few hours once or twice a week and running at low load constantly for 2-3 days with fair frequency?
One would think some evaporation is occurring at 161f and with a tier 3 or 4 engine little unburnt fuel leaves the chamber. What’s the reality? What am I missing?


Typed this as R was posting. Thanks R

I'd be curious what your oil temps (measured at the pan) are at both of those speeds. If the oil temps are hot enough, the 160-ish degree coolant temp may be fine. If the oil temps are also low, then it's a potential concern.



Being a modern enough engine that's still in production (in updated form), it may be worth asking Cummins what their opinion on sustained running around 1000 RPM is. They likely have some test data for it.

 

[HeadedToTexas]

Does the Cummins QSB have an oil thermostat to allow the oil to heat up during extended periods of light load?

 

[O C Diver]

The comparison of truck to marine just isn't valid. Here are some of the differences. Much different ancillary equipment. Much different environment. Different usage pattern as trucks are run much more regularly. Trucks also regularly run hard through acceleration as stop and go and passing enter in. That's versus a marine diesel starting at low load and never increasing it. Your load is far more than 38% as you accelerate from 0 to 70. Your load is higher when you pass other vehicles. Truck load isn't constant.

Think that's all a matter of how you drive whether boat or truck. Probably 80% of my truck miles were highway, get in the right lane, set the cruise control, and run for an hour or six. See a significant amount of long haul truckers driving the same way. Also see a fair number of people drive their boats on the AICW like they're in rush hour traffic. The stretch from Miami to Stuart comes to mind. Think they are very comparable as long as you pick appropriate ones to compare.

Ted

 

[Hippocampus]

Gone through the parts list and exploded views and don’t see a dedicated oil temp sensor for the QSC 8.3. Need to crawl around when I get to the boat to see if one was added. Perhaps someone here who has a marinized version of this engine would know. The oil cooler doesn’t look like it has a spot for one on the diagrams. Didn’t use the IR thermometer on the oil pan during sea trial. Every place else so that was a oversight. Will ASAP.

 

[DDW]

There are no objective tests as that would require two large control groups and we don't have them. However, don't dismiss anecdotal. Experienced engineers observe. Just like experienced doctors on medical.

The comparison of truck to marine just isn't valid. Here are some of the differences. Much different ancillary equipment. Much different environment. Different usage pattern as trucks are run much more regularly. Trucks also regularly run hard through acceleration as stop and go and passing enter in. That's versus a marine diesel starting at low load and never increasing it. Your load is far more than 38% as you accelerate from 0 to 70. Your load is higher when you pass other vehicles. Truck load isn't constant.

Now, I've never argued if you don't do it, your engine will die. Neither have others with knowledge, that's old dock talk. What I do argue is that it won't perform at peak. I'll also argue, the little old lady who drives her truck only at 25 mph, won't get maximum performance either if she decides to open it up.

Engines run very little and run at low load aren't going to die. However, you decide to run them faster and they won't perform at peak either. That's fine if you never have that desire.

I outlined what we do to get peak performance our of planing and semi-planing boats. If I only ran at 20% load and 100 hours a year, I wouldn't worry.

All of that is your (or someone's) "feeling". The tested reality might be quite different. Limited observations even by engineers can be quite wrong (and doctors, for that matter). I'm pretty sure the manufacturers have some of this data but have not published it. You'd only have to run a few engines to their end of life to know, well within an engine development budget. A good example is ring break-in procedures - everybody has their favorite, mostly talking about light load with occasional momentary heavy loads for the first N miles. Then some guy actually tested it (this is on motorcycles) by installing new rings and trying different procedures. Did about 20 tests. The results are far from what you would expect, and conventional wisdom. Tony Anthens "feeling" is that marine diesels run at low loads last longer (in hours). He suggests that a better gage of lifetime is how many gallons of diesel you can run through an engine. So a particular engine might be 10,000 hours at 80% load and 20,000 hours at 40% load. My "feeling" is that is is closer to right. Note that the same amount of work is done in both cases at the same cost, but replacement comes sooner on the heavily loaded engine - and the R&R itself is expensive.

Automotive engines tend to run hotter under light load. They often have no oil cooler or an oil/coolant heat exchanger. Coolant thermostats are typically hotter as well. This is all related to automotive engines not seeing the same sustained heavy load marine engines can, but instead just see relatively short bursts of heavy load. That duty cycle is also why the same engine in a heavier duty truck will typically have a bigger cooling system than in a lighter truck, as it's expected to be worked harder.

The Volvo in my sailboat runs at 197 deg varying almost nothing from that regardless of load. It does take longer to get it there if it is just idling. The Cummins QSB in the trawler runs at 174 deg. It will never get there at idle, but does fairly quickly at even 20% load. It will only gain about 5 degrees at WOT. I can see no difference in application needing different thermostat settings. These engines seem very well regulated in temperature.

Does the Cummins QSB have an oil thermostat to allow the oil to heat up during extended periods of light load?

I've measured the oil temp in the sump, it runs within a few degrees of the coolant temperature. I assume the oil cooler keeps it that way.

 

[rslifkin]

The Volvo in my sailboat runs at 197 deg varying almost nothing from that regardless of load. It does take longer to get it there if it is just idling. The Cummins QSB in the trawler runs at 174 deg. It will never get there at idle, but does fairly quickly at even 20% load. It will only gain about 5 degrees at WOT. I can see no difference in application needing different thermostat settings. These engines seem very well regulated in temperature.

Some engines get bigger cooling systems and colder thermostats for heavier load applications, as keeping the cylinder heads and other components a bit cooler helps to get more heat out of exhaust valves, gives more detonation margin for a gas engine, can reduce EGTs a little to help keep a turbo alive, etc. Things that aren't an issue for a car going WOT for 20 seconds at a time can be a big problem when running the engine hard for a couple of hours continuously.

 

[BandB]

He suggests that a better gage of lifetime is how many gallons of diesel you can run through an engine. So a particular engine might be 10,000 hours at 80% load and 20,000 hours at 40% load. My "feeling" is that is is closer to right. Note that the same amount of work is done in both cases at the same cost, but replacement comes sooner on the heavily loaded engine - and the R&R itself is expensive.

Fuel usage is considered the ultimate measure of engine life. CAT has long encouraged people to base maintenance on gallons, but as those numbers are not readily available to most or not relatable to others, then tables of hours have been created based on estimated fuel consumption.

There's so much misinterpretation when people use simple, flat statements. No one really suggests running the same hours at high load vs. low load will extend life. People suggest balance and suggest some higher load and management of usage.

Just using your above example, let's say 40% load is 6 knots and 80% load in one example is 9 knots but in another example is 18 knots.

So now in example 1, we're talking 20,000 hours at 40% but to achieve the same distance of 120,000 miles, 13,333 hours at 80% and a great deal more fuel burned. On the surface, it would appear running at 40% would give longer life.

However, in example 2, we're talking 20,000 at 40% or only 6,666 hours at 80% so far less fuel burned. This would give the opposite answer because for this boat the higher load and speed is more efficient.

The reality is that 95% of this is irrelevant to the average boater as they're going to run specific loads and speeds based on desire and habits and they're not going to own any boat long enough to have to replace the engines.