Engine Longevity

[DDW]

The actual propeller load tells you little about the percentage load on the engine, as these things are normally defined. What you have calculated in your table is the power the engine is producing, compared to what it is capable of at maximum rpm. Usually percentage load is defined as the actual torque, compared to the maximum torque available at a given rpm. Torque is a measure of the forces on the pistons, bearings, etc. The Cummins data like the one I've posted does not show fuel consumption at maximum power output, which could be used to construct your table.

If I installed a prop on the QSB in the table that only allowed it to reach 1500 rpm at full "throttle", it would burn only about 10 gal/hr compared to 20 gal/hr the same engine will do at maximum power output. So even though the engine is producing all the torque and power of which it is capable (at that rpm), would you call that 50% load? Note that it will also be near its peak torque output. I would call that 100% load. So would the engine instruments on an electronic engine.

For example, on my QSB, I cruise at 1300 rpm burning 1.9 gal/hr. The Cummins ECU reports this as 33% load. That is correctly illustrated in the Cummins graph, which shows about 50 hp absorbed by the prop and 150 available from the engine. But using the formula that Eric suggests, it would be 1.9/20.1 g/h or 9%. I believe the Cummins reported figure is correct.

 

[Simi 60]

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.

Yep, mixed up/read kW as HP , silly me.

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.

I knew that and am more than happy with what she has but I get the impression some are implying that you only need the HP required to push the hull.
As we use about 90hp to get to hull speed a smaller engine would in theory be sufficient.
Our sister ship from another mother (same style on McLaren hull) has a 127hp 6lxb Gardner and while it obviously does the job I suspect it is working a fair bit harder to do it.

To take that to extremes, why not a 2 litre yanmar or a 90hp outboard? They'd probably be dead in a day.
Obviously more is needed than just HP.
Bucketloads of torque and engine weight is also a requirement for longevity.

 

[Rufus]

The actual propeller load tells you little about the percentage load on the engine, as these things are normally defined. What you have calculated in your table is the power the engine is producing, compared to what it is capable of at maximum rpm. Usually percentage load is defined as the actual torque, compared to the maximum torque available at a given rpm. Torque is a measure of the forces on the pistons, bearings, etc. The Cummins data like the one I've posted does not show fuel consumption at maximum power output, which could be used to construct your table.

If I installed a prop on the QSB in the table that only allowed it to reach 1500 rpm at full "throttle", it would burn only about 10 gal/hr compared to 20 gal/hr the same engine will do at maximum power output. So even though the engine is producing all the torque and power of which it is capable (at that rpm), would you call that 50% load? Note that it will also be near its peak torque output. I would call that 100% load. So would the engine instruments on an electronic engine.

For example, on my QSB, I cruise at 1300 rpm burning 1.9 gal/hr. The Cummins ECU reports this as 33% load. That is correctly illustrated in the Cummins graph, which shows about 50 hp absorbed by the prop and 150 available from the engine. But using the formula that Eric suggests, it would be 1.9/20.1 g/h or 9%. I believe the Cummins reported figure is correct.

The Cummins definition by itself isn't a predictive tool, which is what the author of the Passagemaker article was attempting to provide.

 

[DDW]

Not sure what that means, but if you are going to run the engine at some specific percentage load the definition matters. As I've shown, comparing the fuel burn rate only to the maximum at max power and rpm, can yield a large variety of answers, where there really ought to be only one.

I can't access the Passagemaker article, perhaps S.A. will chip in and clarify what he intended, but I'd be surprised to find he does not use the same definition that the engine manufacturers use.

 

[sunchaser]

I can't access the Passagemaker article.

Post #48

 

[DDW]

Ah, didn't see the link. Now I've read the article, I'll say that in my opinion is theory is flawed but his conclusion is correct. The theory would be valid if you were running near max rpm all the time, in a trawler (especially a SD trawler) you are typically not. If operating a SD at displacement speeds with a normal prop, you have no choice but to reduce power. That is a consequence of the propeller absorption curve. On a FD trawler, operating in the upper rpm region where the power curve is flat, his formula is correct. At lower rpm, where the power curve has a significant slope, his formula will lead to significant errors.

 

[Nomad Willy]

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.

For comparison (our boats are so similar) even the same engine.

I cruise at 2300rpm. Minimum vibration and noise is brobably somewhere around 1100rpm. My speed is 6.15 knots and HS is 7 knots.

I think you’re on to something by calling it an intuitive thing .. what feels best. There has been at least hundreds of times on TF skippers have refered to the “sweet spot” and I always assume they mean an rpm that is smoother than most other options and this should be quieter than others also. Next time I’m out I’ll try 1850rpm. 1850rpm will obviously be quite slow.
What gear do you have? Me .. BW w 2.57-1 ratio and an 18X13 prop

 

[sunchaser]

Ah, didn't see the link. Now I've read the article, I'll say that in my opinion is theory is flawed but his conclusion is correct. The theory would be valid if you were running near max rpm all the time, in a trawler (especially a SD trawler) you are typically not. If operating a SD at displacement speeds with a normal prop, you have no choice but to reduce power. That is a consequence of the propeller absorption curve. On a FD trawler, operating in the upper rpm region where the power curve is flat, his formula is correct. At lower rpm, where the power curve has a significant slope, his formula will lead to significant errors.

DDW
You might want to connect with Steve Z direct and query him as to your thoughts. He is very approachable and given his desire to publish in an established magazine I'd guess feedback is important and may give him pause.

My take is to not overthink this question about longevity too much. Stick to the well proven basics and that is the best most of us can do do. Especially since we have presumably bolted in iron that we'd be a bit loathe to pull out to satisfy a theoretical and generalized prop curve.

 

[Rufus]

Ah, didn't see the link. Now I've read the article, I'll say that in my opinion is theory is flawed but his conclusion is correct. The theory would be valid if you were running near max rpm all the time, in a trawler (especially a SD trawler) you are typically not. If operating a SD at displacement speeds with a normal prop, you have no choice but to reduce power. That is a consequence of the propeller absorption curve. On a FD trawler, operating in the upper rpm region where the power curve is flat, his formula is correct. At lower rpm, where the power curve has a significant slope, his formula will lead to significant errors.

Of course it has potential for errors. The whole premise is an approximation. It's a hypothetical, comparative, rule of thumb, predictive type of tool. If this....then this (in terms of engine life....not an estimate of mechanical loading/stress at a single power point (the Cummins ECU) . His point is that IF you ran an engine in a SD at full power all the time (back in the day when fuel was free, owners did that), then this is a comparison of longevity between that and operating at low power. No, it doesn't fit the example of an oversized engine in a FD very well. But the vast majority of recreational "trawlers" are planing or SD.

 

[AusCan]

For comparison (our boats are so similar) even the same engine.

I cruise at 2300rpm. Minimum vibration and noise is brobably somewhere around 1100rpm. My speed is 6.15 knots and HS is 7 knots.

I think you’re on to something by calling it an intuitive thing .. what feels best. There has been at least hundreds of times on TF skippers have refered to the “sweet spot” and I always assume they mean an rpm that is smoother than most other options and this should be quieter than others also. Next time I’m out I’ll try 1850rpm. 1850rpm will obviously be quite slow.
What gear do you have? Me .. BW w 2.57-1 ratio and an 18X13 prop

Eric - My gear ratio is 1.93 : 1, with a 17 x 10 prop. At higher rpm I start developing some minor harmonics. I think it is shaft related as the old Volvo did the same. Maybe the shaft is very slightly out of balance.

2300 rpm does give me an extra 0.5 - 0.8 knot over 1850 rpm. That is usually as hard as as push my engine if I am rushing to get to an anchorage before dark.

 

[Nomad Willy]

OK good AusCan,
I had harmonic problems w my previous trawler ... an Albin 25.
Do you have an intermediate propeller shaft bearing? I didn’t on the Albin and chased many other posibilities. Your boat and the Albin have that in common so jury rigging one in may produce results. For a location avoid the half way point. But that could work too by doubling the excited frequency.
The Albin had a 1” shaft and at times would act like a snake.

I have no vibrations anything like that on the Willard. The engine mounts are connected to the stringers that are connected to the hull and the hull is 3000lbs of ballast at that point. Could have alot to do w the smoothness.
Another factor could be my “Poly-Flex” engine mounts. Not rubber but plastic I think.

I run mine at 2500rpm as long as I like and 2700 for 5-10 min. WOT gives me 3000rpm.

 

[DDW]

DDW
You might want to connect with Steve Z direct and query him as to your thoughts. He is very approachable and given his desire to publish in an established magazine I'd guess feedback is important and may give him pause.

My take is to not overthink this question about longevity too much. Stick to the well proven basics and that is the best most of us can do do. Especially since we have presumably bolted in iron that we'd be a bit loathe to pull out to satisfy a theoretical and generalized prop curve.

Is this the Steve Z in Virginia? I've met him and spoken to him in the past, my boat was in his yard for more than a year.

As a rule of thumb I don't disagree with his conclusion, and in a properly set up boat the result is going to be the same. In an SD boat, you really only have a couple of choices, displacement speed when you will be running at low load and planing when you will be in the flat part of the power curve where his logic makes sense. The grey area is continuous running at an unfortunate speed in between the two, the engine might be highly loaded but still not consuming a lot of fuel, as in my 1500 rpm case. But most people would not do that for other reasons.

On a FD boat the available HP is less, and you will likely always be running it in the flat power section of the curve, and again his logic makes sense there.

There are other boat/engine/prop combinations where this is not true, and so a correct understanding of what is going on will prevent grief in those situations. And if you want to calculate your load to see where you are, you want to use the correct formulas. On my boat at cruise I'm at 33% load, not the 9% predicted by his method - that's a fair difference, though both predict a long life.

The Cummins ECU (and any common rail ECU) will give you percent load at any operating point updated in real time.

 

[Rufus]

The Cummins ECU (and any common rail ECU) will give you percent load at any operating point updated in real time.

Unless they have a strain gauged output shaft a some other direct torque measuring device, they're making a calculation using a prop curve and/or fuel flow....not much different than looking at the prop chart (at least for our boat).

 

[Ski in NC]

Unless they have a strain gauged output shaft a some other direct torque measuring device, they're making a calculation using a prop curve and/or fuel flow....not much different than looking at the prop chart (at least for our boat).

The computer engines are better than that. They don't measure actual fuel flow, they calculate that based on fuel pressure, rpm, injector open times, etc. They have lots of dyno data with actual flow meters and can say if fuel pressure is X and injector times Y, burn is Z. Experience from tank fills shows the calculated fuel burn is real close, within a few %.

Also in the computer is a data map where the calc'd burn rate and other parameters pinpoint right on the map where the engine is operating.

Powerful technology. Just stay away from lightning!

 

[Rufus]

My point is that it's a calculation ultimately based on fuel consumption and not torque, which tells the real story. Sure, their charts are more precise than the prop chart fuel burn, but then so what.... Why do I really care that the engine is x% away from maximum potential output if it can't be accessed anyway (unless I have a variable pitch prop). If the manufacturer is gathering that data and using it in some constructive manner, then that's another story. Doubt that they do unless there's a warranty issue and they can use it against the owner/operator. Give me a torque gauge over a calculated number any day.

 

[Mako]

There have been a few comments here about the key to long engine life being keeping the oil clean with aftermarket bypass filters, Gulf Coasts, centrifugal polishers, etc.

However I've always thought that just running the engine properly, monitoring EGT and sticking to the manufacturer's recommended oil change intervals should be good enough, without having to resort to expensive aftermarket items or synthetic oils.

Is there really some magic to these aftermarket items?

 

[mcarthur]

Perhaps the question to ask manufacturers is - did they test their engines for a significant time on 5yo diesel stored in rusty old steel drums left out in the rain? If the answer is yes, then you are probably right to trust them.
Or you could trust that the previous owners have always used manufacturer test-quality fuel and that you will as well. For some people in first world locations that get regular new fuel supplies stored well, it’ll probably be ok!

 

[FF]

The percent of power as described works OK but just as important is the percent of power used compared to the engine rating at operating RPM.

This will be a much higher percentage than when measuring against WOT

Both of these methods stink , compared to using a fuel map or BMEP chart to set HP/RPM for efficient cruise .

In the "bullseye" the least fuel burn for the HP required was found by engine builder testing, , the "sweet spot" where the engine works hard enough not to slobber , and easy enough to not be overloaded.

 

[Mischief Managed]

Interesting thread. Got me thinking though, has anyone here actually worn out a 4 stroke diesel or gas engine in a recreational boat? I don't want to include catastrophic failures, just simple wear from normal use with proper maintenance. I did not include 2 stroke diesel engines because they do seem to wear out faster, but are easy and cheap to rebuild. I also did not include 2 stroke gas engines because they often fail due to inadequate oil in the gas/oil mix.



In my immediate family's collection of vehicles (that I maintain) we have:


One turbo diesel V8 pickup with 6000ish hours (209,000 miles) This one has an hour meter which is how I can easily estimate hours on the other vehicles based on miles.


Two 4 cylinder gas-powered with 6000ish hours (200,000 to 220,000 miles)


Two V6 gas-powered cars with 4000 to 5000 hours (145000 to 180000 miles)


One 4 cylinder sport motorcycle (Ninja ZX9R) with 3000ish hours (111,000 miles)


One V8 gas-powered boat with 1000 hours (real hour meter on this one too)



All of them run like new. I have never actually worn out an engine on anything and I keep stuff a really long time.

 

[Rufus]

Both of these methods stink , compared to using a fuel map or BMEP chart to set HP/RPM for efficient cruise .

In the "bullseye" the least fuel burn for the HP required was found by engine builder testing, , the "sweet spot" where the engine works hard enough not to slobber , and easy enough to not be overloaded.

If the engine is in a boat with a fixed pitch prop, there will be places in the boat/engine operating envelope where it is impossible to get the engine to the bullseye. The overload issue is mute for a boat with the appropriate (fixed pitch) prop. ECUs have mitigated the slobber problem to some extent.

 

[BandB]

There have been a few comments here about the key to long engine life being keeping the oil clean with aftermarket bypass filters, Gulf Coasts, centrifugal polishers, etc.

However I've always thought that just running the engine properly, monitoring EGT and sticking to the manufacturer's recommended oil change intervals should be good enough, without having to resort to expensive aftermarket items or synthetic oils.

Is there really some magic to these aftermarket items?

The items you mention and others are typically designed or used to address a specific issue. If everything is ok, clean oil and fuel, good maintenance, then extras likely not of value, but to address specific issues, they are a bit magical.

 

[BandB]

Interesting thread. Got me thinking though, has anyone here actually worn out a 4 stroke diesel or gas engine in a recreational boat? I don't want to include catastrophic failures, just simple wear from normal use with proper maintenance. I did not include 2 stroke diesel engines because they do seem to wear out faster, but are easy and cheap to rebuild. I also did not include 2 stroke gas engines because they often fail due to inadequate oil in the gas/oil mix.

How many human beings do you know who just wear out? We don't die from miles or age. We have one or more parts or organs that fails on us.

Wear leads to catastrophic failure. You can't ignore it. 2 strokes don't just wear out either, something goes on them and then other damage occurs. So, 4 strokes don't fail simply from old age, but they do fail from one or more parts failing due to use or age.

I recall seeing "blown" outboards when I was young. Often they'd have something like a broken crankshaft and it would be amazing how parts of the engine would look almost brand new. But the engine was trash at that point.

 

[psneeld]

From a well respected marine engineer...

"How many angels can dance on the head of a pin?

Unless you measure propeller shaft torque, rpm fuel flow, and SOG in real time and use that data to calculate propulsive efficiency (fuel burned vs distance traveled) there is little point in the ongoing discussion.

By its nature, a propeller will impose a widely varying load on the prop shaft/engine. It varies with sea state, wind, and speed through the water, plus depth of prop immersion. It varies rapidly and moment by moment and only over a period of hours can a useful average be obtained and that is only really useful for boats making long enough passages for such data to really matter.*

The discussion is just about as useful as the anchor discussion and singles vs twins.*"

 

[sunchaser]

PSN
I was wondering when the marine engineer would chime in. Nice to know RickB is still alive and well.

 

[Gordon J]

Wow!

Interesting thread. Got me thinking though, has anyone here actually worn out a 4 stroke diesel or gas engine in a recreational boat? I don't want to include catastrophic failures, just simple wear from normal use with proper maintenance. I did not include 2 stroke diesel engines because they do seem to wear out faster, but are easy and cheap to rebuild. I also did not include 2 stroke gas engines because they often fail due to inadequate oil in the gas/oil mix.



In my immediate family's collection of vehicles (that I maintain) we have:


One turbo diesel V8 pickup with 6000ish hours (209,000 miles) This one has an hour meter which is how I can easily estimate hours on the other vehicles based on miles.


Two 4 cylinder gas-powered with 6000ish hours (200,000 to 220,000 miles)


Two V6 gas-powered cars with 4000 to 5000 hours (145000 to 180000 miles)


One 4 cylinder sport motorcycle (Ninja ZX9R) with 3000ish hours (111,000 miles)


One V8 gas-powered boat with 1000 hours (real hour meter on this one too)



All of them run like new. I have never actually worn out an engine on anything and I keep stuff a really long time.

You are my new hero. That you keep the vehicles so long is amazing in this current throw-away society.

Gordon

 

[Delfin]

There have been a few comments here about the key to long engine life being keeping the oil clean with aftermarket bypass filters, Gulf Coasts, centrifugal polishers, etc.

However I've always thought that just running the engine properly, monitoring EGT and sticking to the manufacturer's recommended oil change intervals should be good enough, without having to resort to expensive aftermarket items or synthetic oils.

Is there really some magic to these aftermarket items?

I don't recall that anyone has suggested that bypass filteration is a key to much of anything other than cleaner oil, but I might have missed it. The reduction of wear with cleaner rather than dirtier oil seems more like physics than magic, but you're quite right - "good enough" is, well, good enough for some of not the everyone.

 

[DDW]

Saying that fuel used is an indirect measure of torque on a diesel engine is in some sense backwards. It is the fuel injected that causes the torque, not the other way around. The torque map in the ECU of a modern common rail engine is very accurate, the engine operation absolutely depends on it being so.

 

[Rufus]

The quote didn't say that torque is a measure of fuel burn. But a torque measurement is required to establish actual horsepower at a given rpm (impossible to measure HP directly). Do these ECU engines measure torque? If they don't then a BMEP approach (FF's post) is not in the cards.

 

[Ski in NC]

Seems like a circular discussion. Torque is not measured. Fuel flow is not measured.

ECU has rpm measurement, crank position sensing, charge air measurement, along with various temps and pressures. Input rpm request from helm. Output from fuel map is injector open times, timing, rail pressure, etc.

But in the above algorithm, the data points can be compared to data taken when the (or same model) engine was on the factory dyno. On the dyno they DO measure torque and fuel burn with lab quality equipment. Those dyno data are stored in the ECU.

So where ever you are operating the engine, ECU can look up in its tables and see what the torque and fuel burn should be based on the dyno data and current operating parameters. That is what is displayed. No need to actually measure. Measuring torque and burn directly requires expensive and sensitive equipment.

 

[Rufus]

Seems like a circular discussion. Torque is not measured. Fuel flow is not measured.

ECU has rpm measurement, crank position sensing, charge air measurement, along with various temps and pressures. Input rpm request from helm. Output from fuel map is injector open times, timing, rail pressure, etc.

But in the above algorithm, the data points can be compared to data taken when the (or same model) engine was on the factory dyno. On the dyno they DO measure torque and fuel burn with lab quality equipment. Those dyno data are stored in the ECU.

So where ever you are operating the engine, ECU can look up in its tables and see what the torque and fuel burn should be based on the dyno data and current operating parameters. That is what is displayed. No need to actually measure. Measuring torque and burn directly requires expensive and sensitive equipment.

No it's not circular. Obviously ECU engines work. My remarks were specifically in response to FF's comment about using the BMEP charts for engine operation. Todays ECUs basically load prop chart(s) in memory for more precise engine control. But they don't have the entire BMEP matrix loaded up, nor can they send the engine to the best point on that matrix to achieve a desired hull speed. Impossible to do with a fixed pitch prop. Add a variable pitch prop and the BMEP scenario becomes realistic. Instead of an rpm demand, the parameter input by the operator would likely be hull speed. The prop computer and the engine computer would talk to one another and arrive at the best place to take the engine/propeller combination. I don't believe that could be done without actual torque as an input parameter. For one thing torque limits on drive line components would have to be respected. This is a common scenario with turbine helicopter drive trains.