Cruising RPM research

[smokey73]

Thank you to @Don Lucas for challanging long held "beliefs"

Continuing the research I couldn't find any specific details from Yanmar but did find the following from CAT - Impact of Generator Underloading
While it is for generators, they get into continuous duty diesels, which for most cruisers this would apply (except those fortunate enough to only use the diesel to get into and out of the marina). By the way, the conclusion supports @Don Lucas operating practices, including "blowing it out" now and then.

The Impact Of Generator Set Underloading | Cat | Caterpillar

While power systems vary in operation, application and load profile depending on the purpose and complexity, all power systems are designed with common goals: providing reliable power and maximizing system efficiency. To achieve these design goals, it is important to understand system operation...

www.cat.com

Here are some selected quotes:
GENERATOR SET RATED LOADS First, it is important to understand that generator sets are designed to run and, to be specific, they are designed to run with load. This may seem trivial, but loading a generator set properly is essential to availability, healthy engine operation and long engine life. The ideal operation targets of each generator set will depend on the application and rating. Generally speaking, standby- and prime-rated diesel generator sets are designed to operate between 50 and 85 percent of the full nameplate, while continuous-rated diesel generator sets are optimized between 70 and 100 percent load.

Operating a diesel generator set at load levels less than 30 percent of rated output for extended time periods impacts the unit negatively. ....Running at high idle with little or no load reduces the heat in the cylinder, allowing unburned fuel and oil deposits to leak through the exhaust slip joints.

If maintained properly, diesel and gas generator sets can operate at light loads for long periods of time with no harmful effects. After operation at low load levels, each impacted generator set should operate under increased load to raise the cylinder temperature and pressure, which cleans the deposits from the combustion chamber. In addition, if low load operation is expected to occur regularly, a more aggressive maintenance plan will help to ensure that there is no excessive component wear and the chances for unplanned downtime are minimized.

Engine Load Time Limit 0 to 30 percent 1/2 hour
31 to 50 percent 2 hours
51 to 100 percent Continous

Still some interpretation is needed: From the last sentence of the first paragraph above. It appears that continuous rated diesels are "optimized" to operate between 70 and 100 percent load. If we used the "continuous rating" which is what I interpret the % value referring to (I can't imagine operating at 100% of max rated 3600 continuously) then the optimum cruising rpm would be 2100 rpm to 3000 rpm based on a max rpm of 3600 and a continuous rating of 3000.

Using the table instead of the paragraph it would be 1836 to 3000 (interpreting that the % relates to full rated here in the table and is limited by the manufacturer's continuous rating)

Since a sailboats load can vary widely with hull and prop fouling or even dragging a dingly, I would surmise that these numbers apply only to a boat with a clean hull and properly sized prop and no additional load. I say that because I once had a badly fouled prop and hull and could only get to about 2600 rpm max getting from the mooring to the pier and believe me that engine was overloaded! Luckily it was a short distance.

CAT's white paper isn't perfectly clear but its a lot better than Yanmars. So my conclusion is that stay at or above, 1850 RPM (for a 3000 continuous rated diesel), blow it out now and then, and cruise at the rpm that gives you the best compromise between speed and fuel consumption. After I do my RPM vs speed vs fuel consumption test I'll settle in on a number.

 

[Davidasailor26]

I haven’t read the original thread yet, but I’m not sure the generator analogy will address all of the variables. A generator generally runs at constant RPMs to maintain the output AC frequency, but a propulsion engine varies the load with changing RPMs. I don’t know if or how that affects the durability, but it seems like there would be differences.

 

[smokey73]

I haven’t read the original thread yet, but I’m not sure the generator analogy will address all of the variables. A generator generally runs at constant RPMs to maintain the output AC frequency, but a propulsion engine varies the load with changing RPMs. I don’t know if or how that affects the durability, but it seems like there would be differences.

Good point and there are certainly differences. There are similarities too. In a generator, the output frequency whould change as load is applied or reduced, thus, requiring an incease in throttle as the load on the generator is increased so the RPM remains the same with increasing loads. So the loading discussions (% of load) would seem to be at least analgous with the primary difference the way in which the load is applied. If electrically, requiring an increase in throttle to maintain the same speed with an increasing load. Mechanically, with an increase in load as a result of increasing speed requiring an increase in throttle. Probably not exact but I think there are still enough similarities to make some reasonable comparisons. That is why I said it "requires some interpretation." Unfortunaltly the Yanmar service manual does not have much info. At present this is the best reference on loading I could come up with. I welcome other references that would enlighten us all.

In any case, we are dealing with "loading" and I am not sure there is a significant difference in the source of the load but as you point out there may be. I would relish a better source. As JVISS pointed out in an earlier thread, the relationship between actual load and RMP is am approximation at best. Thinking on it there is an assumption you can reach full rated RPM with your boat and drivetrain configuration (prop and hull condition and external loads like dragging a dingy being the major contributors). Then to have a good analogy, you don't want to change the external loads so they don't require a change to maintain speed.

Thanks for pointing out the possible holes in the analogy though and I'll continue to look for propulsion diesel references.

 

[smokey73]

Doing some more research found this reference for marine propulsion diesels, but it should be noted that it is clearly for large ship propulsion and the comparison needs to taken with a grain of salt as @Davidasailor26 pointed out.
How to Tackle Low Load Operating Conditons of a Marine Engine?

How to Tackle Low Load Operating Conditions of Marine Engine?

Marine Insight - The maritime industry guide.

www.marineinsight.com

There are some marked similarities to the CAT generator discussion. Selected quotes are below are:
"It is considered that marine engine operating above 60% Maximum Continuous Rating (MCR) will have all its parameters and parts normal as compared to being in low load operation.'

In order to be in sync with the need of the hour, an engineer must make sure of the following points when the ship’s main engine is operating at low load (below 40%) for a long period of time:

These numbers are not too far off from the CAT generator discussion but without ranges as in the CAT discussion.
60% of MCR for a Yanmar 4JH2E would be 1800 rpm - similar to the CAT min for continuous operation
<40% defined as "low load operation" similar to the CAT recommendations

From reading both of these sources it would seem that it is narrowing down to a normal cruise RPM of =>1800 or so and then, again, a compromise of speed and fuel efficiency as the determining factor. Add a little gravy for the uncertainty of the load vs RPM relationship and it makes =>2000 RPM sound like a good starting point to maximize fuel effeciency and moving up from there to some number less than the Continuous Rating, depending on how hard you want to push your engine and how fast you want to get there. Again in line with @Don Lucas operating practice.

 

[Rich Stidger]

I have a 50hp Volvo diesel in my sailboat. I had an opportunity early on to have an engine trial with the east coast Volvo tech rep. The engine was brand new and his job was to verify that the installation and performance of the engine was correct.

During the sea trial the engine was run at various rpm speeds and to my alarm he pushed the engine to max rpm at which is 3000 for this engine. When I asked if that was a good idea for a brand new engine with less than an hour on the clock, he told me that he needed to be sure that the engine could actually achieve WOT. I further asked at what speed the engine should be operated for normal cruising. His reply was that the engine was designed and was fully capable of running at WOT-400 rpm (2600 rpm) 24/7.

His recommendations were two-fold-
First, operate the engine at 2400-2600 rpm at all times unless in an area restricted by speed. This is basically 80-87% of WOT. Avoid long periods of idle or low speed.
Second, on every outing or at least once per week, run the engine at WOT for 10 minutes. He said that by doing this high speed operation it would assure that the engine was capable of such speed; it would keep the exhaust passages clean; it would identify issues like a fouled prop; bad fuel, or partially plugged fuel filters. Thus I could then be assured that should the need arise I could depend upon the engine to perform at maximum output.

So that became my operational guide. After 23 years and 2700 hours, my original exhaust elbow is clean, and the engine starts and runs very well. I must be doing something right.

 

[smokey73]

@Rich Stidger That is very consistant with both the CAT white paper and the Marine Propulsion paper. The numbers =>1800 (for a 3600 rpm max engine) are just the minimum cruising rpm and all the way up to (MCR) Maximum Continuous Rpm. From your 3000 WOT and hs WOT-400 it seems your MCR looks like it is 2600 so operating at 2400 to 2600 is in that range and consistent with the recommendations referenced. By the way, I open it up to WOT (3600 rpm) for 5 minutes every day it is operated for the same reasons. I normally do it on return to the club right after I clear the bridge.

 

[kloudie1]

quick thoughts.. Attached is the curve set for the Yanmar 3GMF .. Note that the continuous rating is 20 HP so 50% is 10 HP .. That would occur with an ideally pitched prop at 2700 RPM..
Diesels are strange in that the "throttle" actually is an RPM setpoint that the governor will hold (try to) by adding or subtracting the amount that the injectors squirt in.. at lower loads, the injectors are cut back a lot to hold the power to the place that satisfies the RPM setpoint.
The prop is a pump, essentially and its power requirement is a cube function on RPM .. the motor makes its power in a square function kinda way so that is why the prop power requirement rises much faster than the engine power curve. with a perfectly matched prop, the prop power required and the power that the engine makes comes together at the engine max rating.. A dirty prop or boat bottom shifts the prop power curve to the left and it gets so that the engine can't make enough power to satisfy the RPM setpoint; at that point, the governor is maxing out the injectors but the air available is insufficient to burn all of the diesel.. black smoke is the result..

 

[dlochner]

Hey guys, are you all bored retired engineers?

The simple guidance I have followed for 20+ years is to limit no load engine idling and when reasonable run a diesel at 80% of WOT throttle. If the engine, prop, and boat are properly matched and everything is clean, then 80% of WOT should get you to about hull speed in flat water with some reserve for adverse wind and current.

Diesels manufacturers build a limited number of blocks and then change the innards to fit the job. Something like a crane that runs a hydraulic pump with intermittent use may use the same block as a farm tractor or or a sailboat, but the innards are different due to different task demands. Likewise for vehicle motors, diesels in trucks, buses and cars need to run at high speed for for long periods of time with relatively little load while boats need to run at lower speeds with higher torque loads.

One common fallacy among boat owners is more diesel power is better. If the boat is over powered, then it will not run at its maximum efficiency which will cause the problems others have outlined above.

 

[smokey73]

@kloudie1 Great insights. Different way of looking at it. I'll go back to my 4JH2E curves and see what it results in. Should I be looking at the DIN6270A curve or the Propeller Power Curve to determine the "HP load on the engine" I

50% load for Propeller HP it is 2700 RPM
50% load for DIN HP it is 1780 RPM

60% load for Propeller HP it is 2900 RPM
60% load for DIN HP it is 2050 RPM

80% load for Propeller HP it is 3180 RPM
80 % load for the DIN HP it is 2650 RPM

The increase in HP (load) for a given RPM increase is much sharper for the Propeller HP curve (particularly at the higher RPM) so I would surmise that Propeller HP Curve is the one to use for Load on the Engine VS RPM. @kloudie1 am I interpreting this correctly? I've been wrong before so please give me your thoughts.

 

[smokey73]

Hey guys, are you all bored retired engineers?

Yes - you caught me .

Just trying to understand if 80% of WOT corresponds to 80% load on the engine itself as per @jviss comment earlier.
Agree with everything you said, including that I run mine at 75% WOT (WOT is 3600) an at 75% I am at 2700 rpm. You are absolutly correct that "bigger isn't necessarily better" since the diesel will be chornically underloaded. I'll have to assume the design loading of the power train is optimized (unless I know otherwise).
With that, the 75% WOT would be 2700 RPM and according to the Propeller HP curve for the Yanmar 4JH2E this is only 20HP or 42% engine load. This is pretty close to a "lightly loaded" diesel"

On the other hand, if the proper curve to use is DIN6270B with Marine gear, then 75% WOT (still 2700 RPM) would be 40HP or 83% engine load.
Although it is counterintuitive, I"m thinking that the Propellor HP curve is the one to use for determining actual HP applied to the diesel since I intuitively know that for the next increment of only 900 RPM up to Max RPM, the load increases at a very rapid rate resulting in much much higer fuel burn and that should equate to higher load. Does that seem like a reasonable conclusion?

Yes I'm afflicted with "engineeritis" and see why I'm trying to understand this.

 

[dlochner]

Yes - you caught me .

Just trying to understand if 80% of WOT corresponds to 80% load on the engine itself as per @jviss comment earlier.
Agree with everything you said, including that I run mine at 75% WOT (WOT is 3600) an at 75% I am at 2700 rpm. You are absolutly correct that "bigger isn't necessarily better" since the diesel will be chornically underloaded. I'll have to assume the design loading of the power train is optimized (unless I know otherwise).
With that, the 75% WOT would be 2700 RPM and according to the Propeller HP curve for the Yanmar 4JH2E this is only 20HP or 42% engine load. This is pretty close to a "lightly loaded" diesel"

On the other hand, if the proper curve to use is DIN6270B with Marine gear, then 75% WOT (still 2700 RPM) would be 40HP or 83% engine load.
Although it is counterintuitive, I"m thinking that the Propellor HP curve is the one to use for determining actual HP applied to the diesel since I intuitively know that for the next increment of only 900 RPM up to Max RPM, the load increases at a very rapid rate resulting in much much higer fuel burn and that should equate to higher load. Does that seem like a reasonable conclusion?

Yes I'm afflicted with "engineeritis" and see why I'm trying to understand this.

I'm not a retired engineer, just a retired educator. So I get a little less involved with the details relying on engineers figure out the numbers, while I try to understand the big concepts.

The 80% number I used is probably designed for guys like me and may or may not be very precise, but as they say, "close enough for government work."

As I understand this issue, there is a relationship between RPM, Torque, and HP. although I can't describe the nature of that relationship in any great detail. I think what happens is as the resistance increases or the amount of torque required increases, the RPM at which a given HP is developed changes. Thus in a low resistance (low torque) application, say a bare shaft or a small prop, the maximum HP will be developed at much higher RPMs. When resistance is increased, more torque is needed, so the max HP is developed at a lower RPM. This understanding sort of matches what you are describing. Transmission gearing is also going to play a factor, maybe that is what the "marine gear" curve is using.

You're on your own when it comes to the calculations.

 

[JamesG161]

Hey guys, are you all bored retired [retarded in my case] engineers?

Horsepower delivered to propulsion is the key, or even to a genset's AC generator head.
_____

After 23 years and 2700 hours, my original exhaust elbow is clean

We have the same engine, but less hours on mine.

Jim...

PS: There is sweet spot for each engine and each boat for WOT. Lowest fuel vs. hull speed.

 

[kloudie1]

Prop curve is the one to look at if considering load.. The DIN curves are with the engine being loaded into a dynamometer, making the most horsepower at a given RPM.. the prop curve is the load on the engine from the propeller, close to what the engine is having to produce to satisfy the RPM target that is set by the "throttle"..
Yup retired engineer here too but hardly bored!
My engine always seemed happy at an indicated 2100 RPM; the Yanmar tacho under reporting.. I checked with an optical tach and my indicated 2100 was/is actually 2800.. which agrees with most of the guidance for engine happiness.. .

 

[Davidasailor26]

As another data point, here are some excerpts from the Yanmar YM Series operation manual -

It is recommended that new vessels be propped so the engines can operate at 100 to 200 rpm above the Maximum Rated Power Output rpm (3700 to 3800) to allow for some added weight and hull resistance. The engine must be able to reach the Maximum Rated Power rpm (3600) under full load at all times.

The YM series engines are designed to be operated at maximum throttle (3600 rpm) for less than 5% of total engine time (30 minutes out of every 10 hours) and cruising speed (3400 rpm or less) for less than 90% of total engine time (9 hours out of every 10 hours).

During the first 10 hours of operation, the engine should be operated at maximum rpm minus 400 to 500 rpm (approximately 60 to 70% of load) most of the time. This will ensure the sliding parts break in properly.
NOTICE: During this period, avoid operating at maximum engine speed and load to avoid damaging or scoring sliding parts.
Do not operate the engine at low idle or at low speed and light load for more than
30 minutes at a time. Since unburned fuel and engine oil will adhere to the piston rings when operating at low speeds for long periods, this will interfere with proper movement of the rings and the diesel fuel consumption may increase. Low idle speed does not allow break-in of sliding parts.
If operating the engine at low speed and light load, you must race the engine to clean the carbon from the cylinders and the fuel injection valve.

“Once past the initial 10 hours until 50 hours, the engine should be used over its full operating range, with special emphasis on running at relatively high power settings. This is not the time for an extended cruise at idle or low speed. The boat should be operated at maximum speed minus 400 rpm most of the time (approximately 70% load), with a 10 minute run at maximum minus 200 rpm (approximately 80% load) every
30 minutes and a 4 to 5 minute period of operation at WOT (wide open throttle) once every 30 minutes. During this period, be sure not to operate the engine at low speed and light load for more than 30 minutes. If operating engine at low speed and light load is necessary, race the engine after low idle operation.

“Periodically operate the engine near maximum speed while underway. This will generate higher exhaust temperatures, which will help clean out hard carbon deposits, maintain engine performance and prolong the life of the engine.”

I think it’s interesting that they say 400 to 500 RPMs below max is 60% to 70% of max load, and 200 RPMs below max is 80% of max load. They don’t seem to show a power/rpm curve, but these numbers seem to suggest that cutting RPMs to 80% of maximum is actually cutting load much more significantly, to significantly less than 60%.

 

[smokey73]

Wow- I love it when a plan comes together and people can openly discuss thoughts.

So for my 4JH2E rated at 48HP, 500 below Max RPM (3600) would be 3100 rpm for about 60% load on the engine. Using 3100 as the entry point on the Propeller HP curve, it shows 30HP. Low and behold 30HP divided by 48HP equals 62.5% load, just as predicted.

Now I may not run at 3100 all the time. My past practice of 2700 to 2750 (46%) and opening it up to 3600 rpm for 5 minutes every day I take it out seems like a good compromise between fuel economy and operating like the manual says to. I still want to do my RPM vs Speed runs and make a table of RPM, HP, Speed, fuel usage, usable range as a useful guide for planning for passagemaking and long hauls where I may be forced to use the engine.

When all is said and done @Don Lucas may still be right in that for most cruisers we'll never accumulate the hours necessary to wear out our engines based on operating speeds and the biggest concern may very well be obstructing the exhaust elbow and "blowing it out" periodically may take care of that. None the less, it is alway better to understand than to blindly accept.

Thank you all for your participation in this exercise! Your final grade will be detrmined by class participation and the length of time your Yanmar lasts before major overhaul.

Remember the Engineer's lament:

"The Engineer's Lament"

A shorter Version:
Its not for me to say how far or fast the ship will go.
Its not for me to to ring the bell or even the whistle blow
Its not for me to steer the ship or even the helmsman tell.
But let the d$#n thing start to sink
And see who catches H#$L

 

[Davidasailor26]

Incase we need more somewhat confusing numbers, here is the Yanmar YM Service manual section on continuous and maximum speeds (excerpted for the 30 HP 3YM30) -

So, straightforward enough - you can run continuously at 3489 RPMs, and make 20.1 kW which is 27.3 HP. Max out at 3600 RPMs and 30 HP. But then from the Operating Manual for the same engine -

So the Operating Manual seems to say the continuous rating is the same HP and RPM as the Max rating, but the kW output is less???

 

[Rick D]

OK, so my Yanmar 4JH2E is rated a top of 3600 RPM. When I installed my new prop, she would only do 3400 RPM. I checked with a Yanmar engineer. He said 3400 WOT won't cause any issues. I used to cruise at 3100 RPM. I now cruise at 2800 RPM. The temp increases if I run it harder, but note I am towing a 1000 pound dink in an open seaway too, so it's loaded up. 2870 hours. I may have the prop tips shaved slightly. It can't be re-pitched.

 

[smokey73]

@Rick D When I first got my 40.5 the Max Prop had a bit too much pitch and I could only do 3400 rpm (which I checked with a remote hand held tach on the crankshaft pully) with a clean hull and prop. The first time I had it on the hard I pulled, cleaned and relubed the MaxProp and reset the pitch by 2 degrees and reassembled. It now does 3600 RPM on the button at full throttle and temperature is steady. If I have a really badly fouled prop and hull it will heat up a little if I can't get to 3600 rpm. Towing the dink will put an extra load, as will fouling on the prop or hull. When you have a clean hull and prop ditch the dink for a short trip and see what your max rpm is. I am sure a good prop shop can recommend how much to shave.

I take it you have a fixed prop so I'd be careful in shaving the tips since if you overdo it it can't be unshaved. Measure twice, cut once. Good luck

 

[Rick D]

@Rick D When I first got my 40.5 the Max Prop had a bit too much pitch and I could only do 3400 rpm (which I checked with a remote hand held tach on the crankshaft pully) with a clean hull and prop. The first time I had it on the hard I pulled, cleaned and relubed the MaxProp and reset the pitch by 2 degrees and reassembled. It now does 3600 RPM on the button at full throttle and temperature is steady. If I have a really badly fouled prop and hull it will heat up a little if I can't get to 3600 rpm. Towing the dink will put an extra load, as will fouling on the prop or hull. When you have a clean hull and prop ditch the dink for a short trip and see what your max rpm is. I am sure a good prop shop can recommend how much to shave.

I take it you have a fixed prop so I'd be careful in shaving the tips since if you overdo it it can't be unshaved. Measure twice, cut once. Good luck

Nope. Two blade geared folder.

 

[smokey73]

Nope. Two blade geared folder.

Still though, measure twice cut once.