Cruising RPM.
- Thread starter Waltsailor
- Start date
Yanmar describes continuous as all the Time. Your engine should work non-stop at this speed.
And then: maximum RPM where your engine could overheat... Or under lubricate.
And then: maximum RPM where your engine could overheat... Or under lubricate.
Of course it is. The question isn't whether you *can* run it at that all the time but whether it is optimum to do so. The engine won't blow up but it may not be the best thing for it--or at least that is the question being addressed here. Maybe it is best to run it at this all the time, but that is clearly higher than the generally accepted optimum percentage of max RPM that holds for diesels (i.e., ~80% of max). Hence, my original question as to whether anyone could find this as a *recommended* rpm at which to run the engine in any of Yanmar's printed literature vs. the verbal statement that was offered.
Post #11 provides that clear context to the discussion.
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You know, I see this a lot, and I think it's a misnomer. I think there's an optimum power output for a diesel, and it's happiest at 80% of power output, not 80% of max RPM. Running a diesel at 80% RPM and no or low power output isn't good for it, and will glaze the cylinder walls.
Lets think about this a little. Assuming the diesel is properly sized for the boat, is connected to a shaft and the shaft is connected to a properly pitched prop how can you be at 80% RPM and "no or low power output?"
It is generally advised that the proper pitch for the prop is such that you can get to rated Max RPM listed by Yanmar for your diesel. Assuming that as you can get to rated MAX rpm then you are probably at or very near MAX rated power at that point. You will know that is the case if your prop and/or hull gets severely fouled and not matter what you do to the throttle, you can't get to MAX RPM (ask me how I know
) If that happens, then it is more likely than not, when you are at MAX RPM with a clean hull and prop you are at or near MAX Power for the diesel (assuming your governor is working properly).Now true, the RMP vs Power curve is probably not linear so 80% RPM is probably not exactly 80% Power, but its probably close enough for government work. If my logic is incorrect, please let me know where I'm wrong. I accept correction without protest.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,862
With your boat set up as identified you will be rewarded when running, as little or no oily soot will appear on your stern. When I first got my boat, as I opened the throttle the wrong prop/pitch caused the stern to be pulled down as much as a foot and the boat struggled to reach hull speed. There was a troublesome black oily line across my boats stern. I put on a new prop that was matched to my boat and engine. Magically the line disappeared and has not returned.
Moral... as you search for than magic perfect RPM remember diesels have been observed to work best when they are run hard. To get the best performance out of your boat under power, you need to match the engine, transmission, shaft, and pitched prop to the boat.
Thanks @jssailem, I forgot to add the transmission into the drive line. The point is that the diesel power (HP) and all the drive train clear to the prop impacts the efficiency of the engine as it dictates the maximum and ideal RPM (and by extension load) for the combination. Hopefully, the boat engineering design team took all this into account when sizing your diesel and selecting the output components (transmission, prop and pitch). Any change in these by mod or by accident (fouling) will drastically impact diesel performance and life. There are available diesel performance curves for the Yanmar diesels and looking at them is enlightening.
A good example is a power boat. It takes a lot more HP and RPM to get the boat up on a plane. Once on a plane, the drag (one component of load) decreases dramatically and the throttle (hence RPM) can be reduced and still stay on a plane.
Your overall points are well taken @smokey73. Since our boats are equipped with tachometers and not dynamometers, these suggested rule of thumb RPM numbers serve as a proxy for how much to load up our engine. And these rules of thumb--such as 75-85% max RPM--are just that: rules of thumb. They give you a good enough idea of whether you are loading up your engine sufficiently or not. And, obviously, they assume you are not running the engine in neutral but running it in gear, with a correctly sized prop, etc.--as you aver.Lets think about this a little. Assuming the diesel is properly sized for the boat, is connected to a shaft and the shaft is connected to a properly pitched prop how can you be at 80% RPM and "no or low power output?"
It is generally advised that the proper pitch for the prop is such that you can get to rated Max RPM listed by Yanmar for your diesel. Assuming that as you can get to rated MAX rpm then you are probably at or very near MAX rated power at that point. You will know that is the case if your prop and/or hull gets severely fouled and not matter what you do to the throttle, you can't get to MAX RPM (ask me how I know
Now true, the RMP vs Power curve is probably not linear so 80% RPM is probably not exactly 80% Power, but its probably close enough for government work. If my logic is incorrect, please let me know where I'm wrong. I accept correction without protest.
As far as I know (and I'm open to correction here), in the case of a diesel, what makes it happy is running it in its sweet spot in the torque curve. Consulting the power curves for the 1GM10 as an example, it's pretty clear that this engine reaches its sweet spot right around 2900 RPM. This is essentially 80% of max RPM. The torque appears to begin dropping somewhat after around 3200 (around 88% of max RPM). 3400 is the continuous rating and 3600 is maxed out. So again, this ~80%-of-max rule of thumb works--for that engine, at least--if you want to run it at or near the peak of its torque curve.
And so back to my original question: A Yanmar rep is reported to have said that one OUGHT TO run the engine at its continuous rating all the time (see post #12). That, based on the "conventional wisdom" for diesels, seems too high. Again, the question is NOT (and never was) whether you CAN run it at the continuous rating all the time without the engine exploding. (And yes, that is why it is called "continuous.") I'm sure you *could* run your engine at its continuous rating in neutral all day as well without spewing parts all over the dock, but no one would suggest that would result in a happy engine in the long term.The question is whether it is BEST to do so (under load), or whether something closer to 80% of max RPM is a better idea.
Aside from a statement from a Yanmar rep at a boat show (and I'm not doubting he said this), is there anywhere in their printed documentation that says this? If so, I've never seen it. But that doesn't mean it doesn't exist somehwere--hence the question. If this can be produced I'd like to see it. And if it can't, then I'm wondering on what basis the Yanmar rep made his claim.
I for one DON'T feel it is BEST to run the diesel at 80% (or some other mysterious high power output) all the time. I have lost count of the number of times I have now seen this topic in various forums. It always becomes a bunch of "I heards" "someone saids" etc. and that may include factory "techs" who at just (face It )knuckle draggers. It doesn't seem to matter how many examples of engines lasting just fine get tossed out, some stick to their story. My Yanmar 4JH3E is basically just a tractor engine with a seawater cooled heat exchanger instead of a radiator. How many farmers have you seen racing the tractor up and down the field?
For me I have run my current engine at 1800-2200 rpm most of the 2500 hours I have put on it. I believe that is the best rpm for my hull/engine/prop combination purely based on the sound of the engine. In that range the engine is purring instead of shouting. I feel this is probably right in the middle of power/torque curve. It always has seemed foolish to be to follow this "cylinder glazing" line of reasoning as an excuse to put EVERYTHING in the engine under higher loads and stresses. Now I do every few hours run my engine all the way up to the stop and run max for 30 seconds or so. This is kind of reading into my manual that says to run it up to 3000 rpm every few hours if running under light load. But that says to do it with transmission in neutral so I think they are really talking about "light load" being basically idle or a little higher like you would do to run the alternator for a few hours if need.
I had to take my exhaust manifold/heat exchanger off the engine to fix a flange last month. I figured that would be a good time to cc
heck and clean my exhaust elbow. But it was clean and scale free and I feel this supports my practice as being just fine.
Far as the max continuous rpm recommendation, it is just that. The MAX CONTINUOUS rpm you should run the engine. Has absolutely nothing to do with BEST.
For me I have run my current engine at 1800-2200 rpm most of the 2500 hours I have put on it. I believe that is the best rpm for my hull/engine/prop combination purely based on the sound of the engine. In that range the engine is purring instead of shouting. I feel this is probably right in the middle of power/torque curve. It always has seemed foolish to be to follow this "cylinder glazing" line of reasoning as an excuse to put EVERYTHING in the engine under higher loads and stresses. Now I do every few hours run my engine all the way up to the stop and run max for 30 seconds or so. This is kind of reading into my manual that says to run it up to 3000 rpm every few hours if running under light load. But that says to do it with transmission in neutral so I think they are really talking about "light load" being basically idle or a little higher like you would do to run the alternator for a few hours if need.
I had to take my exhaust manifold/heat exchanger off the engine to fix a flange last month. I figured that would be a good time to cc
heck and clean my exhaust elbow. But it was clean and scale free and I feel this supports my practice as being just fine.
Far as the max continuous rpm recommendation, it is just that. The MAX CONTINUOUS rpm you should run the engine. Has absolutely nothing to do with BEST.
@Don Lucas You are right, the Yanmar 4JH3E is basically a tractor engine. However, you question of "how many farmers" lacks in depth experience "in the field" (the farm field that is.) Tractors have gears and can change the load and speed traveled over the ground (ie racing down the field) by changing gears. I grew up on a farm, tending to over 600 acres of corn, soybeans and wheat and have a lot of hours behind the wheel, so to speak. I also worked at nuclear power stations and other stations where there were emergency diesel generators (some as big as locomotive engines) and we NEVER ran them at half load and when started, we always ran them up to operating temperature and loaded them to about 75% rated load (which was typically the design basis loading and all we could get on them)
I once purchased a "lightly used" Ford 4000 diesel tractor "from a little old man" who only used it to mow his cow pastures. While I can't say definitively what percent of load that would equate to but I would say (based on experience mowing pastures) that it would equate to somewhere about half or slightly less loaded. When I got it home and tried to load it to pull only a 2 bottom plow, it belched black smoke and bogged down no matter what gear it was in. I had to nurse it for months, slowly increasing the load over time to get it to pull the plow.
Of course what is best for you is your choice. I suspect highly that your practice of fully loading the engine periodically (which is a good practice) is keeping your exhaust elbow clear plus you may be lucky and have a Bronze elbow (like I lucked out and got) which doesn't clog up like a cast or iron elbow. Running at 1800 to 2200 probably is no harm if you blow it out now and then. In addition to what is "best for the diesel" there is the question as to which is most economical from a question of fuel consumed per mile steamed since there is a "sweet spot" for RPM vs speed based on on a lot of factors independent of the engine.
Never mind, people can run thier stuff into the ground and it matters nothing to me. It is rare for sailboats to ever get engine hours high enough for any of this to matter anyway.
Btw you wasted the nuc card with me. I was a nuc plus did lots of other things in life.
Btw you wasted the nuc card with me. I was a nuc plus did lots of other things in life.
Probably true with regard to ever getting high enough for any of this to matter so "best" for life may not be even a consideration. But I stand by my comments on farm diesels and standby/emergency power generators.
Just out of curiosity, what is your average speed over ground at 2200 rpm and, if you get stabilized at 3000 rpm what is your speed over ground at 3000 rpm?
As well you should, because they are quite correct. For another data point, see my experience with these same issues in this thread on the EYO forum. I'm glad I took the advice of the highly knowledgeable mechanics at S&W Diesel, who suggested running the crap out of the engine to see if that might not rectify the oil consumption problem. He said it's an issue they see all the time, and always from owners who think it's a swell idea to under load their engines. While I can't say for certain, I think it's highly likely the previous owner of that engine ran it much too lightly. He did mention something along those lines to me, saying he only needed it to get in and out of the slip to the racing start line. Hmmmm.... I still took a chance on it anyway because I got it for peanuts, and the engine is overall in great shape in terms of compression and so forth. (His boat was totalled, by the way, and he was parting it out.) Started right up and ran/runs well. But the high oil consumption was no bueno. I'm happy to say the issue has not returned, and I'm hopeful it will stay away by not babying it. Light running, if that's what (most likely) caused it, did the engine no favors. Fortunately, it was not so far gone that I had to tear it apart and redo the rings.
What load did you run your standby/emergency diesels at during loading for testing?
There is no question that "lighlty loaded" is not good for the diesels (and 1800 to 2200 is not lighlty loaded). There is no question that fully loaded or "lugged- overloaded" is not good for the diesel. The question is what load is "best" and that is a function of a lot of factors. For me, I don't consider 2700 rpm for normal cruise (75%) as "running my stuff to ground." Yours may seem "smoothed out" at 2000 but for mine, 2700 feels "smoothed out".
Sorry @Don Lucas has opted out of the discussion. I always appreciate counter points because they make us rethink our long held beliefs, but so be it. After some study, it appears I need to rethink some of those at least for marine diesels.
Here are the power curves for the 4JH2E, which is the diesel in my 40.5. I am not sure yet what to make of it but am attempting to get smarter on its meaning. It seems to show that the Engine Torque without Marine Gear is relatively flat and at its highest from 2400 to 3000 rpm. IF the "best" point in operation is to operate at the max on the torque curve then that would suggest that optimum operating RPM from an engine load standpoint would be between 2400 (67%) to 3000 (83%). That is a big IF though since it is withouh marine gear and I am not sure how the marine gear impact that torque curve. Assuming the addition of the marine gear impacts the torque curve the same way it impacts the rating HP output it would shift it a little right. However, that is only an assumption right now. The torque curve is surprisingly "flat" over a wide range so @Don Lucas operating range doesn't look like it would do any harm at all (IF operating at the torque peak is the desired range).
Using the Specific Fuel Consumption and the Propeller Power Curve and crunching the numbers it shows what we expect. At 2200 rpm the fuel consumption is .75 gph. At 2400 rpm, the fuel consumption is .89 gph. At 2700 rpm the fuel consumption is 1.18 gph (which is pretty close to what I am observing on my boat with a clean hull and prop.} At 3000 rpm, the fuel consumption is all the way up to 1.57 gph and at 3400, the "continuous operating limit" it jumps all the way to 2.24 gph. The real thing to consider is the speed for your boat at various rpms what this translates to in nm/fuel consumed.
Next time I go out in calm weather with no current I plan on plotting speed vs rpm to generate my boat specific fuel consumption and effeciency
The question that is probably specific to your boat and how you use it is what speed you do at each of these RPMs and which is more important, maximum range and minimum fuel consumption or higher speed (shortening the transit) and accepting the higher fuel consumption.
Here are the power curves for the 4JH2E, which is the diesel in my 40.5. I am not sure yet what to make of it but am attempting to get smarter on its meaning. It seems to show that the Engine Torque without Marine Gear is relatively flat and at its highest from 2400 to 3000 rpm. IF the "best" point in operation is to operate at the max on the torque curve then that would suggest that optimum operating RPM from an engine load standpoint would be between 2400 (67%) to 3000 (83%). That is a big IF though since it is withouh marine gear and I am not sure how the marine gear impact that torque curve. Assuming the addition of the marine gear impacts the torque curve the same way it impacts the rating HP output it would shift it a little right. However, that is only an assumption right now. The torque curve is surprisingly "flat" over a wide range so @Don Lucas operating range doesn't look like it would do any harm at all (IF operating at the torque peak is the desired range).
Using the Specific Fuel Consumption and the Propeller Power Curve and crunching the numbers it shows what we expect. At 2200 rpm the fuel consumption is .75 gph. At 2400 rpm, the fuel consumption is .89 gph. At 2700 rpm the fuel consumption is 1.18 gph (which is pretty close to what I am observing on my boat with a clean hull and prop.} At 3000 rpm, the fuel consumption is all the way up to 1.57 gph and at 3400, the "continuous operating limit" it jumps all the way to 2.24 gph
. The real thing to consider is the speed for your boat at various rpms what this translates to in nm/fuel consumed.Next time I go out in calm weather with no current I plan on plotting speed vs rpm to generate my boat specific fuel consumption and effeciency
The question that is probably specific to your boat and how you use it is what speed you do at each of these RPMs and which is more important, maximum range and minimum fuel consumption or higher speed (shortening the transit) and accepting the higher fuel consumption.
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got me! Certainly the only way - I should have thought of that. I was referring to the comment of running at 80% rpm "all the time at no or low load" and taking that to be cruising. Not sure you'd need 80% rpm to be at max alternator output though I've never run the diesel just to charge the batteries.
So you don't cruise unless you get a slip and plug in? We are on a mooring and anchor out when we cruise, so the only way we get electrical power is to run the engine. On days we don't motor a lot we have to run it to charge.
Most of the time if I have to charge the batteries I use my Nextgen 5.5KW generator and run my AC powered charger. A lot more amps for the fuel use. I probably should upgrade my alternator though. Otherwise it seems like when I do cruise in the somewhat narrow restrictions we have here I end up motoring long enough on the transit that it takes care of some of the charging and the solor panel finishes it up. And yes, the admiral pefers to get a slip and plug in more often than not. Not my perference but as I have said many times before - If mamma ain't happy ain't nobody happy.
Just for my education, do you really need to run at 80% rpm to reach maximum alternator output? Seems awfully high. I would think more like 1500 to 2000 rpm or so (40% to 55%).would do trick but then again I would appreciate your input on this.