I just got back from a sail from Mystic, CT to Portsmouth, RI with a friend who has the same boat as I, a 1988 Legend 37. He had a problem with his engine failing and had to replace the "mixing elbo" on his 3HM30F at a Brewer's Marina. I spoke to the marina engine mechanic who replaced the elbo and he said that the elbo should be replace every 5 years and no based upon the hours of operation. This seemed a little strange to me. What to you think about his statement. Second item; The mechanic said that the proper normal operating RPM for the 3HM30F was 3,400 RPM with a capability of sustaining 3,600 RPM for one hour. He said that a diesel engine should be run towards to top of it's range. Being new to diesels I checked with others and I get an operating range of 2,400 to 3,000 RPM. Whats right?? I obviously don't want to overtax the engine nor do I want to operate it at to low an RPM and have carbon buildup. Help!!!!
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I believe it will overheat.
Mickey: I am not sure about this, but I would bet that if your friend runs his engine at 3600 RPM's for 1 hour, it is probably going to overheat. 3600 rpms is the max rated for the engine. EVERTHING that I have heard is that the engines are suppose to be run at 70-80% of the MAX (in this case 3600rpm's) which is in line with your figures. 1. Can he achieve 3600 rpms when the boat is in gear? 2. Can he get more than 3600 rpms? I would think that if he can, he may be a little under propped. There is a fine line here on not enough and too much prop. If you cannot get approx 3400 rpm in gear you are over propped. Running these engines at low rpms will cause coking of the elbow. I have heard that changing the elbow every 5 years is recommended.
Mickey: I am not sure about this, but I would bet that if your friend runs his engine at 3600 RPM's for 1 hour, it is probably going to overheat. 3600 rpms is the max rated for the engine. EVERTHING that I have heard is that the engines are suppose to be run at 70-80% of the MAX (in this case 3600rpm's) which is in line with your figures. 1. Can he achieve 3600 rpms when the boat is in gear? 2. Can he get more than 3600 rpms? I would think that if he can, he may be a little under propped. There is a fine line here on not enough and too much prop. If you cannot get approx 3400 rpm in gear you are over propped. Running these engines at low rpms will cause coking of the elbow. I have heard that changing the elbow every 5 years is recommended.
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Tim Schaaf
Mostly agree with Steve
except that the engine should be able to run at its rated speed indefinitely without overheating. However, the recommended speeds are as he said, or maybe 70 - 85%. By the way, do NOT run your engine at greater than the rated speed, even if you can. And you probably can, when in neutral. You can severly damage the engine doing this. The elbow not only cokes up, but corrodes due to the inevitable mix of salt water and engine exhaust gasses. I would say that five years is about right, or maybe even a little optimistic.
except that the engine should be able to run at its rated speed indefinitely without overheating. However, the recommended speeds are as he said, or maybe 70 - 85%. By the way, do NOT run your engine at greater than the rated speed, even if you can. And you probably can, when in neutral. You can severly damage the engine doing this. The elbow not only cokes up, but corrodes due to the inevitable mix of salt water and engine exhaust gasses. I would say that five years is about right, or maybe even a little optimistic.
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Dakota Jim Russell
Ask Yanmar
I would suggest you contact a Yanmar distributor via: http://www.marinedieseldirect.com/yanmar/ or Torresen Marine Diesel Direct [marineengineupdate@marinedieseldirect.com]
I would suggest you contact a Yanmar distributor via: http://www.marinedieseldirect.com/yanmar/ or Torresen Marine Diesel Direct [marineengineupdate@marinedieseldirect.com]
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Paul Akers
Yes, it's mine...
...and I will double-check the Yanmar manual for the rated rpm's and post it here soon. Since the engine has been fixed, I do get more rpm's than I expected. The elbow was terribly coked. I will see if I can post a picture of the elbow soon so that I can share it with everyone as to what happens. There was no gradual loss of power when it happenned. Apparently the coking hit a threshold that just immediately dropped the rpm's to 200-500 and no better. Had to get towed into a marina to get it fixed. All this happened while I was on vacation. But, the major hint was that the engine has been smoking more so than ever before this happened - so beware.
...and I will double-check the Yanmar manual for the rated rpm's and post it here soon. Since the engine has been fixed, I do get more rpm's than I expected. The elbow was terribly coked. I will see if I can post a picture of the elbow soon so that I can share it with everyone as to what happens. There was no gradual loss of power when it happenned. Apparently the coking hit a threshold that just immediately dropped the rpm's to 200-500 and no better. Had to get towed into a marina to get it fixed. All this happened while I was on vacation. But, the major hint was that the engine has been smoking more so than ever before this happened - so beware.
Tim S, FYI
Tim: FYI the Yanmar manual has a rating of "One hour rating output" (this is the 3600 rpm) and then the "Continous rating output" (this is the 3400 rpm). Now whatever one would interpert as a "One hour rating"? All of this is dependent on the prop. If you are over propped and try running at anywhere near the rated engine RPM's you are going to overheat or do something bad to the engine. If you run at these RPM's and you are under-propped you are not going to do the engine any good either. So, there are a lot of variables here to consider. I was told by my YANMAR guy that reving these engines to 4000 rpms in short burst was not going to hurt the engine. They are normally ever going to get to these RPM's because of the govenor that limits the amount of fuel that the engine can get. So every one needs to see how their engine performs under any given RPM's. I suppose that having a gauge in addition to the idiot lights and buzzers is still the best combo of accessories. Having the idiot lights and buzzers gives you plenty of advanced warning. Having a gauge give you some type of indication of the trend if your engine is starting to overheat or is running on the warm side without actually overheating.
Tim: FYI the Yanmar manual has a rating of "One hour rating output" (this is the 3600 rpm) and then the "Continous rating output" (this is the 3400 rpm). Now whatever one would interpert as a "One hour rating"? All of this is dependent on the prop. If you are over propped and try running at anywhere near the rated engine RPM's you are going to overheat or do something bad to the engine. If you run at these RPM's and you are under-propped you are not going to do the engine any good either. So, there are a lot of variables here to consider. I was told by my YANMAR guy that reving these engines to 4000 rpms in short burst was not going to hurt the engine. They are normally ever going to get to these RPM's because of the govenor that limits the amount of fuel that the engine can get. So every one needs to see how their engine performs under any given RPM's. I suppose that having a gauge in addition to the idiot lights and buzzers is still the best combo of accessories. Having the idiot lights and buzzers gives you plenty of advanced warning. Having a gauge give you some type of indication of the trend if your engine is starting to overheat or is running on the warm side without actually overheating.
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Eric Lorgus
One Hour Rating Not Likely 'cuz Overheating
Some interesting food for thought here... for example, marine diesels don't have redlines on the tach. Why not? I've "heard" it's because you'll never be able to run them up high enough to need a redline. As for the one-hour rating, I doubt it's due to the risk of overheating. Fresh water cooled marine engines have thermostats which attempt to keep the coolant temperature within a narrow range. If the engine generates more heat than the cooling system can dissipate, the overheating will likely occur sooner than one hour.
Some interesting food for thought here... for example, marine diesels don't have redlines on the tach. Why not? I've "heard" it's because you'll never be able to run them up high enough to need a redline. As for the one-hour rating, I doubt it's due to the risk of overheating. Fresh water cooled marine engines have thermostats which attempt to keep the coolant temperature within a narrow range. If the engine generates more heat than the cooling system can dissipate, the overheating will likely occur sooner than one hour.
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Tim Schaaf
Hmmmmm, Steve, good food for thought
Interesting. And different from the way my manual states the rating for my 2QM15. My shop manual gives a maximum 3000 RPM's, but does not differentiate between a one hour or continuous rating, It does say "15 HP, 14 HP, continuous". I had assumed that for whatever reason, it might not be able to hold the 15 HP, indefinitely. But, I have assumed that it could do the 3000 RPM indefinitely (although not advisedly!), always assuming that the engine was correctly propped. In my case, the engine will now do about 2850 RPM, and I am about to take about an inch out of the pitch of the prop. Back when my boat was less loaded down with all sorts of cruising stuff (!) and thus generated less drag, the engine would faithfully do its 3000. From time to time, I would blast along like this for an hour to eliminate carbon, per my mechanic's suggestion. Once, I unthinkingly did it for four hours! My engine has also always been able to get up to about 3300 in neutral, which I used to do, momentarily, preparatory to shutting down, per the owner's manual. Actually, the manual said to bring it to "it's rated speed", so I was overdoing it. I now only take it up to 3000. My comment about potential damage came from Nigel Calder's Diesel book, which emphatically states that reving an engine above its rated RPM can do extensive damage. Since Nigel's Bible is down on Casual Water, and not at my side, I can't tell you exactly what disaster he was concerned about, but I think it may have had to do with the valves. I will look. Any thoughts?
Interesting. And different from the way my manual states the rating for my 2QM15. My shop manual gives a maximum 3000 RPM's, but does not differentiate between a one hour or continuous rating, It does say "15 HP, 14 HP, continuous". I had assumed that for whatever reason, it might not be able to hold the 15 HP, indefinitely. But, I have assumed that it could do the 3000 RPM indefinitely (although not advisedly!), always assuming that the engine was correctly propped. In my case, the engine will now do about 2850 RPM, and I am about to take about an inch out of the pitch of the prop. Back when my boat was less loaded down with all sorts of cruising stuff (!) and thus generated less drag, the engine would faithfully do its 3000. From time to time, I would blast along like this for an hour to eliminate carbon, per my mechanic's suggestion. Once, I unthinkingly did it for four hours! My engine has also always been able to get up to about 3300 in neutral, which I used to do, momentarily, preparatory to shutting down, per the owner's manual. Actually, the manual said to bring it to "it's rated speed", so I was overdoing it. I now only take it up to 3000. My comment about potential damage came from Nigel Calder's Diesel book, which emphatically states that reving an engine above its rated RPM can do extensive damage. Since Nigel's Bible is down on Casual Water, and not at my side, I can't tell you exactly what disaster he was concerned about, but I think it may have had to do with the valves. I will look. Any thoughts?
Tim S, again!
Tim: That is interesting. I just looked up a site that showed your engine 2QM15 and (as you stated) it shows 15 and 14HP@ 3000rpms. Don't really understand what they are trying to say. I am wondering if it was a mis-print in the manual. I was under the impression that those engines were rated at 3200 or 3250 rpms? Oh well, what the hell difference does it make if it gets you from point A to point B without any problems anyway. Do you know how many hours you have on your engine? Have you done anything major to it? The YANMAR guy told me that you can expect 12000-15000 hours out of these engines IF (that is IF) you maintain them. Mine is 14 years old and only has about 1000 hrs. I don't think I'll live to see it die (based on this info).
Tim: That is interesting. I just looked up a site that showed your engine 2QM15 and (as you stated) it shows 15 and 14HP@ 3000rpms. Don't really understand what they are trying to say. I am wondering if it was a mis-print in the manual. I was under the impression that those engines were rated at 3200 or 3250 rpms? Oh well, what the hell difference does it make if it gets you from point A to point B without any problems anyway. Do you know how many hours you have on your engine? Have you done anything major to it? The YANMAR guy told me that you can expect 12000-15000 hours out of these engines IF (that is IF) you maintain them. Mine is 14 years old and only has about 1000 hrs. I don't think I'll live to see it die (based on this info).
J
Jim Logan
Tim and Steve
Almost all diesels have different ratings based on the number of hours at certain POWER levels, not necessarily rpm. For example, a common Cummins marine diesel, a 6cylinder 359 ci engine, can be had in various power levels from 165 hp to over 330 hp, depending on whether it is naturally aspirated or turbocharged, among other changes. The engine is basically the same - the difference is, the higher the power level, the less hours total the engine is rated for - so pulling more than one hp per ci out of a diesel will generally make it last only 500 to 1000 hours, while it will last almost forever at 165 hp- same basic engine, different heat loads and power levels, caused by different overboosts and fuel input. So while you can certainly overspeed an engine, the longevity is more related to power extracted (and heat that must be dissipated through relatively thick metal castings) than rpm. Diesels are very sensitive to excessive heat in the cylinders, which because of the robust nature of diesel engines, cannot transfer to the cooling system as quickly as a gasoline engine, thus excessive rpm (increasing piston rubbing speed and friction) at high power levels (overpropped) will cause even the best engine to fail at less than its normal service life. Diesels are actually somewhat delicate and sensitive to operating outside their designed operating parameters. I guess the point is, look at the data plate on the side of the engine, it will have an rpm on it, operate the engine at 70-80% of that rpm most of the time - if it overheats or smokes or won't make that rated rpm, you need to fix it or the prop.
Almost all diesels have different ratings based on the number of hours at certain POWER levels, not necessarily rpm. For example, a common Cummins marine diesel, a 6cylinder 359 ci engine, can be had in various power levels from 165 hp to over 330 hp, depending on whether it is naturally aspirated or turbocharged, among other changes. The engine is basically the same - the difference is, the higher the power level, the less hours total the engine is rated for - so pulling more than one hp per ci out of a diesel will generally make it last only 500 to 1000 hours, while it will last almost forever at 165 hp- same basic engine, different heat loads and power levels, caused by different overboosts and fuel input. So while you can certainly overspeed an engine, the longevity is more related to power extracted (and heat that must be dissipated through relatively thick metal castings) than rpm. Diesels are very sensitive to excessive heat in the cylinders, which because of the robust nature of diesel engines, cannot transfer to the cooling system as quickly as a gasoline engine, thus excessive rpm (increasing piston rubbing speed and friction) at high power levels (overpropped) will cause even the best engine to fail at less than its normal service life. Diesels are actually somewhat delicate and sensitive to operating outside their designed operating parameters. I guess the point is, look at the data plate on the side of the engine, it will have an rpm on it, operate the engine at 70-80% of that rpm most of the time - if it overheats or smokes or won't make that rated rpm, you need to fix it or the prop.
M
Mickey Goodman
I got the Engine model wrong and the top RPMs
I went to the boat today and looked at the manual. It states the rated RPM's as 3,400. I also called Mack Boring, the east coast Yanmar distributor and spoke to someone in service. He said that you can comfortably run at 90% of max or 3,000 (rounded) but you should not run at any one RPM consistently. He said it is better to vary the running speed (RPMs). My boat is an 1988 with only 100 hours on the engine, can you believe it? My concern was that probably the mixing elbo was never changed in the 13 years the boat has been "alive". Up until the last two years that I owned the boat it was a "fresh water" boat and I expect that to carbon problem in the mixing elbo to be less than had the boat been in salt water all those years. When running the engine at all RPMs I don't get any black smoke. A little gray smoke at a particular RPM but when I change the RPM it goes away. The Mack Boring guy recommended that I purchase a elbo to have on hand incase I have a problem. Thanks, guys, for all your help. Mickey
I went to the boat today and looked at the manual. It states the rated RPM's as 3,400. I also called Mack Boring, the east coast Yanmar distributor and spoke to someone in service. He said that you can comfortably run at 90% of max or 3,000 (rounded) but you should not run at any one RPM consistently. He said it is better to vary the running speed (RPMs). My boat is an 1988 with only 100 hours on the engine, can you believe it? My concern was that probably the mixing elbo was never changed in the 13 years the boat has been "alive". Up until the last two years that I owned the boat it was a "fresh water" boat and I expect that to carbon problem in the mixing elbo to be less than had the boat been in salt water all those years. When running the engine at all RPMs I don't get any black smoke. A little gray smoke at a particular RPM but when I change the RPM it goes away. The Mack Boring guy recommended that I purchase a elbo to have on hand incase I have a problem. Thanks, guys, for all your help. Mickey
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Eric Lorgus
To Mickey: Best to Vary RPM's???
Mickey, Did your Yanmar advisor mean don't run same RPM setting day in
Mickey, Did your Yanmar advisor mean don't run same RPM setting day in
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Mickey Goodman
Running at Constant RPM
I understood what he said is during a cruise. At least change once an hour.
I understood what he said is during a cruise. At least change once an hour.
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Tim Schaaf
back to Steve....
Hi Steve, yes, your Yanmar will probably outlive you! Mine doesn't have an hour meter on it, but my best estimate is about 4,000 hours. It might be somewhat more, however. I maintain it regularly, and religiously run it at least once a week, even if that means just putting it in gear in a slip. What have I done to it? Let's see.....I replace the water pump more or less every six years. There comes a time when even a new impeller, cam or wear plate wont do the trick. I have recently installed pump #4. They are not cheap, either! I replace the exhaust elbow slightly more often, and always carry a spare. Hoses less often, and I have had two minor holes, that I quickly spotted. (After turning on the engine, I always open the box, and watch for a moment as it warms up). I have replaced one engine mount. For years, I had trouble keeping them from vibrating loose, and used bigger and longer lag bolts. They seem pretty happy these days. I have put in new injector tips from time to time, but I think I still have the original injectors. Two years ago, I replaced the head gasket, and took the opportunity to do a pretty thorough top-end job, including cleaning out the cooling passages, and then repainting the engine. Belts and zincs seem to last forever, although I do change the zincs, at least every other year. My best modification have been to remove the pan and install a drain in its bottom. This has a valve, and I have attached a hose with the type of bulb used on an outboard motor gas line. As a result, it takes me about thirty seconds to remove ALL of the oil, rather than an hour to get most of it out of the dipstick hole. As a result, the oil gets changed regularly, sometimes more often than recommended. I usually go about fifty hours, and change the filter every other time. Second best modification was to install a remote low voltage solenoid in the wiring harness. So, if there is a voltage drop in the harness (as wire gets older or whatever), the solenoid will still close, and the battery cable that goes to IT, and then to the starter itself, will give good voltage to the starter solenoid. It also gives me a way to start the engine from inside the engine box, without putting a screwdriver across the started. Biggest mystery....I have fiddled with different size alternators, all the way from the original 35 amp alternator to an Ample Power 106 amp (rated hot) alternator. This last is really big for such a small engine, and I have installed a bigger pulley which cuts its output by about 20 - 25 %. The mystery is that when this alternator is REALLY putting out (inverter loads of 1500 watts, for example), and engine RPM's are over about 2000, the oil pressure drops slightly, even though revs are maintained or increased! Switch off the load, and the oil pressure comes back up. Switch it on and the pressure backs off. Every expert to whom I have described this immediately says, "of course!", thinking that the RPM's drop with the load, which would lead to the lower pressure. But I am talking about a CONSTANT RPM. Or, if I put the load on when the revs are below 2000 (and the alternator is putting out less), and then up the RPM's, at about 2100, down goes the pressure. Not much, but noticeable. That is when the mechanics scratch their heads. The best guess I have heard so far is that the load distorts the engine bearings very slightly, and that causes a loss of pressure. The shop manual suggests a maximum take-off load, which more or less corresponds to the big alternator, but they contemplate an auxiliary lay shaft, supported on both sides of the pulley. And, of course the alternator pulley is supported on only one side. My current thinking is to go back to the 55 amp alternator, since my solar array puts out 320 watts, which can more or less keep up with everything. Maybe someone on the HOW can figure out the mystery! Anyway, Steve, it has been a great little engine, and I hope it lasts for at least another twenty years. By the way, the potential misprint that you have theorized, may, in fact be true, because I have accidentally revved up to 3300 RPM neutral.......
Hi Steve, yes, your Yanmar will probably outlive you! Mine doesn't have an hour meter on it, but my best estimate is about 4,000 hours. It might be somewhat more, however. I maintain it regularly, and religiously run it at least once a week, even if that means just putting it in gear in a slip. What have I done to it? Let's see.....I replace the water pump more or less every six years. There comes a time when even a new impeller, cam or wear plate wont do the trick. I have recently installed pump #4. They are not cheap, either! I replace the exhaust elbow slightly more often, and always carry a spare. Hoses less often, and I have had two minor holes, that I quickly spotted. (After turning on the engine, I always open the box, and watch for a moment as it warms up). I have replaced one engine mount. For years, I had trouble keeping them from vibrating loose, and used bigger and longer lag bolts. They seem pretty happy these days. I have put in new injector tips from time to time, but I think I still have the original injectors. Two years ago, I replaced the head gasket, and took the opportunity to do a pretty thorough top-end job, including cleaning out the cooling passages, and then repainting the engine. Belts and zincs seem to last forever, although I do change the zincs, at least every other year. My best modification have been to remove the pan and install a drain in its bottom. This has a valve, and I have attached a hose with the type of bulb used on an outboard motor gas line. As a result, it takes me about thirty seconds to remove ALL of the oil, rather than an hour to get most of it out of the dipstick hole. As a result, the oil gets changed regularly, sometimes more often than recommended. I usually go about fifty hours, and change the filter every other time. Second best modification was to install a remote low voltage solenoid in the wiring harness. So, if there is a voltage drop in the harness (as wire gets older or whatever), the solenoid will still close, and the battery cable that goes to IT, and then to the starter itself, will give good voltage to the starter solenoid. It also gives me a way to start the engine from inside the engine box, without putting a screwdriver across the started. Biggest mystery....I have fiddled with different size alternators, all the way from the original 35 amp alternator to an Ample Power 106 amp (rated hot) alternator. This last is really big for such a small engine, and I have installed a bigger pulley which cuts its output by about 20 - 25 %. The mystery is that when this alternator is REALLY putting out (inverter loads of 1500 watts, for example), and engine RPM's are over about 2000, the oil pressure drops slightly, even though revs are maintained or increased! Switch off the load, and the oil pressure comes back up. Switch it on and the pressure backs off. Every expert to whom I have described this immediately says, "of course!", thinking that the RPM's drop with the load, which would lead to the lower pressure. But I am talking about a CONSTANT RPM. Or, if I put the load on when the revs are below 2000 (and the alternator is putting out less), and then up the RPM's, at about 2100, down goes the pressure. Not much, but noticeable. That is when the mechanics scratch their heads. The best guess I have heard so far is that the load distorts the engine bearings very slightly, and that causes a loss of pressure. The shop manual suggests a maximum take-off load, which more or less corresponds to the big alternator, but they contemplate an auxiliary lay shaft, supported on both sides of the pulley. And, of course the alternator pulley is supported on only one side. My current thinking is to go back to the 55 amp alternator, since my solar array puts out 320 watts, which can more or less keep up with everything. Maybe someone on the HOW can figure out the mystery! Anyway, Steve, it has been a great little engine, and I hope it lasts for at least another twenty years. By the way, the potential misprint that you have theorized, may, in fact be true, because I have accidentally revved up to 3300 RPM neutral.......
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Paul Akers
3HM35F
Mickey, to set these people straight and avoid confusion, the model # of our Yanmar in the 1988 Legend 37 is 3HM35F. Not to be confused with the 3GM30F. Although they are both in the same engine manual.
Mickey, to set these people straight and avoid confusion, the model # of our Yanmar in the 1988 Legend 37 is 3HM35F. Not to be confused with the 3GM30F. Although they are both in the same engine manual.
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Paul Akers
Consulted Manual...
...and he said that the 3HM35F has a high end output of 3200rpm and produces 30 hp at that level. The one hour rating is 32hp/3400rpm. The same manual states that for the 3GM30F the rating id 24 hp/3400rpm with a one hour rating of 27hp/3600rpm. Can anybody explain the "one hour rating"?
...and he said that the 3HM35F has a high end output of 3200rpm and produces 30 hp at that level. The one hour rating is 32hp/3400rpm. The same manual states that for the 3GM30F the rating id 24 hp/3400rpm with a one hour rating of 27hp/3600rpm. Can anybody explain the "one hour rating"?
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Eric Lorgus
Diesel Engine Ratings
Paul, In doing some internet research to check what I read here on HOW against what I thought I already knew, I came up with the following web page: http://www.bfpbooks.com/248.htm Below, I've extracted what the author had to say about diesel ratings. Before I go, I'll also mention that this same author opined that marine diesels like to be run at continuous speeds, which is contrary to one of other posts in this thread. I tend to believe the "continuous RPM's is OK" theory, which makes even more sense when you consider all the fixed-RPM applications for diesels, like generators. Here's the excerpt: ================================== The diesel engine may have any one of several horsepower ratings and it generally conforms rather closely to that particular rating, whichever one is chosen. A Continuous Horsepower or Continuous Duty Rating and an Intermittent Horsepower or Intermittent Duty Rating are two of these choices. The continuous horsepower rating is the amount of horsepower the engine can produce on a steady basis without over working the engine. It is the equivalent of the cruising speed of a gasoline engine. Operating at or slightly below this level, the diesel engine is quite happy. The intermittent horsepower rating is the amount of horsepower the engine can produce for short spurts, a few minutes at a time. This is the full throttle rating of the engine and it is generally higher than the continuous horsepower rating. In some cases it is much higher. At this speed the life of the engine will be much shorter.
Paul, In doing some internet research to check what I read here on HOW against what I thought I already knew, I came up with the following web page: http://www.bfpbooks.com/248.htm Below, I've extracted what the author had to say about diesel ratings. Before I go, I'll also mention that this same author opined that marine diesels like to be run at continuous speeds, which is contrary to one of other posts in this thread. I tend to believe the "continuous RPM's is OK" theory, which makes even more sense when you consider all the fixed-RPM applications for diesels, like generators. Here's the excerpt: ================================== The diesel engine may have any one of several horsepower ratings and it generally conforms rather closely to that particular rating, whichever one is chosen. A Continuous Horsepower or Continuous Duty Rating and an Intermittent Horsepower or Intermittent Duty Rating are two of these choices. The continuous horsepower rating is the amount of horsepower the engine can produce on a steady basis without over working the engine. It is the equivalent of the cruising speed of a gasoline engine. Operating at or slightly below this level, the diesel engine is quite happy. The intermittent horsepower rating is the amount of horsepower the engine can produce for short spurts, a few minutes at a time. This is the full throttle rating of the engine and it is generally higher than the continuous horsepower rating. In some cases it is much higher. At this speed the life of the engine will be much shorter.
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