I own a boat with a diesel for the first time...a Yanmar 2GM20, and I love it. It's a bit underpowered for my boat (Hunter 326), but that's not the engine's fault. Anyway, not being familiar with diesel temperaments, I was wondering about what the recommendation is for running the engine in terms of RPM. I have seen some opinions that say diesels should be run hard (~90 to 95% of max RPM) for best results and engine longevity. This of course is not how gasoline (Otto cycle) engines like to be run. Can anyone provide information on this?
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Re Sam's reply: yanmarhelp.com is not Yanmar....
... but the conventional wisdom is that the engines should be run at 0.8 of the maximum. I have a problem in that if I try to run at above 2100 rpm (4JH-TE) I hit hull speed and my stern squats down at 7.2 knots, and all I do is get a dirty transom because the exhaust is underwater. I will probably resize the prop this winter to increase the working rpm.
... but the conventional wisdom is that the engines should be run at 0.8 of the maximum. I have a problem in that if I try to run at above 2100 rpm (4JH-TE) I hit hull speed and my stern squats down at 7.2 knots, and all I do is get a dirty transom because the exhaust is underwater. I will probably resize the prop this winter to increase the working rpm.
80-90% of max rated.
Derek: If you read the information on the Yanmar Help site, this guy is a guru of Yanmars. The engines are suppose to be run at 80-90% of the max continous rating while cruising. and you are probably correct that your prop is overpitched for your boat. Was this a factory prop or something that has been changed out?
Derek: If you read the information on the Yanmar Help site, this guy is a guru of Yanmars. The engines are suppose to be run at 80-90% of the max continous rating while cruising. and you are probably correct that your prop is overpitched for your boat. Was this a factory prop or something that has been changed out?
Steve, re my problem...
We're a bit off the original question here, but regarding my low rpm problem; the prop is as came with the boat (Pearson 422) when I purchased it last summer. It is labeled as a 17x13x3, which is the correct size according to the Yanmar specs. Here's the question I've been pondering. It obviously takes a certain thrust from the prop to reach hull-speed, regardless of the prop size. We can translate that thrust to engine output (horsepower). My understanding is that the cruising rpm issue is related to potential cylinder wall glazing if the engine is not brought up to temperature when at low rpm. I can understand that, if the engine is operating at light loads, such as charging the battery. But if I'm putting out the maximum horsepower I need at 2100 rpm, surely the engine will be at full operating temperature? BTW - I have checked the Yanmar tach against an optical tach - it reads a little low, but not a lot.
We're a bit off the original question here, but regarding my low rpm problem; the prop is as came with the boat (Pearson 422) when I purchased it last summer. It is labeled as a 17x13x3, which is the correct size according to the Yanmar specs. Here's the question I've been pondering. It obviously takes a certain thrust from the prop to reach hull-speed, regardless of the prop size. We can translate that thrust to engine output (horsepower). My understanding is that the cruising rpm issue is related to potential cylinder wall glazing if the engine is not brought up to temperature when at low rpm. I can understand that, if the engine is operating at light loads, such as charging the battery. But if I'm putting out the maximum horsepower I need at 2100 rpm, surely the engine will be at full operating temperature? BTW - I have checked the Yanmar tach against an optical tach - it reads a little low, but not a lot.
Lot of dynamics here.
Derek: There are a lot of dynamics here. One thing that ususally happens (I'm speaking about Yanmars) is they will start to blow black smoke if you are pushing the engine too hard. They may also overheat. I am not sure that the Yanmar specs for an engine/transmission combo goes. Just because a prop is stamped with specs does not mean that the prop has not been altered either. You need to pull the prop and have the shop check it out. I would consult with a local prop shop and have them give you what they think is the optimal prop. Your engine should be able to rev to about 3400-3800 rpm in neutral. You should be able to get your engine to about 3200-3600 in gear. If you cannot get these RPM you have some problem. It can be the exhaust restriction, fuel, govenor, prop etc. The prop is only going to be a problem when you are in gear. Good luck and let us know what you find.
Derek: There are a lot of dynamics here. One thing that ususally happens (I'm speaking about Yanmars) is they will start to blow black smoke if you are pushing the engine too hard. They may also overheat. I am not sure that the Yanmar specs for an engine/transmission combo goes. Just because a prop is stamped with specs does not mean that the prop has not been altered either. You need to pull the prop and have the shop check it out. I would consult with a local prop shop and have them give you what they think is the optimal prop. Your engine should be able to rev to about 3400-3800 rpm in neutral. You should be able to get your engine to about 3200-3600 in gear. If you cannot get these RPM you have some problem. It can be the exhaust restriction, fuel, govenor, prop etc. The prop is only going to be a problem when you are in gear. Good luck and let us know what you find.
Balance
Scott It's really a question of balance. You are correct that the worst thing you can do is to lug a diesel engine. What you have to consider is the balance between engine speed, your operating temperature, hull speed and fuel consumption. It's not only the rpms. If you can run the engine to 80% (or whatever % of max your own individual engine's rpms in forward, not neutral) and stay within the operating temperature of 165 to 185 degrees on your temperature gauge, that's great. Most boats' diesel engines will overheat a bit when operated at full throttle for extended periods, because the heat exchangers were sized by overly cost conscious engineers (I am an engineer, by the way, but I'm told I'm not that cost conscious!) who kept the HX size down to a bare minimum for cost reasons. Universal, for instance, increased the size of their heat exchanger from two inches in diameter to three when they introduced their 23 hp version of the old 21 hp M25 engine. Pretty much the same engine, but with greatly increased HX capacity. OTOH, operating at that rpm, while staying within the temperature tolerances, may well significantly increase your fuel consumption. Say you can do 6.5 knots at X%. If you do 6.2 knots at a slightly less % of rpms, you save fuel, because that last 0.3 knots could increase your fuel consumption dramatically from 0.4 or 0.5 gallons per hour to 0.6 or 0.7. Only you can answer the question of whether or not those 0.3 knots extra is worth it to you. Suggest you also get either or both of Nigel Calder's books, Boatowner's Manual or Diesel Engine, they're worth their weight and cost for the information you will learn. (All of us, at one time or another, were new to diesel engines.)
Scott It's really a question of balance. You are correct that the worst thing you can do is to lug a diesel engine. What you have to consider is the balance between engine speed, your operating temperature, hull speed and fuel consumption. It's not only the rpms. If you can run the engine to 80% (or whatever % of max your own individual engine's rpms in forward, not neutral) and stay within the operating temperature of 165 to 185 degrees on your temperature gauge, that's great. Most boats' diesel engines will overheat a bit when operated at full throttle for extended periods, because the heat exchangers were sized by overly cost conscious engineers (I am an engineer, by the way, but I'm told I'm not that cost conscious!) who kept the HX size down to a bare minimum for cost reasons. Universal, for instance, increased the size of their heat exchanger from two inches in diameter to three when they introduced their 23 hp version of the old 21 hp M25 engine. Pretty much the same engine, but with greatly increased HX capacity. OTOH, operating at that rpm, while staying within the temperature tolerances, may well significantly increase your fuel consumption. Say you can do 6.5 knots at X%. If you do 6.2 knots at a slightly less % of rpms, you save fuel, because that last 0.3 knots could increase your fuel consumption dramatically from 0.4 or 0.5 gallons per hour to 0.6 or 0.7. Only you can answer the question of whether or not those 0.3 knots extra is worth it to you. Suggest you also get either or both of Nigel Calder's books, Boatowner's Manual or Diesel Engine, they're worth their weight and cost for the information you will learn. (All of us, at one time or another, were new to diesel engines.)
Steve....thats not yanmars that do that
You said" One thing that ususally happens (I'm speaking about Yanmars) is they will start to blow black smoke if you are pushing the engine too hard. "...... All marine diesels that I have used do that. I didn't ever hear that it will "kill" the engine necessarily, but its not the best thing either, I heard it was due to the engine not being able to burn the diesel and it ends up coming outthe exhaust. You don't want to do it too long though.....
You said" One thing that ususally happens (I'm speaking about Yanmars) is they will start to blow black smoke if you are pushing the engine too hard. "...... All marine diesels that I have used do that. I didn't ever hear that it will "kill" the engine necessarily, but its not the best thing either, I heard it was due to the engine not being able to burn the diesel and it ends up coming outthe exhaust. You don't want to do it too long though.....
Tom, You are correct.
Tom: You are correct. If you push these "high RPM diesels" they tend to blow black smoke (fuel). I haven't ever had the pleasure of having a real thumper (low rpm engine 2000rpm cruise)and have no idea how they reach. You are probably corrrect they do the same thing.
Tom: You are correct. If you push these "high RPM diesels" they tend to blow black smoke (fuel). I haven't ever had the pleasure of having a real thumper (low rpm engine 2000rpm cruise)and have no idea how they reach. You are probably corrrect they do the same thing.
Thnaks, and one more comment...
Since the engine is underpowered, I can't get close to hull speed no matter what. The hull speed is 7 knots (I reach this under sail sometimes), but at near full out on the motor, the boat does a little over 6 knots. This makes one tend to push the motor a bit. Even so, the fuel consumption is miniscule.
Since the engine is underpowered, I can't get close to hull speed no matter what. The hull speed is 7 knots (I reach this under sail sometimes), but at near full out on the motor, the boat does a little over 6 knots. This makes one tend to push the motor a bit. Even so, the fuel consumption is miniscule.
Hull speed is not top speed
Scott, If you do the math on the LWL for the H320/326, you will find it to be less that the 7 knts. In fact, you should find it to be about what you get under engine power. As for the top speed, I can hit 7 knots under sail, and once under a double reef and 30% of my jib, I hit and sustained 9 knots on a broad reach (GPS data, not knot meter). The boat under light air and a good trim hits the same speed under motor or sail. Stronger winds and wave surfing can push the speed up, so I disagree that the engine is underpowered. It is not a speed boat, but a sailboat. Steve
Scott, If you do the math on the LWL for the H320/326, you will find it to be less that the 7 knts. In fact, you should find it to be about what you get under engine power. As for the top speed, I can hit 7 knots under sail, and once under a double reef and 30% of my jib, I hit and sustained 9 knots on a broad reach (GPS data, not knot meter). The boat under light air and a good trim hits the same speed under motor or sail. Stronger winds and wave surfing can push the speed up, so I disagree that the engine is underpowered. It is not a speed boat, but a sailboat. Steve
Two thoughts
1. By coincidence, there's an entire discussion going on elsewhere today on hull speed. Short version: don't push it. 2. Speaking of black smoke, I always gun the engine when I straighten up out of the slip to blow out the carbon build-up in the mixing elbow. (Then I back down so as not to scare women and children.) By clearing out the elbow, I hope to prolong having to replace it for coking up. That happens so frequently with Yanmars that my ship's store keeps the things in stock.
1. By coincidence, there's an entire discussion going on elsewhere today on hull speed. Short version: don't push it. 2. Speaking of black smoke, I always gun the engine when I straighten up out of the slip to blow out the carbon build-up in the mixing elbow. (Then I back down so as not to scare women and children.) By clearing out the elbow, I hope to prolong having to replace it for coking up. That happens so frequently with Yanmars that my ship's store keeps the things in stock.
My 4-108
As opposed to conjecture....and speculation. Diesels do not have to be run hard. If that was the case diesels would die faster than gas engines. My Morgan 33OI has a 27 year old 4-108 Perkins in it that has never been rebuilt....had 2421 hours on it when the hour meter died....so I'm not sure how many hours it has...but it fires up immediately. When I am motoring I do so at 23-2400 rpm which with my 3 blade prop gives me 7 knots which is my hull speed according to the 1.34 rule. The 4-108 has a nominal top rpm of 3200 or so.... but if I crank her up to 2800 or more all I do is consume more fuel and push more water....but don't go any faster. So I guess what I am saying is: Don't drive yourself nuts over this. Operate the diesel judiciously...if you can reach hull speed at a moderate rpm...ok. if you reach it at a lower rpm....thats ok too. Be nice to your diesel and it will be nice to you......hammer on it and you will experience the "repowering blues".
As opposed to conjecture....and speculation. Diesels do not have to be run hard. If that was the case diesels would die faster than gas engines. My Morgan 33OI has a 27 year old 4-108 Perkins in it that has never been rebuilt....had 2421 hours on it when the hour meter died....so I'm not sure how many hours it has...but it fires up immediately. When I am motoring I do so at 23-2400 rpm which with my 3 blade prop gives me 7 knots which is my hull speed according to the 1.34 rule. The 4-108 has a nominal top rpm of 3200 or so.... but if I crank her up to 2800 or more all I do is consume more fuel and push more water....but don't go any faster. So I guess what I am saying is: Don't drive yourself nuts over this. Operate the diesel judiciously...if you can reach hull speed at a moderate rpm...ok. if you reach it at a lower rpm....thats ok too. Be nice to your diesel and it will be nice to you......hammer on it and you will experience the "repowering blues".
Beg to differ Rob!
Rob: If you read the literature/manuals on Yanmars they are very specific about running the engine at 80-90% of the max rated RPM. This is not conjecture or speculation. Running at lower rpms causes coking of the mixing elbow and glazing of the piston cylinders. I have no idea about the design of a Perkins but this is how the factory indicates that Yanmars should be operated for maximum operating efficency and longevity.
Rob: If you read the literature/manuals on Yanmars they are very specific about running the engine at 80-90% of the max rated RPM. This is not conjecture or speculation. Running at lower rpms causes coking of the mixing elbow and glazing of the piston cylinders. I have no idea about the design of a Perkins but this is how the factory indicates that Yanmars should be operated for maximum operating efficency and longevity.
My Experience Dictates . . .
otherwise, Steve. I'm not argueing principal, just experience. I understand what the factory manuals read, but I don't think its a hard and fast rule. As Rob indicated, if it was, most of us would have destroyed our diesels (Yanmars included) by now. Like a lot of us, this is my first diesel, a Yanmar 3GM30F. Its in an 89' Vision, now 13 years old, with a fair amount of use. I didn't have the internet to open up a whole realm of ideas back then, so I went with what I knew from having gasoline engines. I didn't run it all that hard. If 2000 rpm gave me 6 knots, and 2800 rpm gave me 7, but with 50% more fuel usage, increased noise, etc, I would normally run at 2000 rpm. I changed out my mixing elbow this last winter for the first time and found it to be in good shape. I'll keep it for a spare. As I said, I don't think that running at 80% of max is a hard and fast rule, but rather a point on the rpm curve that guarantees that the diesel is being loaded enough to bring the operating temperatures up to the point that it runs clean. However, as the diesel is less loaded, it runs cooler and coking starts to occur. Idling for long periods is a definite no-no, as it puts no load on the diesel and would probably cause coking and cylinder wall glazing to happen relatively quickly. But on my engine, 2000 rpm is pushing my boat at a pretty good clip, loading the engine significantly. Another boat that is under propped would be loading the engine much less at the same rpm and might coke up much more quickly. Neverless, I am now running at much higher rpm's much more of the time. But I also won't stay up nights worring about it when I don't.
otherwise, Steve. I'm not argueing principal, just experience. I understand what the factory manuals read, but I don't think its a hard and fast rule. As Rob indicated, if it was, most of us would have destroyed our diesels (Yanmars included) by now. Like a lot of us, this is my first diesel, a Yanmar 3GM30F. Its in an 89' Vision, now 13 years old, with a fair amount of use. I didn't have the internet to open up a whole realm of ideas back then, so I went with what I knew from having gasoline engines. I didn't run it all that hard. If 2000 rpm gave me 6 knots, and 2800 rpm gave me 7, but with 50% more fuel usage, increased noise, etc, I would normally run at 2000 rpm. I changed out my mixing elbow this last winter for the first time and found it to be in good shape. I'll keep it for a spare. As I said, I don't think that running at 80% of max is a hard and fast rule, but rather a point on the rpm curve that guarantees that the diesel is being loaded enough to bring the operating temperatures up to the point that it runs clean. However, as the diesel is less loaded, it runs cooler and coking starts to occur. Idling for long periods is a definite no-no, as it puts no load on the diesel and would probably cause coking and cylinder wall glazing to happen relatively quickly. But on my engine, 2000 rpm is pushing my boat at a pretty good clip, loading the engine significantly. Another boat that is under propped would be loading the engine much less at the same rpm and might coke up much more quickly. Neverless, I am now running at much higher rpm's much more of the time. But I also won't stay up nights worring about it when I don't.
Mike.
Mike: You are running your diesel in fresh water too (I presume). I have run my engine in both fresh and salt water since it was new. We are on the Sacramento River which flows to the S.F. Bay. When I checked out my elbow after 12+ years it was also free of coking and corrosion. My assumption here is the fact that there must be a combination of the exhaust gases (probably acidic and the salt water) that causes this corrosion problem. I believe what Yanmar is talking about is long term. The folks that have worked on my Yanmar are factory trained Yanmar dealers. They have told me that a Yanmar is good for 10,000-12,000 hours IF properly maintained. With this in mind, we have probably not reached the critical number of hours on our engines for some of the symptoms to show up. If you look at the owners of many of the Cherubinis, their boats are getting into that critical time frame and they are rebuilding and replacing engines. My contention is the factory has no reason to tell you these things if they are not in their best interest. It is like oil changes. You can run a car for many years and never do an oil change (just add oil when it's low). The difference is going to be the person that did his regular oil changes is going to have a lot better change of increasing the life the engine vs the person that never changed their oil.
Mike: You are running your diesel in fresh water too (I presume). I have run my engine in both fresh and salt water since it was new. We are on the Sacramento River which flows to the S.F. Bay. When I checked out my elbow after 12+ years it was also free of coking and corrosion. My assumption here is the fact that there must be a combination of the exhaust gases (probably acidic and the salt water) that causes this corrosion problem. I believe what Yanmar is talking about is long term. The folks that have worked on my Yanmar are factory trained Yanmar dealers. They have told me that a Yanmar is good for 10,000-12,000 hours IF properly maintained. With this in mind, we have probably not reached the critical number of hours on our engines for some of the symptoms to show up. If you look at the owners of many of the Cherubinis, their boats are getting into that critical time frame and they are rebuilding and replacing engines. My contention is the factory has no reason to tell you these things if they are not in their best interest. It is like oil changes. You can run a car for many years and never do an oil change (just add oil when it's low). The difference is going to be the person that did his regular oil changes is going to have a lot better change of increasing the life the engine vs the person that never changed their oil.
Steve,
Like you, I have run my diesel in both salt and fresh water, 7 years on the Chesapeake and Atlantic, fresh water since then. I do agree with your latest post and as I stated, I'm being much more careful to follow Yanmars suggestions. However, some of my fellow sailors now think I'm a race nut since I now tend to go at full speed a lot more of the time.
Like you, I have run my diesel in both salt and fresh water, 7 years on the Chesapeake and Atlantic, fresh water since then. I do agree with your latest post and as I stated, I'm being much more careful to follow Yanmars suggestions. However, some of my fellow sailors now think I'm a race nut since I now tend to go at full speed a lot more of the time.
RPM
Steve, Rob's comment is right on. The rpm that the engine runs at is not nearly as critical as whether or not the engine is operating with a load.
Steve, Rob's comment is right on. The rpm that the engine runs at is not nearly as critical as whether or not the engine is operating with a load.
Mixing
The failure of this point of connection is rarely due to the use or non-use that an engine gets, regardless of rpms. It is rather from the metal fatigue at the welded joint where the exhaust water meets the hot manifold exhaust air. Additionally, many boats used the "older" stiff metal reinforced exhaust hose between the muffler inlet and the elbow. This caused significant and constant vibration and torque to be transmitted to the elbow welded joint, "assisting" in the failure of this "made-up" joint. Newer boats are sometimes being manufactured with hump hose. We retrofitted our old exhaust hose from the manifold to the mufler with new hump hose after rebuilding the inlet port to the muffler with Marine Tex. It's not always what's going through the hose that causes the problem, but, rather, what the ends of the hose are attached to. The hump hose eliminates (absorbs) the direct vibration between the exhaust manifold and the stationary muffler.
The failure of this point of connection is rarely due to the use or non-use that an engine gets, regardless of rpms. It is rather from the metal fatigue at the welded joint where the exhaust water meets the hot manifold exhaust air. Additionally, many boats used the "older" stiff metal reinforced exhaust hose between the muffler inlet and the elbow. This caused significant and constant vibration and torque to be transmitted to the elbow welded joint, "assisting" in the failure of this "made-up" joint. Newer boats are sometimes being manufactured with hump hose. We retrofitted our old exhaust hose from the manifold to the mufler with new hump hose after rebuilding the inlet port to the muffler with Marine Tex. It's not always what's going through the hose that causes the problem, but, rather, what the ends of the hose are attached to. The hump hose eliminates (absorbs) the direct vibration between the exhaust manifold and the stationary muffler.
Steve-Regarding Hull Speed
Steve, I don't know what calculator you are using, but hull speed based on the displacement hull theory is just slightly less than 7 knots, or quite a bit faster than what I can push the boat under power. I personally feel a sailboat should have an engine/prop combo that can reach hull speed.
Steve, I don't know what calculator you are using, but hull speed based on the displacement hull theory is just slightly less than 7 knots, or quite a bit faster than what I can push the boat under power. I personally feel a sailboat should have an engine/prop combo that can reach hull speed.
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