Can of worms I know but....

Alright, How come when the question of diesel reliability comes up, ie. How long will my SUCH AND SUCH diesel engines last there is really no right or wrong?
I have spent hundreds of hours on the subject from Boat diesel (excellent source) to every boat forum out there and no one agrees on diesel engine longevity. It really was daunting when I bought my first diesel boat and made me even question the purchase at the time.
I understand that useage,nonuseage,load/underload, turbo, natural there are hundreds of variables.
It just amazes me that there are no useful guidelines. For example I have twin 1989 DD 6-71 Ti's with a little over a thousand hours. I have heard they are grenades and will blow between now and 1500 hours to they'll out live me. Now I run them at a steady pace of 1000-1200 rpms and every 4 hours or so run them up to 1800 to blow them out. At the lower rpms they maintain proper operating temps and I will change the oil and coolant every season. Have only had the boat for a year now so time to service everything.
I have even contemplated bringing them back to Naturals because I dont need 900 HP for the type cruising I do, shes a big, deep, heavy motoryacht and I have no desire to cook along at 18 kts everytime I take her out, but have been told it's notnecessary but with no real clear understanding of why it's not necessary to do so. I thought it would save money on those items from wearing out and needing to be fixed or replaced?
What I really would like to do, but it is not cost efficent is to pull the Detroit iron and install a couple of small 100 hp diesels to move along at 8-10 knots with a decrease in fuel burn. I would then have to add ballast as the DD's and gears weigh in at around 6-7k pounds total. While my boat is not a "trawler" but a motoryacht, She is perfect for the wife and I and I will use her as a "trawler".
I know a long winded rant but something that I would like to find out what others here think.

Knothead-I can;t speak to the efficacy of replacing your 6-71s, that is beyond my limitedknowledge. I can however, say something about the 6-71s. the folks who told you they will outlive you are closer to correct than the 1,500 hour folks. In the late 70's-early 80's 3 of us would buy old Trumpy motorycht and rehab/rebuild them. They all had either 6-71 or 8-71s in them. The boats we dealt with were all built from the mid-50's to the mid-60's. We never had to change out an engine. We did have to rebuild one or two, but that is a relatively cheap and painless process. Those things will run just about forever if well-taken care of. In my mind-the 71 series is one of hte most solid diesel engine ever built. There are still many thousands of them in service of all types all around the world.

Do you have the torque and HP graphs for the engines? Probably not the best to run them at too low an rpm for normal cruising. One alternative is to flatten the props a little so that you turn up the rpm for the same knots. Running the engines in their normal operating range (rpm) at a reduced load is certainly better for the engines and should reduce the carbon build up for running at slower rpms. You shouldn't see any meaningful change in fuel consumption as the increased rpm is offset by the engines running more efficiently in their normal rpm range. Certainly this is a lot cheaper to try than your other alternatives.

Ted

For example I have twin 1989 DD 6-71 Ti's with a little over a thousand hours. I have heard they are grenades and will blow between now and 1500 hours to they'll out live me.

The problem with guesstimating engine hours is the loads and maint are unknown.

For longest service a genset at 1200rpm, with oil changes on the fly will go longest.

Take the same engine , put on a turbo and after cooler and run it at 2300 for its life and it wont last as long.

The fuel burn will change from 5HPH to about 30GPH!

A 6-71 TA is a sport fish choice as its light and will make its rated HP for a season.

The simplest concept is to figure an engine will burn so many gallons of fuel in its lifetime.

Run that 6-71 as a prime 1200rpm genset and the service life will run astronomical, 20,000 hours with good oil and very carefull service of the air filters is not unheard of, tho 10,000 is more common.

While your engine setup would certainly NOT be the first choice for 1200-1400 cruising, as long as the turbo is making positive pressure at cruise , your engine service life will be closer to the gen set than the fish killer.

It will probably out live you , but the turbo may suffer a bit if not spooled up.

Underloading could be a "problem" but so what?

Your 10,000 hour engine looses 50% of its service life , its STILL 5000 hours !!!

Sure at 5000 it will smoke and burn more oil , and fuel, but these are minor compared to any engine swop.

ENJOY

I agree w THD and FF,

It may be a blessing to have the turbo's. When you run them up the resulting load and temps will probably help w the under loading issue ... if it even exists ... especially w the DDs. The type of rings used in the DDs will probably tell some about the under loading tolerance. Enjoy your beautiful boat. The cost of a repower will almost certainly not be cost effective but the fairly obvious fact that you don't have the boat that fits your needs right now indicates that you should sell her and get the boat designed to go slow BUT that probably won't be cost effective either. Depends entirely on how how long you'll be boating as you are now and you don't even know that. I'd make a very honest assessment of the future and try to make a decision based on what you know. The downside w continuing to do what you're doing is almost entirely just the cost of the extra fuel and it's almost certainly the most cost effective thing to do. BUT if you can live happily w the cost of changing boats and that's what you really want to do then go for it. I have the feeling you're asking us to validity your desires. It just money.

Diver,
That's an interesting question. It could result in even greater under loading as it's like shifting into a lower gear. On each power stroke ther'e will be less pressure and forces that aren't dependent on engine speed. More energy will be exerted and consumed moving pistons and other reciprocating parts back and forth but which will be the greater is a good question. Basically that would be going from very low load and rpm to low rpm and even lower load. But the load is ultimately determined by fuel consumption and if under propping results in lower fuel consumption it may not be effective re the OP. But if fuel consumption went up that would confirm that load went up and indicate that your idea works but it would probably be better to just increase engine speed/load with the correct propellers. Then when the correct load/speed is used to "blow them out" or whatever the engines would be run as designed and intended.
Another thing to consider is that one can't "lug" a boat engine propped correctly so underpropping may work very well unless a helmsman got a hold of the boat that didn't know about the underpropped condition. Placards don't always work.
Regarding this I personally prefer an engine slightly underpropped about 50 to 75rpm
Also one could use the underpropping solution to optimize a sweet spot of low vibration. This could be a very valuable side effect.

I agree with most of what has been said here: run a turbo charged engine hard and it won't last so long. Run a turbocharged engine at low rpms and power and it will outlast you.

I do not agree that flattening the prop and increasing rpms will improve engine life. Excactly the oppositie is true. A diesel pulls in air in proportion to rpm and boost pressure. At higher rpms and boost you are pulling in more air but with the same diesel injection quantity, so combustion and exhaust gas temps will be lower. So the engine will run cooler, which isn't good for long life.

Unless the turbo is seawater inter cooled (don't know about the 671 ti) there isn't that much more to go wrong on a turbo engine, except for exhaust sea water intrusion. That risk is determined by exhaust system design.

David

Nothing mechanic outlives a 6-71. My point of view? Bring them to natural or better yet, take the intercooler and tune down the turbos and your problem is solved.

manyboats said:

Diver,
That's an interesting question. It could result in even greater under loading as it's like shifting into a lower gear. On each power stroke ther'e will be less pressure and forces that aren't dependent on engine speed. More energy will be exerted and consumed moving pistons and other reciprocating parts back and forth but which will be the greater is a good question. Basically that would be going from very low load and rpm to low rpm and even lower load. But the load is ultimately determined by fuel consumption and if under propping results in lower fuel consumption it may not be effective re the OP. But if fuel consumption went up that would confirm that load went up and indicate that your idea works but it would probably be better to just increase engine speed/load with the correct propellers. Then when the correct load/speed is used to "blow them out" or whatever the engines would be run as designed and intended.

Click to expand...

djmarchand said:

I do not agree that flattening the prop and increasing rpms will improve engine life. Excactly the oppositie is true. A diesel pulls in air in proportion to rpm and boost pressure. At higher rpms and boost you are pulling in more air but with the same diesel injection quantity, so combustion and exhaust gas temps will be lower. So the engine will run cooler, which isn't good for long life.
David

Click to expand...

Ok, first of all you are going to burn a little more fuel at higher rpm for the same knots of speed. There is a cost to spinning all the engine's moving parts, transmissions, and prop shafts faster. Secondly, the OP stated that the engine was running at 1,000 to 1,200 rpm with proper operating temperature. Think of this in the application of a commercial grade 25KW generator. They are designed to run with minimal to full load capacity at the same rpm for weeks on end. The objective here is to get the motor to run in the normal rpm operating range and at the normal operating temperatures. If you have normal oil and coolant temps, and proper lubrication from running the motor at a high enough RPM, the reduced load shouldn't be a problem.

I have a pickup with a 220 HP diesel in it. I can tow a trailer with a combined vehicle weight to 20,000+ pounds at 60 mph and burn 5+ gallon per hour (85 +/- HP). The same truck running empty at 60 mph burns 2.4 gallons per hour (40 +/- HP). Both scenarios work because everything is kept with in the proper operating ranges. Truck has 310,000 miles on it and uses no oil!

Ted

knotheadcharters said:

Alright, How come when the question of diesel reliability comes up, ie. How long will my SUCH AND SUCH diesel engines last there is really no right or wrong?

Click to expand...

It's an impossible question to answer because the primary variable is human and there is no way to predict human behavior. The longevity of anything--- marine diesel, car, camera, sink faucet--- is largely dependent on the people using it. Even if you set up an engine in a lab and ran it under carefully calculated loads and power settings to duplicate its service in a boat, you would get a meaningless longevity figure because you took the human out of the equation.

You see it on this forum all the time. One person has X-engine and has enjoyed years, even decades, of trouble free boating. Another person has X-engine and has constant problems. Why? I think it mostly boils down to how the engines are run, how they are maintained, and so on.

The Ford Lehman 120 is said to be a 12,000 to 14,000 hour engine between total overhauls in recreational boat service. I expect a lot of them have done this. I know a lot of them haven't. According to Bob Smith, who while at Lehman did much of the marinization development and design for this engine, the FL120 has gone 25,000 hours between overhauls in service on Washington State ferries, I assume running generators or pumps or something.

The only "solution" is to try to find out what the people who are getting long, trouble-free lives out of their engines are doing--- how they run them, how often they service them, and so on--- and try to emulate it. If you can find someone with a lot of real world experience with the type of engine you have in terms of servicing, maintaining, and fixing them, all the better.

Case in point---- a number of years ago I became acquainted with a fellow from the UK who had made a career out of servicing, repairing, and overhauling diesel engines, many of them Ford of England products like the Dorset engine that is the base engine for the FL120. While he was a boater at the time I met him, his lifetime ofexperience with these engines was land-based, in vehicles and in industrial and agricultural equipment. He saw how these engines failed and under what circumstances. As a result, his "do this, don't do this" advice was based on real experience in the real world these engines live in.

A person like this I will listen to and act on the advice they give me. As opposed to someone like Steve D'Antonio who, while he certainly knows a lot and has (I assume) a fair amount of hands-on experience, seems to me to offer a lot of armchair theory, particularly when it comes to older engines. Which is why I pay no attention whatsoever to his notions on engine loading and such.

O C Diver said:

Think of this in the application of a commercial grade 25KW generator. They are designed to run with minimal to full load capacity at the same rpm for weeks on end.

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And running them at synchronous rpm for long periods at low loads is the death of most of them. If they are not loaded properly at high rpm they will fail in short order.

They don't normally fail catastrophically, they fail in a manner that might be described as congestive heart failure in a human.

If you have normal oil and coolant temps, and proper lubrication from running the motor at a high enough RPM, the reduced load shouldn't be a problem.

Click to expand...

That is misleading. Reduced loading at high rpm is destructive. It is the most common reason for replacement or overhaul of marine generators.

Too much to respond to in a restaurant on my I-pad but as always I've got something to say.

The first may be totally incorrect but I suspect not. My suspicion is that at higher rpm and the same power output the forces from combustion would insure that the rings did not exert as much pressure against the cylinder walls. Also the piston (because of the position of the crankcase journal) would press against the cylinder wall less due to far less combustion pressure and these two things would probably cause or help to cause cylinder glazing. You could say that the cylinders would get polished by these conditions. I don't know anything of the kind but I do suspect it. Perhaps someone deeper into this can confirm it or reject it. Thanks in advance.

Marin,
I disagree. I think engines run in a lab on a dynamometer could answer these questions completely regarding rpm differences w the same power output. One could monitor fuel consumption, power output and enough temps to tell the tale.

Eric--- That's not the question. Yes, a lab would give you those answers. But it would not answer the OP's original question which is how long of a life can one expect out of the engines in a boat you buy because the lab does not have any way of factoring in how the operators of that engine will operate it. So the lab can give you a theoretical answer. But it's not an answer you could count on in the real world of boats, many of which have had multiple previous owners who did Lord-knows-what in terms of engine operation and maintenance.

I stand corrected by my friend Marin. But the likelihood of the engine life being cut short is/may be directly proportional to engine loading and speed. Not measured exactly of course but generally speaking things to avoid or embrace can be identified and addressed or rejected.

RickB said:

And running them at synchronous rpm for long periods at low loads is the death of most of them. If they are not loaded properly at high rpm they will fail in short order.

They don't normally fail catastrophically, they fail in a manner that might be described as congestive heart failure in a human.


That is misleading. Reduced loading at high rpm is destructive. It is the most common reason for replacement or overhaul of marine generators.

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Rick, most commercial 25KW generators aren't running at high RPM. They run at a significantly lower rpm (2/3 to 3/4 of rated continuous duty). That is why I said "commercial" as opposed to the recreational generators such as what is found in most yachts and pleasure trawlers.

O C Diver said:

Rick, most commercial 25KW generators aren't running at high RPM. They run at a significantly lower rpm (2/3 to 3/4 of rated continuous duty). That is why I said "commercial" as opposed to the recreational generators such as what is found in most yachts and pleasure trawlers.

Click to expand...

WTF are you talking about?

A 25kW generator is pretty small no matter where it is used and last time I looked there wasn't any difference between a "recreational" generator and one installed on a charter boat or tug or a fishing vessel.

Diesel powered generators, or more correctly - alternators - turn at a synchronous speed related to the number of poles and the desired output frequency. A high speed diesel in the power range we are talking about here that drives a generator to produce 60Hz AC power will commonly run at 1800 rpm or 3600 rpm. Larger units might operate at 900 or 1200 rpm.

Maybe where you come from they only use variable speed generators but the rest of the world relies on conventional synchronous alternators.

71 series life span

I lived in the Bay Islands of Honduras for 20 years. Large schimp fleet there and lots of 71 series engines. I know those guys use to beat everything to death and the 71's took it.
I also ran a 2-71 generator for 5 years ( 10 hrs/day X 365 days/yr) without a problem. You have to clean the air box out regularly...

RickB said:

WTF are you talking about?

A 25kW generator is pretty small no matter where it is used and last time I looked there wasn't any difference between a "recreational" generator and one installed on a charter boat or tug or a fishing vessel.

Diesel powered generators, or more correctly - alternators - turn at a synchronous speed related to the number of poles and the desired output frequency. A high speed diesel in the power range we are talking about here that drives a generator to produce 60Hz AC power will commonly run at 1800 rpm or 3600 rpm. Larger units might operate at 900 or 1200 rpm.

Maybe where you come from they only use variable speed generators but the rest of the world relies on conventional synchronous alternators.

Click to expand...

Rick, since you clearly have a limited knowledge of "commercial" generators, let me explain it to you. The generator end for a direct drive generator is wound to match a specific rpm. As an example, if you had a 4-71 DD you could get the generator wound for 1,800 or 1,200 rpm. The 1,200 rpm unit will have a lower KW rating as the 4-71 can't produce as much HP at that RPM. The lower rpm / KW rated generators have longer lives and significantly greater duty cycles between servicings as a result of lower rpm, lighter loads, and less fuel consumed.

If you look at the generators on most trawlers / yachts, the rpm is at the higher end of the continuous duty rating for the engine. This is done to produce the most KWs with the smallest engine, least amount of weight and lowest engine cost.

Ted

All good insight but again the answer is widely varied. I appreciate the comments though. As I plan on keeping this boat for an indefinate time, I am going to continue running as I do and deal with problems as they crop up.

As far as generators, I was in Iraq all of 2011 and we had these huge shipping container sized gennys that ran our 20 CHU's (Containerized Housing Units) I don't know what they were rated at but had to be something ridiculous and they ran underloaded for over 4 years they were in place. Once a week they would shut them down for about an hour and a half for maintence then right back on line. Never had an issue.

So far I have had no issues with my DD's and they crank right up and very little smoke for about the first minute then no more. Yes I would like to have small 100hp motors, I could have a hell of a work space in the engine room with those. As I plan on keeping this boat for a long time I will be able to speak about the longevity from experiance.

O C Diver said:

Rick, since you clearly have a limited knowledge of "commercial" generators, let me explain it to you.

Click to expand...

I'll try one more time. A marine alternator operates at a synchronous speed precisely as I stated. It runs at that speed regardless of load. It will run at 900, 1200, 1800, 3600 or whatever rpm it was made to operate to produce (in those cases) 60Hz alternating current. It will turn that rpm at 25kW or 2.5kW output.

When the unit is operated for long periods at 2.5kW the engine suffers serious problems due to underloading. This is a fact of marine generator operation and those of us who live and work with these devices on a daily basis rather than pretend on the internet are fully aware of the costs and problems created by low loading of generators.

When it comes to marine diesel generators, there is no such thing as a "commercial" vs some other type. We have DGs installed in our yachts which range in output from10 to 250kW and everything in between. Some of these yachts operate privately and some operate commercially ... the generators are all the same and work the same and even cost the same and fail the same way for the same reasons.

Low loading of generators is a different issue than the perception of low loading of propulsion engines that dominates the threads on forums such as this one.

So Rick let me ask you then. I have a Phasor 15kw genny in the boat, when underway all it really needs to power is the home style refrigerator that I have. Now I usually run the AC/heat depending on weather so I assume she is severely underloaded. Do I need to turn on a bunch of stuff to try and properly load it? If so instead of just one AC unit should I run all 3? My last boat I ran the genny rarely so I am trying to keep things right to stay away from ruining expensive equipment. Thanks

Ideally, yes, you should increase the load as much as possible to bring it up to 10kW or higher. Realistically, that is a pain in the keel and costs a lot in fuel.

We sometimes use loadbanks to keep the load up when the "hotel" load is low. I personally despise the things because they waste fuel to heat seawater in order to make up for a poorly considered build specification. But, considering that long term underloading of synchronous generators leads to high emissions, soot, oil sheen, and smoke to the point where some yachts have been asked to find another marina and have had to pay for cleaning their neighbor's hull and waterline, it is a workable solution.

I have been installing particulate filters with great results but that only hides the symptoms, it doesn't cure the problem.

In your case, it might be worth considering a good house battery and an inverter to run the trivial fridge load and give the DG a break. Your neighbors will appreciate it as well.

The lower rpm / KW rated generators have longer lives and significantly greater duty cycles between servicings as a result of lower rpm, lighter loads, and less fuel consumed.

Perhaps ,

The slow speed keeps down the piston miles , and the smaller output will consume less fuel.

I believe their longer life may be due to the HIGHER percentage of their permissible loading during normal operation.

I would assume its harder to underload a small unit than a big one.

The "best" setup if noisemaker engine life is desired is probably a DC unit matched to a large sized inverter.

4KW or 6KW inverter is not too expensive and a 300A 24V bus alt could be coupled to a light industrial engine (Kubota?) and set to operate at only the RPM required simply.

The Big inverter would probably have a load sense setup , so would parallel the dock power when loads of air cond require starting.

RickB said:

I'll try one more time. A marine alternator operates at a synchronous speed precisely as I stated. It runs at that speed regardless of load. It will run at 900, 1200, 1800, 3600 or whatever rpm it was made to operate to produce (in those cases) 60Hz alternating current. It will turn that rpm at 25kW or 2.5kW output.

When the unit is operated for long periods at 2.5kW the engine suffers serious problems due to underloading. This is a fact of marine generator operation and those of us who live and work with these devices on a daily basis rather than pretend on the internet are fully aware of the costs and problems created by low loading of generators.

When it comes to marine diesel generators, there is no such thing as a "commercial" vs some other type. We have DGs installed in our yachts which range in output from10 to 250kW and everything in between. Some of these yachts operate privately and some operate commercially ... the generators are all the same and work the same and even cost the same and fail the same way for the same reasons.

Low loading of generators is a different issue than the perception of low loading of propulsion engines that dominates the threads on forums such as this one.

Click to expand...

Rick, you just like to argue with people for the sake of argument. Nobody here was talking about running an engine at 10% of capacity for a specific RPM / HP. The OP was talking about pushing a 48' motor yacht at hull speed. At say 1,400 rpm, the load on the engines is likely 25% +/- of the capacity of those engines at that rpm.

Ted

RickB said:

WTF are you talking about?

A 25kW generator is pretty small no matter where it is used and last time I looked there wasn't any difference between a "recreational" generator and one installed on a charter boat or tug or a fishing vessel.

Diesel powered generators, or more correctly - alternators - turn at a synchronous speed related to the number of poles and the desired output frequency. A high speed diesel in the power range we are talking about here that drives a generator to produce 60Hz AC power will commonly run at 1800 rpm or 3600 rpm. Larger units might operate at 900 or 1200 rpm.

Maybe where you come from they only use variable speed generators but the rest of the world relies on conventional synchronous alternators.

Click to expand...

That is absolutly 100% correct.

Furthering Rick's discussion of loading vs engine life we have found that a disel fired prime mover in a generator application will get a acceptable mean time between overhauls if it is loaded at least 30% and under 80% of its rated capacity. We measure rated capacity of a generator set as the point where the governor no longer has control of the RPM.

O C Diver said:

Rick, you just like to argue with people for the sake of argument.

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I don't generally waste my time arguing with pretenders. I posted to correct your inaccurate statement that "commercial" (whatever that means) generators are suited to operate for "weeks at a time" at very low loads.

Real life operators of those systems know that is false and I responded to advise readers of that fact, not to argue with you.

FF's last post pretty much nails it, I think. He is correct.

O C Diver said:

Rick, since you clearly have a limited knowledge of "commercial" generators, let me explain it to you. The generator end for a direct drive generator is wound to match a specific rpm. As an example, if you had a 4-71 DD you could get the generator wound for 1,800 or 1,200 rpm. The 1,200 rpm unit will have a lower KW rating as the 4-71 can't produce as much HP at that RPM. The lower rpm / KW rated generators have longer lives and significantly greater duty cycles between servicings as a result of lower rpm, lighter loads, and less fuel consumed.

If you look at the generators on most trawlers / yachts, the rpm is at the higher end of the continuous duty rating for the engine. This is done to produce the most KWs with the smallest engine, least amount of weight and lowest engine cost.

Ted

Click to expand...

Ted, you are correct in theory, but not in practical application.

There are VERY FEW 1200 rpm generator sets in service. This is not a common RPM, although I have used it for continous duty applications at remote microwave sites a few times with great results. But, I had to design the engine/generator set myself as it is not a commercially available item.

Again, looking at commercially abailable generator sets you cannot but "small" generator sets in 1200 RPM. That makes them impractable for any reasonable dissussion.

Knothead

You are correct to be concerned about the 6-71 450 HP versions and longevity. Currently there is a good thread on boatdiesel about 1500 hour J&Ts at 1500 hours and associated issues. If you browse the boatdiesel archives some good info will show up on your question. Hopefully your engines are not the tuner builds but DDs, the DDs generally do better.

Derating is not cheap or easy, it can be done but many bits and pieces are involved. Assuming you are not the original owner, how these engines were operated by the PO will play a role in their remaining life.

Sunchaser,
I am on Boat Diesel and I spent the better part of a week solid reading every Detroit post on there and like I said in the original you get everything from good to bad. I guess I'm not going to worry about it and just run them as I see fit. Everyone has an opinion based on there experiances so I am going to do the same.
Yes not the original owner, 3rd actually. The question I posted about the genny was the PO had installed the Phasor 15kw about 2 years before I bought the boat and I am wondering why 15KW, it seems a little overkill unless you want to run everything in the boat at the same time. A 10KW would probably do fine. I know most of the Californians I've seen on the market have had 12.5KW gennys in them so I think that was the size they went with originally. As I've said in the past I'm new to diesels and I am just trying to get a better understanding of them, all my life I've been around and worked on gas, so diesel just seems foreign. I guess after a few more years with the DD's I'll know for sure. I do have to admit, I love the sound, can't even get a big block V-8 to sound as throaty as the 6-71 at idle.