Turbo Diesels

In my mind, for a planing hull, pick an engine with a good reputation and size so that normal planing cruise is a bit below max continuous. That gives you a bit of sprinting ability and is gentler on the engines. Any power above max continuous is emergency power.

Based on that philosophy, for my own boat in a diesel repower I'd pick the QSB at either 355 or 380 HP. The current 340 HP gas big blocks are plenty of power, but a higher max continuous rating would be nice. As is, I cruise just below max continuous and often end up there (which is 18 kts at best with the current props) on a following sea. So a bit more wouldn't hurt.

Swfla said:

I guess, on paper, running rpms below target speed will encourage soot buildup. Maybe in actual use, it's not an issue until you've reached 5000 hours (random number). This "on paper/in theory" is a trap I don't wish to fall into. But it's hard to tell which posts are real world vs theory.

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That one is very engine dependent. Some tolerate running at light load well, others hate it and crud up quickly.

Swfla said:

I guess, on paper, running rpms below target speed will encourage soot buildup. Maybe in actual use, it's not an issue until you've reached 5000 hours (random number). This "on paper/in theory" is a trap I don't wish to fall into. But it's hard to tell which posts are real world vs theory. Most everyone states their opinions as black and white fact.

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If what I said is true...that the QSB500 and 600...are the same engines, then my opinion is fact...hahaha. Seriously. I do know the QSB330 and 380 are exact same engine with only a tune. There’s a lot about this on the Seaboard website. Anyway, the common rail engines do not suffer the soot problems or carbon build up like a mechanical engine might. And while soot might be “normal”, it should be a flag that the engine is working hard. Maybe too hard. I have had two Cummins powered boats. None ever had soot. And I always ran them hard. I was always under propped. On the Carver, significantly so. I say “hard”....400 below max RPM whereas MCP is 200 below max.

Swfla said:

I guess, on paper, running rpms below target speed will encourage soot buildup. Maybe in actual use, it's not an issue until you've reached 5000 hours (random number). This "on paper/in theory" is a trap I don't wish to fall into. But it's hard to tell which posts are real world vs theory. Most everyone states their opinions as black and white fact.

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Regarding your last sentence in the above paragraph; you have brought up several misconceptions about diesels in this thread and have received comments. We are quoting people like Tony Athens because he is a recognized expert in this area. Check out his website at SBmar. Ski is also someone who’s opinion we respect and he has answered one of your questions. Boat diesel is also a good source as previously mentioned. I am no Diesel expert and don’t pretend to be one, especially on the web. I have based my replies to you on running diesels, reading credible sources, the manual, and talking to engineers at times at Cummins HQ and that includes one of the topics you have brought up.

Thanks for letting me know that you are using those references. I wasn't pointing fingers at anyone in particular regarding validity of opinions. That's just a frustrating fact of forums in general (for me at least). As I learn more, it gets easier to figure out the actual importance of real life vs paper theory. Also, as time goes by, I learn about other members who post and where their knowledge comes from. We've all seen people post stuff that they've pulled out of their ass. Baker just made a very helpful point about his personal experiences. I'm grateful for everyone who cares enough to respond to my threads.

If you're concerned about keeping things lubricated, dig around for an electric oil pump setup. Years ago I had one on my Ford Excursion. "Simple" electronics box tied into the ignition switch: turn key to on and the pump would run for 30 seconds, turn key to off and the pump would run for an adjustable 0-5 minutes. It drew oil from the pan (Tee fitting at the drain plug) and returned oil via a "custom" fitting that inserted between the oil filter mount and the oil filter. Turn the key to on, watch for the oil pressure gauge to rise, then start. You'll learn a lot about your choice of motor oil with a gadget like that.


Fair warning: no idea if I did something wrong when doing an oil change some time later, but the fitting it uses to return the oil to the system came loose and I didn't have a wrench available that was anywhere near large enough to retighten in. Alas, out it came.

If you want a specific engine to last, turbo or not, look at the sales brochures and find one rated for continuous duty. In my case I have a Cummins 8.3 6CTA rated at 450hp. Turbo, aftercooler, the whole nine yards. Rated for recreational duty. Limited run hours per year, limits on cruise rpm, limits on how much at full power. Run that engine at near its full power, and it will not last long. Maybe a few thousand hours...

But the same basic engine was available in continuous duty form. Somewhere around 200-230hp, 1800-2000rpm. In that form it can run all day everyday at full power. That is 8000-odd hours per year, and would probably make a few years. That's like 20k hours. Kind of like tractor trailers at 14 liters able to make a million miles. That's about 20k hours too.

When I set my boat up, I figured it needed about 200-250hp to cruise at 20kts. I could have picked a 6BTA (5.9 liters) but an 8.3 came up for sale used at a good price and I snapped it up.

Glad I did. Turns out my 20kt cruise is right at 11gph and 1900rpm. Right where the continuous version runs.

So look at the continuous rated versions of your fave engine. Run it there and it will last. Even if it is turbo'd to twice the power at full.

Another way to look at loading (especially non-turbo engines) is look at the dyno charts and find what rpm at full power it makes max torque. Where it makes max torque at full power is an rpm at cruise power where it is happy. For mine, I think it is around 1700rpm. For the Ford Lehman, I think it is around 1500-1600. Similar with the Cat 3208NA. Cruise at that rpm at 1/4, 1/2 half power and it will run for years. Whether it has a turbo or not, does not matter.

If it has an aftercooler, that has to be babysat, but does not change the longevity if run right.

Aren't full-time, wide-open-throttle engines de-powered versions? I'm sure if the maximum RPM was reduced from 2400 to 2000, my engine could run full time wide-open throttle without undue wear.

Hippocampus wrote;
“Would think trawlers are more like sailboats. Both are full displacement vessels.”

Sailboats are (most all) but not so for red trawlers. Most all trawlers are SD.

Mark I’m quite sure you’re right. My Mitsubishi is offered by three marineizers, Westerbeke, Vetus and Klassen. None are listed w the same hp. Westerbeke 44hp, Vetus 42hp and Klassen 37hp. I have the Klassen and belive it to be an industrial rating.

The engine assemblers could provide the fuel map or BMEP graphs for their products and quite informed selections could be made.

Sadly most refuse and only give HP numbers and prop curves.

All sellers will provide HP and torque curves , usually the torque peak is close to the "sweet spot", low fuel burn , long engine life .

I have never left hose hooked up for the above reasons also so that I can flush hot old water out of the hose and dock pipes before putting it into my boat.

Swfla said:

Thanks for letting me know that you are using those references. I wasn't pointing fingers at anyone in particular regarding validity of opinions. That's just a frustrating fact of forums in general (for me at least). As I learn more, it gets easier to figure out the actual importance of real life vs paper theory. Also, as time goes by, I learn about other members who post and where their knowledge comes from. We've all seen people post stuff that they've pulled out of their ass. Baker just made a very helpful point about his personal experiences. I'm grateful for everyone who cares enough to respond to my threads.

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Morning Swfla, I agree, you need to be careful about things posted on the web and that definitely includes this site as well. I have also learned over time who to read, and who to ignore.

My manual instructing me to run my Yanmar turbo 315 HP engine under no load before shutting down makes no mention of the turbo. I assume it was just referring to ENGINE cool down which just happens to include the turbo.

Fletcher500 said:

Morning Swfla, I agree, you need to be careful about things posted on the web and that definitely includes this site as well. I have also learned over time who to read, and who to ignore.

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The thread about LLC is a glaring example of this. I think it's up to 6 pages!!

rgano said:

My manual instructing me to run my Yanmar turbo 315 HP engine under no load before shutting down makes no mention of the turbo. I assume it was just referring to ENGINE cool down which just happens to include the turbo.

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Rich, my 4LHA instructed me to do the same thing. I honestly think it is to clear any accumulated water that could potentially be in the exhaust system to prevent backflow into the turbo. My Carver with Cummins had one engine that would build up water and then blast it all out in one blast.....build up....blast....repeat. So since I could hear it, I would always throttle up until I heard that blast of water and then immediately shut it down knowing the exhaust system was free of accumulated water. I think if you shut it down right as it was filled up, add a little continuous rolling motion and you might have that backlfow issue everyone talks about.

Baker said:

Rich, my 4LHA instructed me to do the same thing. I honestly think it is to clear any accumulated water that could potentially be in the exhaust system to prevent backflow into the turbo. My Carver with Cummins had one engine that would build up water and then blast it all out in one blast.....build up....blast....repeat. So since I could hear it, I would always throttle up until I heard that blast of water and then immediately shut it down knowing the exhaust system was free of accumulated water. I think if you shut it down right as it was filled up, add a little continuous rolling motion and you might have that backlfow issue everyone talks about.

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That's an interesting thought; however, my exhaust flow, quite unlike my trawler which ran much like your engine's exhaust, gives a very steady flow with no surges.

rgano said:

That's an interesting thought; however, my exhaust flow, quite unlike my trawler which ran much like your engine's exhaust, gives a very steady flow with no surges.

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But Yanmar has no clue how the engine is installed or how the exhaust system is designed. So the put a “blanket procedure” in to cover that problem. I even think mine said to run up to full RPM in neutral before **** down. Not totally sure about that but I do know it said to run it up.

I’m no expert but have done several diesel courses. This what I’ve been taught.
Black smoke or discharge water means incomplete combustion. Assuming injectors and the rest is tuned correctly this most likely occurs when the engine isn’t sufficiently loaded. Many things can carbonize if this is allowed to occur with any frequency. My Yanmars, and Lehman manuals strongly recommended to run at WOT for 1/2h periodically in order to achieve full loading and decarbonize . Agree common rail solves much of this issue but you have another black box to potentially fail.
I’ve further been taught cooling and lubrication are the failure points shortening engine life. Even running full synthetic this remains true. Most wear occurs at start and shut off. Having the lubrication/cooling systems running while engine cools is helpful. Synthetics help at start. The demands on these failure points for a detuned commercially rated engine is less than in an engine set up to produce maximal HP.
Perhaps I’m wrong as regards very recent engines. If so please explain why.

Hippocampus said:

I’m no expert but have done several diesel courses. This what I’ve been taught.
Black smoke or discharge water means incomplete combustion. Assuming injectors and the rest is tuned correctly this most likely occurs when the engine isn’t sufficiently loaded. Many things can carbonize if this is allowed to occur with any frequency. My Yanmars, and Lehman manuals strongly recommended to run at WOT of 1/2h periodically in order to achieve full loading and decarbonize . Agree common rail solves much of this issue but you have another black box to potentially fail.
I’ve further been taught cooling and lubrication are the failure points shortening engine life. Even running full synthetic this remains true. The demands on these failure points for a detuned commercially rated engine is less than in an engine set up to produce maximal HP.
Perhaps I’m wrong as regards very recent engines. If so please explain why.

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Black smoke usually is caused by “overfueling”...so your statement is kinda correct. You get black smoke when there is more fuel than air. A good example, since we are talking about it, would be a failed turbocharger or anything that would restrict intake air. Also overloading. Like being overpropped, a foiluled hull, etc. “Incomplete combustion” would more likely manifest itself as white smoke and a sheen on the water. Think a fouled injector here. Improper spray pattern and the charge fails to ignite or burns poorly.

And yes, marine diesels usually fail due to other reasons than just wearing out. Overheating would probably be the number one cause. Overpropping is a major cause in high strung turbo aftercooled engines that are run hard. See the bottom three engines in my signature. Another failure point is aftercoolers. If one leaks, you’re blowing saltwater strait into the engine...assuming a raw water cooler. I don’t think there is ever really a reason to run your engine at WOT if your engine is loaded fairly well while underway. I run my engines in the 70-80% range all the time. I seriously doubt they suffer from carbon build up. Now if I was puttering about at near idle speed all the time, then yes it would be advisable. My dead idle in gear speed is about 6,5 knots. At 1000rpm I’m doing 7.5....the speed most people on here cruise at.

Info from marine engineer....

"... or discharge water means incomplete combustion."

Combustion of a hydrocarbon fuel produces close to a pound of water for every pound of fuel burned. In the case of a "wet exhaust" system that water will be mixed with the cooling water and never even noticed. Look at a car exhaust after starting on a cold day to see a great example, look at the contrail of an airliner at altitude for another example. If a dry exhaust shows water it is because the piping is cooled enough to condense the water vapor in the exhaust. That is another reason to ensure a dry exhaust system is well insulated, to keep it dry and reduce corrosion.....


"Having the lubrication/cooling systems running while engine cools is helpful."

Circulating cold raw water through the heat exchanger of a fresh water cooled engine (as most readers engines are configured) does nothing to cool the engine. If the engine uses raw water or keel cooling that technique would only create more moisture condensation in the crankcase and valve covers. Subcooling the engine rapidly is a very bad idea for that and a few other reasons. It is better to keep the engine hot as long as possible after shutdown. Continuing to circulate lube oil does nothing to benefit the engine unless the lube oil is heated as a means to keep the engine warm and dry while shut down.

I followed up on the previous post about idle cooling to prevent coking of passages and got this advice from the same engineer....

"I may have misunderstood his message, I read it as circulating oil or raw water after shutdown. Of course it is a good idea to idle the engine for a brief period before shutdown, if for no other reason than to let the turbo bearings and hot section cool a bit.

Shutting down a turbo'ed engine from high idle is really bad advice. All that does is spool up the turbine then remove all oil flow!"

Oil going thru a turbo doesn’t just provide lubrication it removes heat.
When I was an engineer aboard tugs the ones without water cooled turbos had an electric oil pump that would run through the turbo for about 20 minutes to cool it down. Tapping the start button before startup would also start the same pump to ensure oil to the turbo when starting. If that pump didn’t work we would idle for 1/2hr before shut down.
My 200tdi land Rover has an exhaust temp gauge. I don’t shut it down until the exhaust temp Is 300 degrees or below.
My Diesel Duck has a Cummins B3.9M. No turbo with 75 Big Horses.

Believe that the increase of turbo-engines verses naturally-aspirated id mostly due to governments' concern for air pollution which virtually mandates turbos. ... Thankfully, living naturally-aspirated without noticeable pollution.

'Lo All, My old Albin 43 had twin Cummins 6BT5.9M 210 HP engines. Power required to put the boat at hull speed (where I usually ran her), was calculated at about 54 HP (IIRC). I was considering investigating detuning the engines by removing the turbos and changing the injectors. Hurricane Michael obviated the question. At any rate, I believe that the boat was way overpowered. For whatever reason, it didn't plane very well, unless the power was way up, where she guzzled fuel. I did have one turbo freeze up due to carbon in the bearings, but we were able to work it free and it performed properly afterwards - but I always opened the throttles on occasion. And yes, when that turbo froze up, there was a black cloud following us unless the engine was essentially at idle power.

on Detroit it is a scavenger pump, They have intake ports , no intake valves. They will not pump air in to the cylinders with out the pump. If you remove and mount on a gas engine then it becomes a supercharger

Some real pearls here. Thank you. I’ve learned a thing or two. For the last 3 decades I’ve carried spare injectors, alternator, pumps, belts, filters, some hoses, rescue tape and fluids. Are there other things I should be carrying when off the grid? Does the list change with NA, turbo, or common rail?

Performance does not come at the expense of long term reliability

The more one pushes to achieve more torque at higher engine loads the greater the general degradation of long term reliability. It doesn't matter if you are talking a turbocharged diesel or a human being.

My advice based on experience is simple. If you are using your engines at greater than 90% load a majority of the time get every extension to your warranty available to you. About six months before the expiration of the last extension get a good engine survey done and get repaired what needs repair and then sell the boat

Carlr219 said:

The more one pushes to achieve more torque at higher engine loads the greater the general degradation of long term reliability. It doesn't matter if you are talking a turbocharged diesel or a human being.

My advice based on experience is simple. If you are using your engines at greater than 90% load a majority of the time get every extension to your warranty available to you. About six months before the expiration of the last extension get a good engine survey done and get repaired what needs repair and then sell the boat

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As mentioned in previous posts, the % load a diesel can safely operate at is all in the Rating. Genset duty, river tugs, dirt moving equipment etc come to mind when referring to continuous full load operation.

For example, one of the best and most popular industrial diesels out there is the Cat 3406. At an A Rating it is designed for 100% load full time putting out around 350HP. Cummins, Scania, Perkins Sabre, JD, Volvo and others make their versions of continuous duty "A" "M" etc rated diesels. All of course have turbos.

Interesting conversation.

However it does beg the question; what is everyone's definition of continuous duty, and the difference in that definition between commercial and recreational?

Is there really any difference between the boat fishing the grand banks for a month at a time and the recreational boat running from Charleston to Bermuda when it comes to running continuous duty?

BTW my diesels are turbo'd but do not have aftercoolers.

To me, continuous duty means you can run it at WOT until it's time to change the oil. And do that again and again while still getting a good lifespan out of the engine.