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Old 03-01-2013, 06:37 AM   #1
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Underloading

What we call underloading the generator folks call wet stacking.

Here is one engineers view on its cause and effects .
A close look at wet stacking

Wet stacking: How it happens, what it does, and how to avoid it.


http://www.plantengineering.com/indu...ca8f03440.html

Tom Divine, PE, Smith Seckman Reid Inc., Houston

“Wet stacking” is a term that originally described a diesel engine dripping a thick, dark substance from its exhaust pipes or, as they’re often called, “stacks.” The dripping exhaust stacks were called “wet stacks,” and the engine was said to be “wet stacking.” The condition is caused by operating the engine at light load for extended periods, sending unburned fuel and soot into the exhaust system. Today, the term refers to an engine that isn’t completely burning all the fuel that’s delivered to its cylinders. Over a prolonged period, this condition can seriously degrade engine performance.
Most standby generators for facilities have a diesel engine as the prime mover. Many of these generators are routinely tested at no load or at light loads, for a variety of reasons. Building operators are reluctant to interrupt critical loads for transfer to generator and back to utility. Data center operators often refrain from switching uninterruptible power supplies (UPS) to emergency power during tests to avoid affecting battery warranties with excessive transfers. Generator sets may have been oversized in anticipation of load growth that didn’t materialize. Whatever the reason, diesel generators that aren’t regularly exercised at a significant fraction of their nameplate capacities are at risk for wet stacking.
Electrical design personnel, who typically make generator selections, are not always well-versed in the internal workings of diesel engines. The mechanisms responsible for wet stacking aren’t intuitively obvious. Consequently, wet stacking can be mysterious: It impacts generator selection, but it’s not well understood in the electrical profession.
The intent of this article is to demystify wet stacking by describing some of the fundamentals of diesel operation, how wet stacking happens in an engine and how it affects performance, solutions for existing facilities, and solutions for facilities in the design phase.
Diesel engine fundamentals
To understand how wet stacking happens, one must have at least a cursory knowledge of the operation of a diesel engine. Here’s a simplified description—looking at a single cylinder—of how a four-stroke, turbocharged diesel engine works:
  • The intake stroke: As the piston travels downward, the intake valve opens, and the turbocharger delivers compressed air to the cylinder.
  • The compression stroke: When the piston reaches the limit of its downward travel, it reverses direction, the intake valve closes, and the piston compresses the air in the cylinder. The temperature and pressure of the air in the cylinder rise dramatically.
  • Fuel injection: With the piston at the top of the compression stroke, the fuel injector sprays a fine mist of fuel into the cylinder. The air in the cylinder is hot enough to vaporize and ignite the fuel. The burning fuel adds heat to the air in the cylinder, and its pressure and temperature rise further.
  • The power stroke: The hot compressed gas in the cylinder pushes the piston downward. The force on the piston is transmitted to the crankshaft through the tie rod, turning the crankshaft.
  • The exhaust stroke: The exhaust valve opens, and the piston pushes the hot gas out of the cylinder, through the exhaust valve, into the exhaust system. A portion of the exhaust gas is diverted through the turbocharger to drive compression of the intake air, and the cycle begins again.
Wet stacking
When a diesel engine runs without load, it develops only enough power to drive its accessories and overcome internal friction. It uses very little fuel and consequently develops little heat inside the cylinder. The cylinder’s internal surfaces stay considerably cooler than when running at higher load.
In the theoretical diesel cycle, the compression stroke is called “adiabatic.” That’s a thermodynamic way of saying that there’s no heat transfer between the air in the cylinder and its surroundings: the piston, cylinder head, and cylinder wall. In reality, though, the hot compressed air does lose heat to those surfaces. With no load, the cylinder’s interior is cooler than the engine’s design temperature, and the compressed air loses more heat to the engine than it would with the engine running under load.
A diesel engine doesn’t use spark plugs. It relies on the hot compressed air in the cylinder to vaporize and ignite the fuel. With the air cooler than the design temperature, conditions for combustion are less than ideal. The fuel ignites and burns, but it doesn’t burn completely. What remains are vaporized fuel and soot—small, hard particles of unburned carbon. Fuel condenses in the exhaust system, and soot deposits on surfaces inside both the exhaust system and the engine.
In the exhaust system, fuel vapors condense and mix with soot to form a dark, thick liquid that looks like engine oil. It may ooze from the turbocharger or drip from the exhaust outlets. The appearance of liquid on the exhaust stacks leads to the term “wet stacking.”
Inside the cylinder, soot can form hard carbon deposits on the fuel injector nozzle. The nozzle is designed to atomize the fuel, delivering a fine mist that vaporizes readily. When the injector nozzle is fouled with carbon, its ability to atomize the fuel is compromised, and it delivers larger droplets to the cylinder. The fuel consequently vaporizes less efficiently, more fuel remains unburned, and more fuel passes into the exhaust system. Wet stacking is a progressive condition; it tends to lead to more wet stacking.
It’s generally believed that prolonged operation at low loads can lead to permanent engine damage, requiring a major engine overhaul. Costs of an overhaul can run so high that replacing the unit is the most economical option.
NFPA 110-2010, Standard for Emergency and Standby Power Systems, paragraph 8.4.2, requires that units be tested monthly with adequate load to maintain the manufacturer’s minimum exhaust temperature, or at 30% or more of their nameplate rating, for at least 30 min. The explanatory material in Annex A states that these testing requirements are intended to reduce the likelihood of wet stacking. It’s worth noting that a provision in the 2005 version of NFPA 110 allowing testing at lighter loads was deleted in the current code. Building operators no longer have the option of testing generators at light loads.
The general cure for wet stacking is a few hours of operation at a load of about 75% of the generator’s nameplate rating or more, raising the exhaust temperature high enough to vaporize the unburned fuel in the exhaust system and blow out the soot. But, the exhaust temperature at that load is well above the auto-ignition temperature for diesel fuel, and on rare occasions fuel and soot can ignite within the exhaust system. If a unit has a history of extended operation at low load, or if there’s no documentation that it’s been exercised recently at adequate load, it’s important to have a professional generator maintenance expert manage the load testing procedure.
Existing facilities
Existing facilities with inadequate available load for generator testing will have to find a method to comply with the requirements of NFPA 110-2010. For facilities with loads considered too sensitive to for the momentary interruptions required by generator testing, the low-cost alternative is to re-evaluate the sensitivity of those loads, and include them in the test protocol. An alternative with low first costs is to engage a professional maintenance firm to manage load testing, with temporary load banks as required to meet code requirements.
Possibly the least disruptive path to compliance is to measure exhaust temperature while powering the available building load. Many generators reach the manufacturer’s recommended exhaust temperature with loads less than 30% of their nameplate rating. This method requires measurement of exhaust temperature for each test, requiring the installation or purchase of measurement equipment, or the services of a generator technician. Compliance isn’t certain, but if the existing test load isn’t far from 30% of nameplate, this technique may well deliver compliance without significant changes to the facility, or to its existing test protocol.
A load bank with enough capacity to increase the total test load, including the facility’s minimum available load, high enough to reach either the recommended exhaust temperature or the prescribed 30% level, is adequate for compliance. This option requires a circuit breaker on the emergency system, a means for disconnecting the load bank if utility power fails during the test, and space for the load bank in a location where its heat can be safely rejected. The load bank may be permanently installed, a temporary bank owned by the facility, or a rental unit engaged for each test.
A facility may choose to connect nonessential loads to the emergency system to increase the test load, essentially using those loads in lieu of a load bank. This option will require one or more additional transfer switches or motorized circuit breakers, along with a load shed controller, to de-energize those loads if utility power fails. It may also require reconfiguration of the distribution to collect preferred loads on appropriate circuits. National Electrical Code 700.9(B)(5)c prohibits serving optional loads and emergency loads from the same vertical switchboard section. If a spare breaker position in an appropriate section doesn’t exist in the generator distribution, this option will require an additional section, switchboard, or disconnect.
New facilities
All of the options available to existing facilities are applicable to new facilities. Generating systems should be specified with exhaust temperature monitoring, as this option is relatively inexpensive to provide with new units. Provisions for connection of a load bank adequate to test generators at 100% of their ratings, with automatic disconnection if utility power fails, should be part of any new installation. Nonessential loads can be connected to the emergency system, with appropriate load shed functions, at facilities where adequate generator loading is a concern. And, options exist at the design phase that are generally not economically feasible for existing facilities.
When a concern for adequate test load exists, the design team should consider using smaller generator units in parallel, with a “load demand” function. This function, implemented in the paralleling switchgear, compares online capacity to actual demand after the generating system stabilizes, and shuts off or restarts units as required to support the demand with a reasonable operating margin. Adequate test loads can be achieved with a fraction of the load required for a single-unit installation.
As an example, a system with two paralleled units can achieve 30% of a single unit’s capacity at only 15% of the total system’s capacity. The disadvantages of this approach include additional cost for paralleling gear and the space it requires, higher system complexity, and extended testing times required to test all of the units at adequate load. Code requirements for quick connection of essential loads will typically dictate the minimum size of paralleled generators at a level that’s adequate to support those loads with the smallest single unit.
A generating system that is capable of paralleling with the electric utility never lacks test load, as the utility acts as a nearly infinite sink for excess generator power. This option will generally be selected for reasons other than maintaining adequate test load, as it comes with a number of disadvantages. A facility may choose to parallel with the utility in order to do peak shaving, reducing its ratcheting demand charges, or to take advantage of payments from the utility for power delivered during system emergencies. Paralleling gear equipped for interconnection with the utility requires a number of functions that aren’t needed for island operation, including under- and overvoltage relaying, reverse power relaying, and redundant components specified by the utility. These functions significantly increase the cost of the paralleling gear.
Wet stacking is a serious condition affecting diesel generators that operate for extended periods at light load. It can be avoided by proper generator selection, and by properly performing routine generator testing.
Divine is project manager and electrical engineer at Smith Seckman Reid Inc.



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Happily for boat owners if we want to increase the load , a push on the throttle is all it takes.
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Old 03-01-2013, 10:19 AM   #2
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Or a "cruise" prop.
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Old 03-01-2013, 11:08 AM   #3
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Thats a good article FF

As a big part of my life I work in the generator industry, covering everything from operations and maintenance to design engineering, to control system integration.

I'm not a trained diesel mechanic, my expertise is the electrical side.

What I have seen in the field is exactly what the article said. Most backup generators are underloaded. Fortunately most of these units do not get enough hours on them to make wet stacking as prevalent as it could be.

Part of the problem is a misunderstanding of the National Electrical Code requirements. Part of the problem is the variable and intermittant loads applied to generator sets, and part of the problem is simple operator error when setting up the weekly or bi-weekly test paramaters.

Lets dig into this further...

The NEC article 700 requires all automatic generator sets to be capable of supplying the full load of whatever it is protecting. This leads to mistakenly oversizing the generator set. Lets take a typical home for example.

If you want to run your entire house on backup generator then many electricians assume that you need a generator capable of producing 200 amps (for NEC compliance), which is the size of most main breakers. Thats a 50KW generator.

The problem is that you'll never draw 200 amps. You'll actually in many houses, depending on air conditioning loads rarely exceed 50 amps.

Its easy to see that this leads to a severe underloading condition. The trick is to properly size the generator and still comply with the NEC. The way to do this is to perform a load calculation on the home and buy a generator that fits those needs. That will often still result in some underloading but not nearly as much.

Now lets take a business user and their weekly generator test.

People incorrectly choose to exercise their generators without a load being applied. In many areas with pretty reliable power, the weekly test represents the vast majority of hours on the generator. This unloaded running time can and does lead to wet stacking.

The facility operators run the weekly test unloaded because they generally do not want to stress their equipment with the brief outage that occures as part of the switchover process.

What the operators fail to realize is that almost every major brand of Automatic Transfer Switch now has a feature called "phase monitoring" built in. What phase monitoring does is wait for the generator and the utility power feeds to come into sync of their power waveforms and then switch very quickly. This minimizes the outage time to a very quick blink of the lights, somewhere around 50 miliseconds. since the power sources are in sync, there is no stress to any equipment.

Again, great article, and thanks for posting it FF
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Old 03-01-2013, 12:26 PM   #4
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Yes it is an interesting article. The $64,000 question is at what reduced load does this phenomenon become evident.

Surely a Ford Lehman 120 or a Perkins 6.354 can run at 1,400-1,500 rpm burning 3 gph and not experience this problem. Many of our forum members cruise at that power level all day long.

That is about 50 hp and about 40% of maximum power output. 50 hp is enough to heat up the cylinder walls to operating temperature so the diesel burns cleanly as described in the article.

So ratchet things up to a Yanmar 6LY or a Cummins 6BTA at 370 hp. 50 hp is a much lower percentage of full hp but it is roughly the same hp per liter as the Lehmans/Perkins. So won't the cylinder walls also get up to operating temperature. You might have to run at a little higher rpm to make 50 hp because these engines have a higher rpm range and are geared differently than the Perkins/Lehman- perhaps 1,600 or so.

But I think that the high powered engine will do just fine all day long at 1,600 rpm.

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Old 03-01-2013, 02:11 PM   #5
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march,
Unless they are over propped the Lehman's should burn closer to 2 gph. Three if considerably over propped. Or did you mean twins?

Re the modern engines I think they have different ring designs to cope w the higher heat, pressures and speeds. But maybe even more influential is the computer controlled injection.

Some of those Yanmar's make incredible power and I wonder how close to max power they can be run continuously?
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Old 03-01-2013, 02:58 PM   #6
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Eric:

I have had a good conversation with Tony Athens of boatdiesel fame about the maximum safe continuous cruising rpms for high output engines like the Cummins 6BTA and the Yanmar 6LY.

Tony is a very conservative guy and he will be the first to tell you that Yanmar's spec of running their engines continuously 200 rpm off of max hp rpm is pure BS. Yes you can do that and they will probably last through the warranty period, but we generally want our engines to last longer than that.

According to Tony if you want to use an engine continuously, then run it at no more than 35 hp per liter. That is about 210 for the Cummins and 180 for the Yanmar. His basis for these numbers are the commercial engine manufacturers like John Deere's ratings for continuous duty. In this case the warranty means something as these commercial guys are going to put a 1000 hours per year on their engine or more and if it fails the first year, the manufacturer is on the hook. And according to Tony, most diesels will go at least 5,000 hours and often 10,000 hours at 35 hp per liter power loading.

Having said all of the above, I do run my Yanmar 6LY at about 40 hp per liter but never for more than a few hours at a time (a fast run to Ocracoke) and always with lots of low speed cruising built into the mix for an average power loading of about 20 hp per liter.

So in summary, the higher the power loading the shorter the life. Conversely the lower the power loading the longer the life down to about 10 hp per liter where the curve probably turns around.

I suspect the Lehman/Perkins can operate a bit lower since their injectors don't have to operate at three times the fuel burn rate like the Cummins/Yanmar so they can maintain a good fuel pattern at low rates which helps with wet stacking. Common rail injection also helps maintain a good fuel injection pattern at low rates.

David
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Old 03-01-2013, 04:05 PM   #7
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I think I've been following many of the same conversations on this as David. My boat has Cummins QSC engines which are high output, and I did some digging to see exactly what Cummins says about low power operation.

They have lots of detail on high power operation, and are very clear about correct prop loads and how to confirm that you are not over loading the engine. And they are very clear about the allowable full-power duty cycle.

But for low power operation, the only requirement is that you operate the engines such that they maintain minimum operating temp, which in this case is 160F. I only need to run my boat between 800 and 1000 RPM to maintain 160F. So why the difference compared to the article posted? I think Dave has identified some of them, namely:

- A common rail engine injects at a significantly higher pressure than a non-common rail engine. Does the finer atomized spray allow complete fuel burn at lower cylinder temps?

- If an engine achieves sufficient exhaust temp at N hp/liter, that value of N occurs earlier in the power curve for a high-output engine than for a lower output engine.

- At least some common rail engines inject multiple pulses of fuel, not just one big shot. This helps generate more complete fuel burn and lower emissions, and in turn presumably reduces the exhaust materials that cause wet stacking.

- The article is in the context of generators where the engine is always running at a constant 1800 or 1500 RPM. Is the wet stacking problem more pronounced running an unloaded engine at higher RPM versus 800 or 1100, or 1200?

What else? There is probably more.

Every answer goes hand in hand with some set of context. I don't dispute the wet stacking issue, but all indications are that there is a shifting context where it is a problem. From what I've seen, common rail engines provide a lot more latitude that previously possible.
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Old 03-01-2013, 04:36 PM   #8
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Yes you are absolutely right. The high pressure, finer spray pattern and on some- multiple pulses of common rail injection all contribute to better combustion of the diesel, even if the cylinder walls are cold.

Generators are particularly prone to dry stacking and cylinder wall glazing. When a generator is used at night to power the A/C it often only sees load 30% of the time. And generators are always running at relatively high rpm- 1,800 to 3,600 rpm. This high rpm means it is sucking lots of air and 70% of the time at night it is only getting enough fuel to keep it turning. This almost certainly won't keep it up to operating temperatures.

I am reminded of a conversation I had with the head of Bayshore Marine in Annapolis. He makes a nice business overhauling gensets of boats that spend the winter in the Carribean. They often come back with glazed cylinders, resulting in high oil consumption. The fix is easy- hone the cylinders. But it takes a lot of labor to get there.

Wet stacking, glazed cylinders is a real phenomenon, but it is mostly limited to gensets.

David
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Old 03-01-2013, 05:19 PM   #9
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Another factor at play is oil temperature. Some main propulsison engines are equipped with oil coolant HXs so that running at low load will still allow the oil to get heated up to circulating coolant temperature or higher. The magic number I've heard is oil temps in the 175 to 190 range are OK for burning off volatiles and water. At low power loads, say 7 or 8 hp per liter and 1650 RPM , my oil temps are in the 185 range on the Perkins Sabre 225TIs.
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Old 03-01-2013, 05:20 PM   #10
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I've seen and heard of many diesel engine failures but never one do to underloading.
Usually no, low, or bad oil, perforated or no air filter, water in cylinder, no water in cooling system stuff like that is common, I'm not so sure about underloading.
Maybe I'm wrong, is it a common failure elsewheres, first hand info please?
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Old 03-01-2013, 05:51 PM   #11
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I've seen and heard of many diesel engine failures but never one do to underloading.
Usually no, low, or bad oil, perforated or no air filter, water in cylinder, no water in cooling system stuff like that is common, I'm not so sure about underloading.
Maybe I'm wrong, is it a common failure elsewheres, first hand info please?
Steve W
The growing trend from what I have been reading is that once a diesel is broken in....that ANY load is OK and will not cause serious problems....no load yes...but even small loads are enough.

I'm not sure I buy that totally.... as almost any operation (normal operation) brings the temps up sooner or later....and there's a propotional load to the RPM.

It's when you have higher RPMs and no load that starts getting into bad habits....such as unloaded (even very underloaded) generator use.

So there's enough doubt in my mind as there is definitely a spectrum of opinions out there on this subject.
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Old 03-01-2013, 07:19 PM   #12
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march,
Unless they are over propped the Lehman's should burn closer to 2 gph. Three if considerably over propped. Or did you mean twins?
Wouldn't the boat's size, shape, weight, and hull condition make a difference in fuel burn?
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Old 03-01-2013, 08:03 PM   #13
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Ron,
Of course. I was throwing that out assuming all would assume a typical TF trawler. If I listed all the variables on all these discussions my posts would only be a small part of my posts.

psneeld,
Yes the underloaded engines that are destined to fail from under loading don't because the UL problem isn't so severe that they fail from it before the engines fail from lack of use, maintenance or all the other evil things a boat engine usually suffers. So we're not planning on ruining our engines but we're destined to do so so we may as well under load them while ther'e alive and well. Is that what I'm hearing?
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Old 03-01-2013, 08:19 PM   #14
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What I don't understand is, from all the warnings on web forums, just about every production diesel trawler is vastly overpowered and is underloaded in normal operation. Why do the manufacturers overpower these boats (assuming that the web forum talk is correct)?
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Old 03-01-2013, 09:15 PM   #15
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Originally Posted by djmarchand View Post

I am reminded of a conversation I had with the head of Bayshore Marine in Annapolis. He makes a nice business overhauling gensets of boats that spend the winter in the Carribean. They often come back with glazed cylinders, resulting in high oil consumption. The fix is easy- hone the cylinders. But it takes a lot of labor to get there.

Wet stacking, glazed cylinders is a real phenomenon, but it is mostly limited to gensets.

David
An easier way to deglaze? 3 years back I glazed the bores of my Onan MDKD genset trying to run the eutectics compressor which had died, lots of blue smoke, I was worried. Internet search found " Nulon Heavy Duty Diesel Engine Treatment", a doubled dose with fresh oil would fix glazing. It did, oil consumption normalized, blue smoke ceased. No idea what`s in it, but it`s clearly not sand. I continue a normal dose in the genset to avoid recurrence risk,I see no reason to put it in the Lehmans.
Nulon is a major local lube oil, and additive, supplier. No connection, except a satisfied user.
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Old 03-01-2013, 09:16 PM   #16
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from all the warnings on web forums, just about every production diesel trawler is vastly overpowered

Ron, you put far too much credence in web chatter. Vastly? I'll have to think about that until I finally hear a satisfactory defintion on a trawler. Better yet, pick on KSanders who repowered his trawler with big a$$ twin (yes twins!!) Cummins a few years ago. All he needed were two 40 HP Yanmars, right?
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Old 03-01-2013, 09:26 PM   #17
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march,
Unless they are over propped the Lehman's should burn closer to 2 gph. Three if considerably over propped. Or did you mean twins?
I have run my Lehmans for over six thousand miles at 1400 rpm burning 1.05gph in our current boat. About ten thousand miles in our last boat. Never an internal engine problem. The flow scan pic is a 120 Lehman in a friends 40' Marine Trader Sedan single.
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Old 03-01-2013, 09:33 PM   #18
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Ron,
Of course. I was throwing that out assuming all would assume a typical TF trawler. If I listed all the variables on all these discussions my posts would only be a small part of my posts.

psneeld,
Yes the underloaded engines that are destined to fail from under loading don't because the UL problem isn't so severe that they fail from it before the engines fail from lack of use, maintenance or all the other evil things a boat engine usually suffers. So we're not planning on ruining our engines but we're destined to do so so we may as well under load them while ther'e alive and well. Is that what I'm hearing?
No
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Old 03-02-2013, 06:34 AM   #19
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"What I don't understand is, from all the warnings on web forums, just about every production diesel trawler is vastly overpowered and is underloaded in normal operation. Why do the manufacturers overpower these boats (assuming that the web forum talk is correct)?"

CO$T is frequently the prime mover for the boat assembler.

Sure he has to sort of match the engine size next guy is sticking in a similar sized boat as ,
as a lots of newbes buy new boats and have ZERO idea that bigger might NOT be "better".

Its very hard on a production cookie to have a selection of engines aviliable , costs too much unless the boat is popular enough for two production lines.

The usual Rules of thumb on diesels like load to 80% load at 90% rated rpm are fine for boats with industrial sourced engines.

Heavy duty with various Cont ratings the mfg specifies.

When dealing with auto or light truck or farm/yard implement sourced engines, they survive low loadings mostly because the" Rated HP" is totally unrealistic advertisement jibberish .

No one would expect a 500 Hp Corvette to last in an 18 wheel truck.

No one would hook a Ford Econo Power conversion to a generator head or water pump that required 120 or 135HP .

For most Displacement boats 1 hp for every 2240 lbs of boat will produce the long range cruise speed most operate at. SL 1 or so.

Over 5 hp per ton just makes waves and heats the water .

OVERPOWERED ? You do the math.
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Old 03-02-2013, 08:29 AM   #20
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Quote:
When dealing with auto or light truck or farm/yard implement sourced engines, they survive low loadings mostly because the" Rated HP" is totally unrealistic advertisement jibberish .
No, they avoid low loadings by being run through multi gear manually or automatically shifted transmissions, that's how.
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