Minimum Cruising RPM

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ghost0070

ghost0070

Member
Joined
Mar 2, 2014
Messages
21
Location
US
Vessel Name
Rosie
Vessel Make
American Tug 34
Forgive me if this has been covered before. 2002 AT34 with the older 370hp Cummins. (6BTA 5.9 M3) It seems max cruise would be 2400 rpm, 80%. This is loud, very fast and not fuel efficient.
My bigger concern is the best minimum cruise. Currently I use 1600rpm, but I wonder if this is too low for good turbo health. I speed up once a day somewhere around 2200rpm.
Thoughts?
 
If there is a positive boost you are fine.

The hassles with a turbo come when its just an exhaust obstruction , doing nothing for the boat.
 
How do I know there is positive boost?
 
1600 is fine for a 6bta 370. Could go a good bit lower and still be fine.

My 6cta 450 has logged a couple thou hours at 950-1050. Sometimes I run on plane at 1800-2100, but don't like the fuel bill. Engine has about 3600hrs on it now and seems fine. Never been taken apart.
 
What does your owners or service manual say?

We run our Yanmar a bit past where the turbo gets a good whine on. The manuals give directions on what rpm to take the engine, and for how long, if running it below where the turbo is happy.
 
The manual speaks mostly about servicing. Not much about best operating practices.
 
1600 is my 8 kt rpm for my Cummins 6bts. Many hundred hours up and down to Alaska with no ill effects.:thumb:
 
1600 is just fine. You're probably very close to peak torque at that rpm. The only concern could be if you noticed increased oil consumption if you ran too lightly loaded for extended periods of time. but it's easy to fix just run it at a higher load for 15 minutes +/- and it should clean it up. Then you can go back to normal.
 
Our 37 Nordic Tug cruises with its 6BTA 5.9 M3 mostly at 1300-1400. It seems to be a very happy engine. 1600 hours on it in the last two summers.

Our previous boat's 260 hp Volvo KAD44P cruised mostly at 1300-1400 too, working just hard enough to keep operating temps close to the normal range. We put 6500 hours on it before we sold it.

I think Cummins guru Tony Athens of Seaboard would say not to worry.
 
I thought this forum established that lube oil temp was the prefered way to tell if an engine was loaded high enough to gaurd against varnish buildup and whatever else happens at temps too low.
Seems to me it was 170 degrees. And an EGT gauge in the exhaust is a fairly good way of telling if an engine is working hard enough.

One thing’s for sure. It’s not an rpm issue or problem but a matter of enough heat.

If there’s such a thing as a “happy engine” the road to this happeness is through maintaining or exceeding minimal operating oil and exhaust gas temp or/and not exceeding maximums.
 
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Nomad Willy said:
I thought this forum established that lube oil temp was the prefered way to tell if an engine was loaded high enough to gaurd against varnish buildup and whatever else happens at temps too low.
Seems to me it was 170 degrees. And an EGT gauge in the exhaust is a fairly good way of telling if an engine is working hard enough.

One thing’s for sure. It’s not an rpm issue or problem but a matter of enough heat.
Click to expand...



I typically run my QSB 5.9 at 1,400 to 1,500 rpm. Usually towards the lower end. My coolant temp stabilizes at 172 degrees.
 
RCook said:
Our 37 Nordic Tug cruises with its 6BTA 5.9 M3 mostly at 1300-1400. It seems to be a very happy engine. 1600 hours on it in the last two summers.

Our previous boat's 260 hp Volvo KAD44P cruised mostly at 1300-1400 too, working just hard enough to keep operating temps close to the normal range. We put 6500 hours on it before we sold it.

I think Cummins guru Tony Athens of Seaboard would say not to worry.
Click to expand...



Richard are you in Booker also with Salty
 
1150rpm gives us 7.2ish knots.
Water sits at 80c or 176f
Oil temp sits at 75c or 167f

Cummins nta855m
 
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Speaking of Tony Athens, here is one of his takes on the subject:

As for life expectancy on a typical modern high speed diesel engine (probably an old one too), when run at 10-50 % of its rated hp, you’ll get longer life (substantially in many cases) when measured in engine hours, everything else being equal..As a general statement, Detroit diesels of WWII design built their “long life” reputation on running slow..Most of my work in the last few years has been replacing Detroit 2-strokes in commercial vessels with Cummins C series engines that are set up to run at approx 30-40% of their rated hp when cruising for long/extended periods. This WILL allow these engines to accumulate approx 20,000+ hrs of engine time before rebuild due to ring/cylinder, head and bearing wear.. The Cummins 6BT 210 is a 10,000 hr+ engine in an application that lets the engine cruise at about 50-60% of rated HP with the remainder of the hours between idle and 50% of load.. I’m also making the assumption that the engine doesn’t fail due to maintenance/installation problems. I personally own a 4BT (2 cylinders less than a 6BT) that had a “top- end” at 13000 hrs and is now approaching 30,000 total hrs and still has all of the original base engine parts in it.. By far, most of its hours have accumulated at well under 50% of its rated HP.

To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less..Yes, there are many other parts of the equation that leads to the life of a diesel engine, but I know from experience that running them slow (i.e. cruising for days on end at hull speeds or less) is NOT a cause for concern. But some obvious things to watch out for (when running slow) are being sure your coolant temp stays up to spec. and watching for any signs of wet stacking or slobbering..Some Detroits seem to do this, but I think it’s more of the general condition of that particular engine and is sometimes related to the design of the exhaust system.. A few minutes a day at higher cruise HP levels should be all that is necessary to clean things up should it be needed..
Click to expand...
 
You want to keep the bow down not worry about RPMs At low RPMs that engine will outlive your grandkids. If the coolant temp is too low use higher temp thermostats. Lower temp stats will raise low load temps but not affect full load temps.
 
Nomad Willy said:
I thought this forum established that lube oil temp was the prefered way to tell if an engine was loaded high enough to gaurd against varnish buildup and whatever else happens at temps too low.
Seems to me it was 170 degrees. And an EGT gauge in the exhaust is a fairly good way of telling if an engine is working hard enough.

One thing’s for sure. It’s not an rpm issue or problem but a matter of enough heat.

If there’s such a thing as a “happy engine” the road to this happeness is through maintaining or exceeding minimal operating oil and exhaust gas temp or/and not exceeding maximums.
Click to expand...

Good points Eric. Many marinization builders include a coolant to oil heat exchanger to insure light loading oil temperatures stay in the minimum range of coolant temperatures, say 175F. And lightly loaded is common as noted by other posters.
 
Ten years ago a PM guru (Steve Antonio) said to run at 70% load 70% of the time.

Starting to appear now it dosn't matter what one does???

I run my engine at 50%. Much more and it's a bit too noisy over time. And I'm a believer in keeping the oil temp up. So it's 50% for me. Of load that is .. not rpm as many seem to think.
 
The Cummins engines being discussed have an oil to coolant heat exchanger (oil cooler) so temps tend to stay in pretty narrow band unlike the sea water oil cooled brands (Volvo, Yanmar, Lehman). At low load the Cummins oil temp will be near coolant temp, at high load typically gets to about 20-30F higher than coolant temp. No concerns there.
 
On my FD boat with a JD4045DFM70:

100% load, equal to maximum fuel consumption of 4 GPH is at 2400 RPM.

Maximum boat speed is achieved at 2200 RPM which represents a 75% load (3 GPH vs. 4 GPH max.)

Normal cruise speed, one knot below max. is achieved at 1800 RPM, a 40% load.

Max. torque is achieved at 1400 RPM, a 25% load.

Believe anything between 25% and 75% load (based on relative fuel-consumption/work) on this engine will result in long engine life.
 
"Believe anything between 25% and 75% load (based on relative fuel-consumption/work) on this engine will result in long engine life."

Believe what you want , but bugger the engine converter for a BMEP or fuel map and you can be assured of operating in a long life ,efficient band.
 
bayview said:
You want to keep the bow down not worry about RPMs At low RPMs that engine will outlive your grandkids. If the coolant temp is too low use higher temp thermostats. Lower temp stats will raise low load temps but not affect full load temps.
Click to expand...

Good point
 
With some caveats I believe that engine life is inversely proportional to hp produced per liter of engine displacement.

The Gardner 6LX engine produces 127 hp at 1,500 rpm and most are probably operated at closer to 1,200 rpm where it makes about 70 hp according to its prop curve. That engine has a displacement of 10.5 liters so at that load it is making about 6.7 hp per liter. It will last a long, long time at that load.

But if you were to run a Cummins 6BTA 370 hp engine at the same hp per displacement it would be making about 40 hp at about 1,300-1,400 rpm. It should last just as long as the Gardner, if maintained properly.

David
 
David,
A bigger engine (more displacement) developing X amount of power will exert a very low amount of force on engine components like pistons, cylinder walls, rod bearings, crankshaft bearings and have a low piston speed and on and on.

A smaller engine making the same power but w smaller parts will have much higher forces applied to all these parts per sq in of bearing surface. So the forces per sq in is higher.

That in itself would indicate a higher wear rate on the specific engine parts but most of these parts are supported by pressureized oil. That opens the door to the question of “is the bearing wear proportional to the load of the part on the bearing or is there no wear unless there is metat to metal contact”? Under what circumstances is there more .. or less bearing wear? It would seem to me that bearing wear should be negligible (basically non-existant) as long as there was no metal to metal contact.

But for the pistons and cyl bore it could be different. If it is different small engines may need pistons and bore work very often during service or rebuild but seldom and work needed on rod bearings ect.

Then there’s the question of heat and what role (if any) heat plays in the mix.

RPM would seem to increase wear. But from what? Increased piston speed?
An engine making X power at 2000rpm exerts a given force on components. Does a smaller engine at 4000rpm with twice as many strokes would conceivably have half the bearing loads and half the relative speed of parts.

So what is the real (and relative) source of engine wear?
 
Regarding big and small engines producing the same power, an engine with half the displacement running at twice the speed will have longer cumulative piston travel in a given amount of time. It will have a shorter stroke by 20%, but twice as many so 60% greater travel, concentrated on 80% of the wall area, and so 2x the wear on the cylinder wall.

Pressurized oil bearings wear very slowly if at all, though speed plays a small part. The issue there is more how many starts, when there is rubbing prior to the oil film prior to the pressure wedge being established.

Heat would play a role in the valve train and injectors, the smaller engine running at much higher combustion and exhaust temperatures.
 
So DDW,
You’re saying component travel and speed has more to do w it that pressure or force?
But the pressure on the cyl wall would be far less on the smaller engine. Less pressure twice as often. I would think less pressure would be less wear. Opps I forgot about area. But would that (pressure per sq in) be about the same?
 
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I think the force would be about the same. You are getting the same power from half the displacement by doubling the RPM at the same pressures, effectively the smaller engine has to turn twice for every once the larger engine does thus twice the wear. If instead it turns the same RPM and develops the same power, then twice the BMEP, so twice the pressures on everything - cylinders, bearings, etc. It's a good question which results in a longer lasting small engine: twice the pressures or twice the RPM.

A reasonable guess is that at light loads the number of revolutions dominates, as every component is running at well below its designed stress. On a trawler at cruise, even a small engine might be considered lightly loaded. Our sailboat for example has a 75 hp engine driving 30,000 lbs, but at cruise rpm is running at about 30% load according to the engine/prop curves. The trawler has a 380 hp engine driving 20,000 lbs, at cruising rpm running at about 20% load. Neither is high enough to stress the components much more than idling, so its just hours, piston, and bearing travel.

There are plenty of examples of lightly loaded pickup truck diesels running 500,000 miles and more, which would be 10,000 hours or so. What seems to kill lightly loaded boat diesels is calendar time near salt water, and lack of use/maintenance.
 
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