WOT is 100 rpm low

[jrd22]

2003 Nordic Tug 32, 6BTA 270 hp. 3200 hours. New boat to us, engine runs great, starts well when cold, uses no oil (200 hrs), but can only get 2500 rpm at WOT. Normally cruise at 13-1400 rpm/ 7.5 kts, and reportedly that's where the two previous owners ran it. Have not checked turbo pressure but there is little to no smoke at WOT. I'm going to adjust valves and maybe change injectors just because of hours/years. I understand that it should hit 2650 or so and that if it doesn't it's lugging the engine a bit but not sure if I should assume that it's been over propped since new (original owner was meticulous, I have all the paperwork on original prop) or if I should explore something else. Haven't had the boat long so don't want to jump to conclusions. Any suggestions appreciated.

 

[READY]

Unless you plan to run at full RPM often it seems like life is good? Congrats and enjoy the boat!

 

[DDW]

This is the subject of countless threads, with the conventional thought being that if you can't reach full rpm then it is "overpropped". My opinion is that if you never run it at full rpm, and it isn't overloading it at your normal cruise rpm, then it is fine, in fact, probably better than the "correct" pitch. One pretty easy way to get a feel for this is to add an EGT gage, even if it is temporary. Most of the exhaust elbows on Cummins engines seem to already have a drilling for this, you just need the sensor and a gage. They aren't terribly expensive. If the EGT is reasonable at cruise, then I'd leave it.

The physics of prop torque absorption guarantees that the torque required to spin it drops rapidly with reducing rpm, so that the required torque is always under what the engine is capable of. And the fact that you can double your cruise rpm indicates that you are well under the capability. How far (on a mechanically injected engine) is most easily told by EGT.

 

[rslifkin]

I would agree that cruising that far below rated RPM, being slightly overpropped is unlikely to be an issue. Even though you're running slightly above the intended prop curve, it's not by much at that RPM, so the difference in power output vs RPM for what you're doing vs what the manufacturer expected for long term use isn't far off. If you do want to cruise faster sometimes then I'd be more inclined to resolve the issue.

Going over the basics to make sure the engine is healthy and making full power is a good idea regardless. If that doesn't resolve the issue, then taking an inch of pitch out of the prop likely will. Plenty of boats are propped a little bit optimistically from the factory and end up overpropped with a realistic cruising load on board.

 

[DDW]

Even though you're running slightly above the intended prop curve, it's not by much at that RPM, so the difference in power output vs RPM for what you're doing vs what the manufacturer expected for long term use isn't far off.

The prop loading curve is what it is, due to physics. Some mistake this for "this is what the engine manufacturer ideally wants". The same basic engine, run in light trucks, is loaded very differently than a prop curve, and in that case the engine manufacturer has great control over the loading because the powertrain control module selects the gear based on the load (recent trucks have 8 and 9 speed transmissions so pretty fine control). They chose to load the engine more heavily at low rpm than a prop curve would. So when they have the choice, the engine manufacturers "overprop". It's just that in a boat you can't change the pitch (or gear) when you bump the rpm and loading way up (unless you have a variable pitch prop). So there is danger that you will run it overloaded at high rpm. That's where the "must be able to achieve max rpm" standard comes from.

Any of you who fly light planes know that this is routine in aircraft. You set the rpm you want, then set the pitch to load the engine most efficiently.

 

[Endeavor GB 32]

You're within 5-6%. Have a cocktail and look for something worthwhile to worry about..
Regards,
Scott

 

[O C Diver]

If you have the standard Cummins gauge cluster from that time period, the tach should be adjustable from the rear of the gauge.

I would do a search for "adjusting 2003 6bt tachometer", to verify.

Ted

 

[DDW]

Then just set the tach to 2650 at full throttle and Be Happy!

 

[BroadCove]

Also, and maybe this is too obvious to state, ensure your bottom / running gear is clean. Fouling would be a significant impact.

 

[jrd22]

Really appreciate the advice/opinions. Thanks! Bottom and prop are clean (new antifouling last spring and diver replaced zincs last week and said everything looked great). I'll leave the prop as is for now. I might have the turbo rebuilt just for reliability (seals) and see if that makes any difference. The coolant temp does go up if I run it at 2000 (178 at cruise -185/90 at 2000) and goes up even more at WOT. I've only run it at WOT for about 4-5 minutes.

 

[rslifkin]

The coolant temp does go up if I run it at 2000 (178 at cruise -185/90 at 2000) and goes up even more at WOT. I've only run it at WOT for about 4-5 minutes.

That points to a cooling system problem. Make sure the seawater pump and impeller are healthy and exhaust elbow isn't clogged. The heat exchanger might also need cleaning.

 

[tiltrider1]

What is the test equipment? If you are just going off the tachometer, how do we know its accurate? I think you are close enough to not worry. If you are worried I would get a photo tachometer to verify your results.

 

[DDW]

Coolant temp rising 5 - 10 degrees between low power cruise and higher power runs seems to be normal for these engines and most any engine. It has been true of both the QSB5.9 on my prior boat, and the QSB6.7 I have now. It is a regulated system (by the thermostat) and regulation requires some slope with increasing load to stay stable. If it went much above 185 then I'd check the cooling system closely.

 

[Comodave]

First thing I would do is get a photo tach and verify what the actual RPMs are. If they are still low then do the Second thing. Second thing is to read Tony Athens on this topic. He is the Cummins guru and he definitely wants Cummins engines to turn 50 RPM over max rated. That way you are not overloading the engine. The 50 extra RPM are for when the boat is loaded heavy you can still turn max rated RPM. Read what he says about overloaded Cummins engines.

Just because POers did something doesn’t make it right.

 

[SteveK]

You could also confirm that the throttle lever at engine is fully deployed and it is not the morse cable at the end of pull.

 

[jrd22]

You could also confirm that the throttle lever at engine is fully deployed and it is not the morse cable at the end of pull.

Good idea! Will check that.

 

[North Baltic sea]

Hi,

Before delivery, every NT undergoes extensive sea trials, and this is one of the things that is tested.

There are several reasons why that 100 rpm may be lacking. The condition and cleanliness of the bottom, the weight of the boat, cargo, and liquids, the condition of the propeller surface.

Currents in the sea, wind, etc. I wouldn't worry if I were you.

I wouldn't do unnecessary maintenance either, but would just drive and keep an eye on things.

 

[mvweebles]

What is the test equipment? If you are just going off the tachometer, how do we know its accurate? I think you are close enough to not worry. If you are worried I would get a photo tachometer to verify your results.

That was my thought as I went through the thread - maybe the tach is reading slightly low?

Here's a cheap and easy digital tech. You put a strip of reflective tape (included) on the main pulley and point/shoot the digital tach.

Amazon.com

I envy the OP if that's his biggest item on the list.......

Peter

 

[jrd22]

That was my thought as I went through the thread - maybe the tach is reading slightly low?

Here's a cheap and easy digital tech. You put a strip of reflective tape (included) on the main pulley and point/shoot the digital tach.

Amazon.com

I envy the OP of that's his biggest item on the list.......

Peter

Ha! well it isn't the biggest item, but it's a long list.

 

[DDW]

First thing I would do is get a photo tach and verify what the actual RPMs are. If they are still low then do the Second thing. Second thing is to read Tony Athens on this topic. He is the Cummins guru and he definitely wants Cummins engines to turn 50 RPM over max rated. That way you are not overloading the engine. The 50 extra RPM are for when the boat is loaded heavy you can still turn max rated RPM. Read what he says about overloaded Cummins engines.

Just because POers did something doesn’t make it right.

But he is also mostly concerned with commercial and fishing operators running the engines at 2200 or 2400 rpm all day. In that case I'd accept his advice - the available torque and the required torque are getting close there. On Nordic and American Tugs, we run these engines at 1200 - 1400 for years or even decades at a time. How it behaves at 2400 or 2600 rpm is academic, if it never sees those conditions. Having it achieve 50 over rated max is safe for the mechanic and the installer, because the owner will have a hard time abusing it. That doesn't mean it is best for the careful owner, if never run up that high. If you are an abusive owner, then yeah, 50 over.

 

[Lepke]

Most of my life I have run overpropped boats. Usually because I wanted a certain rpm, economy, or speed at a particular engine load. Since I always get more hours between overhauls (usually double) than people running the same engines and similar hulls get, I know overpropping didn't hurt the engine or it's lifespan.
100 rpm off max is nothing and as others have said can be attributed to boat loading and bottom cleanliness.
It never a bad Idea to check valve clearances. Especially if you're still earning.

 

[hollywood8118]

I cannot believe anybody would worry about -100 rpm @ wot!

Its akin to questioning 249 gal. capacity in a 250 gal fuel tank.

Hollywood

 

[Kapn]

Really appreciate the advice/opinions. Thanks! Bottom and prop are clean (new antifouling last spring and diver replaced zincs last week and said everything looked great). I'll leave the prop as is for now. I might have the turbo rebuilt just for reliability (seals) and see if that makes any difference. The coolant temp does go up if I run it at 2000 (178 at cruise -185/90 at 2000) and goes up even more at WOT. I've only run it at WOT for about 4-5 minutes.

When was the last time it hit full rpm?
All of the above mentioned items can add up to 100rpm pretty easily.
Unless you short haul and pressure wash and sand all barnacle husks off the prop, it wouldn't be unheard of to lose some rpm to a dirty bottom that 'seems clean' to a diver.
I wouldn't spend too much time chasing this problem.

 

[Greg QS]

Propping a Cummins Mechanical Diesel Engine

www.sbmar.com

Take a read. See what the pros say. There is a lot of good stuff on the site.

Over propped is over propped though the whole power curve. If you want to maximize efficiency and lifespan set it up right. Shoot for 100 over rated rpm with fresh bottom, full of fuel and water and gear. This way you will keep the engine out of an over stressed condition.
Over propping simply puts the engine outside of the design specification of the engine. A car or truck shifts gears and automatically adjusts the load on the engine. Over propping makes the engine lug all the time. Puts more stress on parts, increases heat ,Increases egt, reduces cooling capacity and hurts fuel efficiency.

Is the 100 under super critical ? I would say no. Its probably within the safety margins of the engineer. But now you have eaten into the safety margin. Ignore it, next thing you know its 150 or 200 low . Talk to the prop shop and tweak the prop and then rest easy.

As far as being over propped its whole life. Probably not. Boats get heavier with age, bottoms roughen up. Gear piles up in the boat . All requiring more power/load on the engine.

 

[DDW]

^^ I'd say this is just wrong. Over propping, as normally defined, does not "make the engine lug all the time" or "through the whole power curve". If it did, then all the many millions of diesel engines in daily use throughout the world, in generators, tractors, trucks, pumps, would all be lugging and ruined in short order as they are routinely loaded higher than a prop would.

The design specification of the engine has no knowledge or consideration of the prop absorption curve. That is but one use case, and a minor one, for most of these engines. Yes, a truck shifts gears to control the torque demand, and the way they are set up they load the engine more, much more, by design, than a boat engine being run at 1300 rpm. My Ford truck for example will happily load the engine to 75% at 1500 rpm for hours at a time. The max rpm is 4000. They designed the powertrain control this way. If they thought it was damaging they could downshift 2 or 3 gears but they don't. The Cummins 6.7 in my boat is virtually identical to the one in Ram trucks, and is managed the same way as the Ford. There are many, many more sold into trucks than into boats.

The only purpose of the "must reach max rpm" mythology is to prevent over loading the engine at or near max rpm, which it accomplishes. It has nothing to do with loading at 1/2 or 1/3 of max rpm which is an unfortunate and undesirable artifact of physics. Even Athens says that if you aren't ever running the engine at or near max rpm that some degree of "over propping" is fine. If it weren't, the a variable pitch prop like the Hundested would be useless as it only purpose is "over propping" at reduced rpm.

 

[rslifkin]

^^ I'd say this is just wrong. Over propping, as normally defined, does not "make the engine lug all the time" or "through the whole power curve". If it did, then all the many millions of diesel engines in daily use throughout the world, in generators, tractors, trucks, pumps, would all be lugging and ruined in short order as they are routinely loaded higher than a prop would.

The design specification of the engine has no knowledge or consideration of the prop absorption curve. That is but one use case, and a minor one, for most of these engines. Yes, a truck shifts gears to control the torque demand, and the way they are set up they load the engine more, much more, by design, than a boat engine being run at 1300 rpm. My Ford truck for example will happily load the engine to 75% at 1500 rpm for hours at a time. The max rpm is 4000. They designed the powertrain control this way. If they thought it was damaging they could downshift 2 or 3 gears but they don't. The Cummins 6.7 in my boat is virtually identical to the one in Ram trucks, and is managed the same way as the Ford. There are many, many more sold into trucks than into boats.

The only purpose of the "must reach max rpm" mythology is to prevent over loading the engine at or near max rpm, which it accomplishes. It has nothing to do with loading at 1/2 or 1/3 of max rpm which is an unfortunate and undesirable artifact of physics. Even Athens says that if you aren't ever running the engine at or near max rpm that some degree of "over propping" is fine. If it weren't, the a variable pitch prop like the Hundested would be useless as it only purpose is "over propping" at reduced rpm.

It's not so much about what the engine can handle, but about the intended / specified use. You can get the same engine rated for different uses, but often at different power outputs. And in vehicles, sometimes low RPM output is artificially restricted or output in some situations gets cut back after some period of time in a given loading condition.

In the case of the 6.7 Cummins, the pickup truck version tops out at 430 HP. And the medium duty truck version only goes up to 325 HP. Yet the marine version comes in ratings up to 550 HP. It's all down to differences in available cooling capacity, expected usage, ability to specify / control how the engine is loaded, etc. For the QSB6.7 at 550 HP / 3300 RPM, Cummins says that assuming it's propped to their spec, max continuous RPM is 3000 with a max continuous output around 400 HP. And if you prop it outside of that spec, Cummins won't provide a warranty as the engine is being operated outside of their intended ratings, so their expected usage vs lifespan and max continuous RPM are no longer valid.

An overpropped engine running well below max RPM is generally safe, but you also don't know what the upper limit of the safe continuous operation zone is. In a situation like a controllable pitch prop, you'd need to consult with the manufacturer to get information on what they consider to be acceptable operation.

 

[jleonard]

The best ( smartest) thing to do is check/ calibrate the tachometer with a handheld as discussed above.
Once you do that then you can decide future changes ( or not).
Hope this helps.

 

[DDW]

It's not so much about what the engine can handle, but about the intended / specified use. You can get the same engine rated for different uses, but often at different power outputs. And in vehicles, sometimes low RPM output is artificially restricted or output in some situations gets cut back after some period of time in a given loading condition.

In the case of the 6.7 Cummins, the pickup truck version tops out at 430 HP. And the medium duty truck version only goes up to 325 HP. Yet the marine version comes in ratings up to 550 HP. It's all down to differences in available cooling capacity, expected usage, ability to specify / control how the engine is loaded, etc. For the QSB6.7 at 550 HP / 3300 RPM, Cummins says that assuming it's propped to their spec, max continuous RPM is 3000 with a max continuous output around 400 HP. And if you prop it outside of that spec, Cummins won't provide a warranty as the engine is being operated outside of their intended ratings, so their expected usage vs lifespan and max continuous RPM are no longer valid.

An overpropped engine running well below max RPM is generally safe, but you also don't know what the upper limit of the safe continuous operation zone is. In a situation like a controllable pitch prop, you'd need to consult with the manufacturer to get information on what they consider to be acceptable operation.

If you are running at max continuous rpm and max continuous power, you should prop inside the spec. That isn't what we are talking about here. Running at 1300 rpm, and 30% of available torque, is a very low load for that engine, putting out about 50 hp. A QSB can be made to make anything from about 250 to 600 hp just by changing the ECU programming. But those are max torque, max rpm figures. The question is, what is Cummins spec for maximum continuous torque at 1300 rpm? I know it isn't 30%.

Even Cummins calls the max rpm spec a "Full Throttle Requirement". We aren't talking about full throttle operation. The engine is capable of about 1000 ft lbs of torque at continuous output. At 1300 rpm and 30% output, it is loaded to about 200 ft lbs of torque - 20% of its continuous spec. If I over prop to increase this to 250 ft lbs, still just 25% of its continuous rating, will it damage the engine? It will not. It would be putting out just 61 hp. From experience with other diesels that would raise the EGT from maybe 450 to 500 deg. Other mythologists will say that isn't being run hard enough and you will damage it, you should load it more. The dualing mythologies of "don't overprop at low rpm" and "diesels like to be loaded to at least 60%" or whatever are hard to reconcile (and both false).

With old mechanically injected engines you used EGT as the gage to set the controllable pitch prop. With common rail engines the ECU will tell you what the loading is, but an EGT is still a good check, and many common rail engines have them as part of the feedback loop. That's how you'd know what the maximum safe operation torque is. An ECU with EGT feedback will begin to derate if EGT goes too high. An ECU has other signals to derate, and might even keep an over propped engine from damage at full throttle - it isn't like mechanical which will overfuel by design. The OP has mechanically injected engine, so feedback is up to the operator.

 

[SteveK]

If you are running at max continuous rpm and max continuous power, you should prop inside the spec. That isn't what we are talking about here. Running at 1300 rpm, and 30% of available torque, is a very low load for that engine, putting out about 50 hp. A QSB can be made to make anything from about 250 to 600 hp just by changing the ECU programming. But those are max torque, max rpm figures. The question is, what is Cummins spec for maximum continuous torque at 1300 rpm? I know it isn't 30%.

Even Cummins calls the max rpm spec a "Full Throttle Requirement". We aren't talking about full throttle operation. The engine is capable of about 1000 ft lbs of torque at continuous output. At 1300 rpm and 30% output, it is loaded to about 200 ft lbs of torque - 20% of its continuous spec. If I over prop to increase this to 250 ft lbs, still just 25% of its continuous rating, will it damage the engine? It will not. It would be putting out just 61 hp. From experience with other diesels that would raise the EGT from maybe 450 to 500 deg. Other mythologists will say that isn't being run hard enough and you will damage it, you should load it more. The dualing mythologies of "don't overprop at low rpm" and "diesels like to be loaded to at least 60%" or whatever are hard to reconcile (and both false).

With old mechanically injected engines you used EGT as the gage to set the controllable pitch prop. With common rail engines the ECU will tell you what the loading is, but an EGT is still a good check, and many common rail engines have them as part of the feedback loop. That's how you'd know what the maximum safe operation torque is. An ECU with EGT feedback will begin to derate if EGT goes too high. An ECU has other signals to derate, and might even keep an over propped engine from damage at full throttle - it isn't like mechanical which will overfuel by design. The OP has mechanically injected engine, so feedback is up to the operator.

Finally we are remembering why this was popular in the 70's & 80's and no one questioned it as that is how it was done, prop to the expected cruise rpm.
As the old timers fade away the book educated insist MUST achieve max rated rpm.

 

[Greg QS]

OP go to the tony Athens SBMAR website there is a ton of great information there for you cummings platform .

The specification I gave him come from the Tony Athens/ smar web site. He is mentioned here quite a bit as and expert. These are his instructions. I recommend following them for the OP's set up. A plaining hull. Lets give him the correct information so he can make the right information for his set up. Again a planning hull not displacement usage, not commercial, not 24/7 duty, not a truck, not electronic engines. Lets answer his specific question and try to give him solid recommendations.

For mechanical engines you set the load of the engine by using WOT. If you are under wide open throttle like the manufacture says. It voids all warranties. You ARE operating the engine outside the specification of the manufacture. They spend MILLIONS of dollars setting up the specifications . Cummings will void the warrantee if not set up correctly. They think it is important.

Yes you are lugging the engine. The engines are designed for a certain rpm to have a certain fuel flow, cooling flow, boost pressure, oil flow, air intake to make horsepower at a certain rpm and keep the engine withing specs. More fuel at lower rpms increases heat and increase ware. That is why it voids the warrantee. If you run slow all the time then no you will not hurt the engine, but no one does that in a planning hull setup.

Smoke especially black is improper air to fuel mixture. Running the engine to rich. Loading the engine too much. Since it is mechanical yes again you are lugging the engine through all power ranges. Too much fuel for the air supply at any given rpm. Poor air to fuel mixture reduces fuel efficiency. So give the girl a tune up and re pitch that prop.



Real world account no 1
My last boat 42' 450c engines mechanicals I repower with running take outs modified v hull. . First set up it was over propped. 120 under WOT . Sluggish to climb onto plane. High heat on turbs especially the hump and about 1800rpm. Normal cruse speed. I set it up + 100 ( increased 220 rpm ) like the specifications said. It made the turbo match the hump and the boat at 33k jumped out of the water. The black smoke at the hump is gone. The subtle smoke in the distance at cruse was gone. Turbo temp dropped across the board. picked up 10% fuel efficiency. Sold the boat with over 5k hours and a high proportion of those hours at speed. 21-22 rpm the sweet spot for that set up.