Prop-pitch

As it relates to a diesel engine?

Eric wrote-- Did that Deere/Lugger suffer?. If not I'll throw in with ya. No glazed cylinder walls, smokin hard starting engine? Ya sure ? I'll quit ragi'n on you guys about the underloading issue.

Eric---

Last week I asked my contact at Northern Lights/Lugger how the engine in the Nordhavn 40 held up in its low-power cruise around the world. Northern Lights/Lugger was one of the sponsors of that trip (they provided the fuel) so they kept a good track of what was going on. The engine, he said, did just fine. No problems due to underloading. No smoke, no hard starting, etc. He's going down to Southern California soon and he plans to check on the current status of the boat, but in terms of the round-the-world cruise the engine experienced no problems as a result of being run at low power settings.

Marin, Chris and others,
Thanks for putting up with me all this time. The green light for UL dosn't extend to .. say .. a turbo Cummins like in a Nordic tug at 7 knots ?. One could use extra detergent in thier oil or/and remove or bypass thier oil cooler but what leads to cylinder glazing?. I feel very comfortable in my situation but I'm sure there are people out there that are vulnerable to this problem. There was a discussion on another site about about derating DD to cruise large yachts very slowly. Most are going slow and have never heard of under loading.

Eric Henning
Willard 30
Thorne Bay AK

Eric---

This is by no means an area of knowledge for me. We're just operating our 35 year old boat the way it's apparently always been operated, and the way we were advised to operate it by our diesel shop and friends in the marine diesel industry. From what I gather from the folks with older GBs on the Grand Banks owner site, pretty much everyone with the kind of engines we have operates them within the same power range--- 1500 to 1800 rpm. Many of these boats are woodies even older than ours and have their original engines. This may be underloading the engines, but these particular engines seen to run forever when operated in this power range. I would not suggest, however, that what works for a 1950s design Ford Lehman 120 will work for some other type of engine.

Nordic Tug markets their boats as being able to cruise economically at seven or eight knots but cruise fast when necessary at twelve or fourteen or whatever knots. Hence their term "fast trawler." We have friends with a brand new Nordic Tug, albeit with a Volvo diesel as they don't use the Cummins in this model (34') anymore. But so far as I know they cruise it at a very economical 8 knots or thereabouts and have not been warned by the factory not to do this. Now it may be that they are supposed to run it harder for x-amount of time periodically. I'll try to remember to ask them next time I see them.

Also, I neglected to mention that my friend at Northern Lights/Lugger told me the engine in the Nordhavn 40 that did the world cruise had been properly broken in prior to embarking on their long, low-power cruise. I don't know if that makes any difference but it may be a factor.

-- Edited by Marin at 00:20, 2008-10-14

" but what leads to cylinder glazing?."

The cylinder seals by the combustion gases getting behind the piston rings and forcing then against the cylinder wall to create the seal.

With very low loads the pressure is too low , so blowby and burnished (lightly polished) cylinder walls result, instead of a good seal.

The burnishing will smooth out the honed condition of the cylinder walls , so the engine will wear faster and burn oil more rapidly.

With square cut rings the seal is better at low loads than the more modern trapizoidal.

The Deere that went round may be suffering this problem , no one has disassembeled it to find out.

Also IF the unit has electric injection the computer does change the timing and squirt size to load the engine (advance the timing) to work a bit harder.

For the Detroit Diesels the 'rating' is changed by the size of the injectors. An 8v92 could be 400 hp or it could be 425 or 350 by the amount of fuel squirted in each time. Different injectors are available depending on how much power you want to produce.

NASCAR uses restrictor plates to limit the amount of air that the gasoline engine can get. I would bet that the airplanes have a similar device if they are derating an engine. Remember that gasoline engines regulate speed by regulating air, diesels by regulating fuel.

The number of and size of explosions in an engine determine it's longevity. While designed to withstand such explosions there is a limit to how many it can stand before it wears out. Take a tennis shoe and drag it across the asphalt. Some wear, but not much. Now put a bowling ball on it and do the same drag. More wear? Bigger explosion in the engine means more pressure on the top of the connecting rod bearings, wrist pin, piston, rings are expanded against the cylinder wall more forcefully, etc.

Modern electronic control engines make things much easier. Fuel can be regulated, timing can be adjusted, lifter openings changed, all by the computer. This is why there is an aftermarket for hop-up chips for your diesel pickup, or your Monte Carlo. You can change the parameters to get more or less of something, usually at some cost somewhere.

30 years ago we put dual quads on our V8's to make them faster. Dual quads allowed us to get more air into the engine and dump way more fuel than the engine actually needed. Mom's car might have the same engine with a 2 barrel carb. She wasn't as fast. She also didn't have to have a valve job as often. Her engine didn't get as much blowby in her crankcase either. So, her oil stayed cleaner longer because the explosion in each cylinder was smaller. Her maintenance schedule reflected the way she drove. My maintenance schedule was dependant on how much money I had. If all things were equal though, I would have changed oil much more often than Mom because I polluted my oil faster.

Ken

Vinny wrote:
Just changing*the time or interval for*valve adjustments, oil change, filter change etc... and then*stating that the engine is now rated at 250hp*@ 3,400 with a TBO of 2,500 hrs*instead of 285 hp @ 3600 with a TBO of 2,000 hrs doesn't make it so.* Something in the engine has to change to slow that puppy down, like valve timing, valve overlap, exhaust restriction something.

Click to expand...

I believe in the case of the PT6A-135 turbine I was talking about earlier, the only things that were changed to derate this 750 shp engine to 600 shp were the power settings used, N1, N2, etc.* (I think that's the correct term for the shaft rpms in a turbine--- I haven't flown one for a long time).

Vinny wrote: Marin

So then in affect the change in*the power settings is like*putting a governor in place thus chopping 150shp.* Now the maintenance schedule can be changed due to the lower RPM's.

Click to expand...

I guess.* The derated version of the engine may use a different prop than the fully rated version as well.

Yep, I think we're all on the same page, just saying the same thing different ways.

Ken, I,m not on the over loading over proping page .. not at all. If you have a Ford engine of 120 hp at 2500 rpm and you over prop it 200 rpm at 2300 your'e obviously very over proped at 2300. But going down the engine speed scale where are you NOT overloaded. You don't know. Nobody knows. At what rpm can you operate continiously and get the maximum power out of your engine? You don't know. Maybe 2000 but I would'nt feel comfortable running that engine at 22 or 2100 rpm. The engine would have a max power of about 85 hp .. maby 90 .. but maybe only 80. If you know you'll only need 80 hp then the correct place to be is with a 90 hp engine, then you can prop the engine correctly, as Marin stated long ago on this thread, and operate the engine at the correct load and engine speed. On a full displacement hull over proping works much much better as the boat resistance drops very fast as one,s speed falls below hull speed. This means that if you slow down even one knot the power required is so much less that the engine experiences a large drop in load that can be overcome by over proping but again one looses the ability to run the engine near max power. If one only needs 60 % of the power in thier boat and they can't afford to repower with lower power engines, then over proping becomes a possible half assed solution. Am I on the same page with you Ken ? I guess so since I've said if your'e willing to suffer the consequences and/or the side effects one can do anything one wants.

Eric Henning

Vinny wrote:
Marin wrote: I believe in the case of the PT6A-135 turbine I was talking about earlier, the only things that were changed to derate this 750 shp engine to 600 shp were the power settings used, N1, N2, etc.* (I think that's the correct term for the shaft rpms in a turbine--- I haven't flown one for a long time).

Click to expand...

Marin

So then in affect the change in*the power settings is like*putting a governor in place thus chopping 150shp.* Now the maintenance schedule can be changed due to the lower RPM's.

Ken,
That was what I was talking about.* Three different injectors three different*HP ratings.* But something mechanical or electrical (new engines) has to be changed.

Thanks guys, I didn't see that in Johns post but I felt that was the case.


Vinny, in many aircraft there is a guage that measures the "work" or load of an engine.* In recips with variable pitch propellors, that would be the manifold pressure guage.* In turboprops, that would be the torque guage(with respect to temps). In pure jet, it would be %RPM(again with respect to temps).* In most of these, you can "overboost" an engine simply by advancing the power lever.* What determines "overboosting" is what is in the manual and/or placarded on the guage.* SO the FAA may say your maintenance interval is "this" if you run the aircraft in this manner.* There are operators of the same exact airframe and powerplant that have maintenance plans and intervals that are totally different.* ANyway, I am oversimplifying it a bit.* In a turbine engine, generally they are capable of doing A LOT more work than they are rated for and many times some of that power can be recognized/recovered with paperwork.

How does this apply to diesels and boats.* In reality, it doesn't really.* In theory a simple placard(colored arcs...ala "red line") at 3200RPMs on a 3600RPM motor....or maybe a restriction that you are allowed to operate from 3200-3600 for no more than one hour.....just things like that.* Pleasure boats are different in that once you own it, it is the owner that is responsible for maintaining it or not.* IN aircraft, you are required by law to maintain it.

My boat's max continuous rating is 190hp@3100rpms*with an alllowance to run at 240hp@3300rpms for no more than an hour.* So do I have a 190hp engine uprated to 240 with restrictions or do I have a 240hp engine derated to 190hp???* The manufacturer expects me to adhere to those numbers and produces maintaenance intervals based on those numbers.* My boat is also underpropped and it will make close to 3600RPMs when everything is clean.* SO most likely, it is making even more power at that RPM even though Yanmar does not recognize that(fuel cutoff is at 3800) nor would they "support" that type of operation.

Aircraft operators have a very close relationship with the FAA and the manufacturer of the plane.* Many times, all it takes is trends/history to apply to the FAA and ask for a longer interval or more power and if the operator has a good case backed up by good engineering, then it may be granted.* And yes, these days it may require plugging a laptop into the engine management computer to tell it to allow more fuel,etc.* In the old days, if you firewalled the power levers in a jet it would give you WAY more than rated power and this is where the term "overboost" comes from.* Newer computer controlled engines do not allow this.


-- Edited by Baker at 22:41, 2008-10-14

It's more than simply changing the governor limit, it's changing the entire fuel controller schedule.* Take for example, the venerable 3208T Cat.* It's rated everywhere from 320 hp to 215 hp.* 320, 300, and 290 hp versions are available at a 2800 rpm redline, and 255 and 215 hp at a 2400 redline.* The differences in output at a given redline are torque differences, and are strictly related to fuel flow (compression ratio, etc are identical for all of these engines).

But this is where the 75/75 business gets so silly.* The same base engine block goes from 435 to 150 hp when you go from lightest duty / turbocharged / aftercooled to heaviest duty / naturally aspirated (OK minor error, the 435 hp version has 15:1 compression, all others 16:1).* So somehow this block is going to be reduced to a glazed-over pile of cast iron if its 435 hp version is run at less than 300 hp, yet the same hardware will be fine if the 150 hp version is run at 120 hp???

Like I said, cough up the couple of bucks and peruse the archives at boatdiesel.* You'll find lots of articles about burned pistons, blown head gaskets, cracked manifolds, seized turbos, etc on boats with big engines run hard... but most of the concerns of the "tractor engine" people are oil leaks and the like (and a LOT of the "tractor engines" have English heritage, so they're SUPPOSED to leak oil).* I can't actually recall reading anything there about an engine needing work because of not being run hard enough -- though I'm sure there are some.

" But going down the engine speed scale where are you NOT overloaded. You don't know. Nobody knows. At what rpm can you operate continiously and get the maximum power out of your engine? You don't know."

A look at most engine operation graphs will show that 10% in rpm or 300 rpm gets virtually every engine in a safe load area.

Folks that want to reaLLY know , CAN SIMPLY SPEND THAT $100 FOR AN EGT GAGE.

What could be simpler . or more sure?

So what am I looking for if I do have an EGT Guage??? I know how it works with airplanes but you have more control over the combustion process.

John--- Our boat has EGT gauges but they aren't as dynamically useful as the same gauges on a plane. Unlike an aircraft, where you use the EGT to set the best mixture, the EGT's on a diesel boat simply tell you how hard the engines are working. And, like the other temperature and pressure gauges, they are one more thing that tells you if all is "normal" or if a problem might be developing.

But when we had our props repitched down, I noticed that at the same cruise rpm the EGT readings were a tad lower, meaining the engines weren't working quite as hard as they were before. The result has been a decrease in fuel usage at the same rpm.

"Ken, I,m not on the over loading over proping page .. not at all."

Sorry Eric, I meant to say on the derating issue, not overpropping. I don't think people will ever agree on overpropping. Those who do it swear by it and those who don't swear at it. Me? I don't care, I prop my boat for what the book says is max RPM under load and run it easy even at that. Lehman 120 runs real nice at 1750.

Ken

Hi Ken,
Thats about exactly where I run my new NA engine ( 2300 .. down 700 rpm from rated max ). I'm quite sure wer'e not at 75% .. but in the vicinity. I think those that strongly support over proping are trying to convince someone of somthing .. mostly themselves. They know it's wrong but if one hears enough supportive talk it seems right. Is that dog better than an autopilot ?

Eric Henning

I would agree with Ken. I think overpropping is one of those issues for which there are so many variables that there is no one-size-fits-all right or wrong answer. American Marine often if not always SLIGHTLY overpropped their Grand Banks boats in the 1960s and 1970s. I do not know if this practice continues today. But back then, the nature of the engines they were using permitted a small degree of overpropping which gave the boats a bit more speed for a given cruise rpm.

The price of fuel was negligible back then--- the reason for the overpropping was to give the boats a tad more speed at the relatively low power settings that were common on diesel engines of this type at the time. Don't forget, back then 120 hp was considered a fair amount for a 36 or 42 foot boat. The days of stuffing a pair of 400-plus hp engines in a GB42 were long in the future.

BUT...... these were "thumper" engines that could run for years and years at 1500 rpm, sometimes under relatively light loads powering pumps and cranes and generators and other industrial equipment with no damage. I was told by Bob Smith at American Diesel that the Washington State ferry system had some boats years ago with Ford Lehman 120s on them. Not as propulsion engines of course--- they powered generators or hydraulic pumps or something. I was told they ran at relatively low rpm with a relatively light load. Some of these FL120s--- which ran continuously when the boats were in service and received prompt service and maintenance--- went more than 25,000 hours before needing a major overhaul.

Fast forward to today and we have a whole different philosophy of engine design. Lighter weight, high-rpm, turbocharged, aftercooled, etc., etc. etc. Underloading these engines--- as I understand it--- can have pretty serious consequences. So I think overpropping--- at least to a degree--- makes sense in certain boats with certain engines operated in a certain way. But as a universal practice, I would agree with Eric that it should not be, and in fact can make no sense or be even detrimental in other boats with other engines operated on other ways.

And, while not related to overpropping but just to show there is no hard and fast rule about anything, a good friend of mine currently runs a small fleet of longline tuna boats out of Honolulu. These 70' boats go out for two or three months at a time, come back, unload, undergo whatever maintenance might be needed, and go back out again. Each of his boats is powered with a single, off-the-shelf Volvo turbocharged, aftercooled marine diesel. Each boat is also equipped with a Northern Lights generator that powers the refrigeration system. My friend's standing order to his boat crews with regards to the main engines is to run them at 1500 rpm. No more, no less. These boats are constantly on the move and the engines are never shut off the entire time the boat is at sea. And my friend told me when I visited his facility a few years ago that if he gets any less than 35,000 hours out of each of these Volvos before it needs an overhaul he gets REALLY pissed off. And so far, he said, he's rarely been pissed off, at least not at the engines.



-- Edited by Marin at 22:40, 2008-10-15

Your pal speced rational engines for the contemplated loads.

Today its the Add dept that specks what they think will match the competition in print adds.

Rationality , longevity, fuel burn or reality are NOT part of the equation.

Is that dog better than an autopilot ?

I carefully cropped out the autopilot boxes from the picture. The dog would have us taking a close look at every bird around. She's not even a bird dog, must be the movement she likes.