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Old 10-14-2008, 09:35 PM   #41
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RE: Prop-pitch

Quote:
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).

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
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Old 10-14-2008, 09:42 PM   #42
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RE: Prop-pitch

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.
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Old 10-15-2008, 04:38 AM   #43
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RE: Prop-pitch

" 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?
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Old 10-15-2008, 09:54 AM   #44
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RE: Prop-pitch

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.
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Old 10-15-2008, 10:30 AM   #45
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RE: Prop-pitch

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.
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Old 10-15-2008, 10:30 AM   #46
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RE: Prop-pitch

"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
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Old 10-15-2008, 06:36 PM   #47
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RE: Prop-pitch

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
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Old 10-15-2008, 09:34 PM   #48
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RE: Prop-pitch

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
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Old 10-16-2008, 04:59 AM   #49
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RE: Prop-pitch

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.
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Old 10-16-2008, 09:53 AM   #50
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RE: Prop-pitch

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.
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