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Old 02-04-2019, 09:31 AM   #21
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Rufus:
That's what I thought as well until Tony Athens told me that a derated engine has a lower maxpower curve than the high HP version max power curve for the same engine because of air flow cooling and other reasons.
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Old 02-04-2019, 10:42 AM   #22
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Good thread.

One comment. We all know diesels like to be loaded. Running the engines at high load periodically gets discussed a lot here, and other forums. For the newer common rail engines it’s not a factor. This is not my opinion, but comes from people who have discussed directly with the manufacturer. I believe someone posted their email exchange on TF with Cummins on this subject about 2 years ago. I am not saying idling is good, but running at a lower rpm on common rail will not cause the same adverse affects. With that said, many people I know who have newer diesels, myself included, do run them up to the high end every couple hours as a best practice because it can’t hurt.
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Old 02-04-2019, 11:31 AM   #23
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Originally Posted by menzies View Post
Final (I think question) for you engine nuts. I have been told that it is healthier to kick an engine onto WOT for a while to "remove carbon deposits" that may have built up when cruising for long times at low RPMs. We spend three months cruising the Bahamas where we very rarely go above the mentioned 1400-1500 RPMs. Should we really kick these engines up for a half hour or so every so often to keep them clear of deposits?

I don't think it is necessary to go to wot for 30 minutes. Tony Athens in the years before common rail recommended running a high output diesel up to 60% load which is typically 80% of wot rpm for about 20 minutes at the end of a long day at low speed to blow out accumulated carbon.


As noted above, I think that the advent of common rail has eliminated this requirement as it provides much more precise fuel injection.


Also I think my Yanmar 370 needed it because I often ran it for long days with only 15% load. Your Lugger is probably routinely loaded much higher which means the fuel injectors are being operated closer to maximum than mine, so it burns cleaner and produces less unburned fuel and soot.


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Old 02-08-2019, 01:57 PM   #24
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Curves for Cat 3610/3208

Anybody have a good chart for the old Cats, normally aspirated 3610 or 3208?
FYIW, it's in a '73 GB 36 Classic.

New owners, first post.
Thanks in advance,
Ted
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Old 02-08-2019, 04:02 PM   #25
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My Cummins 6BTA Medium Continuous Duty diesel engine manual says not to idle the engine for more than 10 minutes to prevent unburned fuel from washing the oil off the cylinder walls and scoring the walls. Putting even a light load on the engine negates this problem. Also, "Medium Continuous Rating" is defined (in the manual) as "This power rating is intended for continuous use in variable load applications where full power is limited to six (6) hours out of every twelve (12) hours of operation. Also, reduced power operations must be at or below 200 RPM of the maximum rated RPM." In my case, max RPM's are 2,500 meaning I could theoretically run the engine at 2,300 or lower RPM's continuously. We run at about 1,500 to 1,600 RPM's continuous, with an occasional run-up to maybe 2,000 rpm if we're running to catch a bridge or something.
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Old 02-08-2019, 04:29 PM   #26
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Originally Posted by TTrep View Post
Anybody have a good chart for the old Cats, normally aspirated 3610 or 3208?
FYIW, it's in a '73 GB 36 Classic.

New owners, first post.
Thanks in advance,
Ted
This may help. If I loaded things right....
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File Type: pdf 3208 na perf curve.pdf (81.9 KB, 27 views)
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Old 02-08-2019, 04:36 PM   #27
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I was confused by the notes on that chart, which say it's for displacement hull shapes...
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Old 02-08-2019, 04:50 PM   #28
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I was confused by the notes on that chart, which say it's for displacement hull shapes...
...and also says E rating for planing hulls.
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Old 02-08-2019, 05:14 PM   #29
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Can someone who knows these things explain the meaning and difference in rated crankshaft fuel consumption and propeller shaft fuel consumption? See below.

Rated Crankshaft Fuel Consumption: This is the measured fuel burn when the engine is making it's maximum >>rated<< horsepower. These numbers are found by putting the engine on a dynomometer and loading it up until just before it starts smoking (e.g. when it can no longer burn all the fuel it is sucking).


Propeller Shaft Fuel Consumption: This is a theoretical (mathmatical) estimate of what the engine will burn if it were connected to a fictional propeller load in a full-displacement hull running at slow speeds.



There are many misconceptions about what these "propeller" curves are good for. The best expert analysis and 'de-bunking' of the misconceptions I have found is by Don MacPherson of HydroComp, here:


https://www.boatdesign.net/threads/p...er-curve.9726/


Scroll down to the fourth post...


Highlights:


"The prop curve is a simplistic representation of what the power-RPM curve might look like if the propeller were sized exactly for rated power and RPM with no margins. In other words, the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats.

It is a reasonably close approximation for slow speed displacement hulls with conventional propellers. However, it is completely unsuitable if you have:

a. semi-displacment boats, planing hulls or catamarans
b. boats under high thrust conditions, such as tugs or trawlers
c. dynamic loading such as acceleration
d. a propeller sized with a power margin (e.g., 85% MCR)
e. waterjets, surface drives, or highly-cavitating propellers
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Old 02-08-2019, 05:50 PM   #30
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Continuous duty is about output, not RPM

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Originally Posted by menzies View Post
Thanks, another question.

Continuous Duty Rating .........FWHP (kW)/rpm 140 (104)/2200

I am surprised that the continuous duty is 2200rpm out of 2400, or 91.6%. It seems very high to me. Is there a design reason for this? Or am I not understanding it correctly?
Yes, this is a (very common) misunderstanding.


Don't think in terms of "percentage of RPM". Look at the horsepower rating. Your maximum engine rating is 174hp, and your continuous duty rating is 140hp. That's 80.5%, which is very typical for this class of engine.


One of the biggest 'conceptual humps' to get over when working with diesel engines is the idea that there is a hard relationship between RPM and horsepower, or RPM and Fuel Burn. Neither is true. You have to add torque into the equation.



The best way to break yourself of this is to always remember that (a) engines always burn fuel in proportion to the horsepower they are making, and (b) that horsepower is always Torque x RPM.


So...let's exercise this concept. Take your engine and spin it up to 2,200 rpm with the transmission in neutral. Because there is no external load on the engine, the engine is producing very little torque (near zero) and therefore is producing almost no horsepower. At 2,200rpm in neutral, you would find that you are burning very little fuel, almost the same as if you were at idle. It takes very little energy just to spin the crankshaft...



Now, let's put your engine in gear while tied securely in your slip. Spin the engine up to 1,100RPM...now you are torquing the engine against the prop, producing something like 40hp and burning something like 2-3GPH. You have cut RPM's in half but you are burning 10x the fuel!


Hope this helps...
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Old 02-08-2019, 06:03 PM   #31
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Originally Posted by djmarchand View Post
I don't think it is necessary to go to wot for 30 minutes. Tony Athens in the years before common rail recommended running a high output diesel up to 60% load which is typically 80% of wot rpm for about 20 minutes at the end of a long day at low speed to blow out accumulated carbon.


As noted above, I think that the advent of common rail has eliminated this requirement as it provides much more precise fuel injection.


Also I think my Yanmar 370 needed it because I often ran it for long days with only 15% load. Your Lugger is probably routinely loaded much higher which means the fuel injectors are being operated closer to maximum than mine, so it burns cleaner and produces less unburned fuel and soot.


David

Exactly right. I typically run both my single Yamnar 6LYA and my (twin) Cummins 6BTAs very gently, 1200-1400 rpm for long periods. Once every 4 hours of so, I spin them up to max rpm for 30 seconds, then back down to 2,400 or so for 5-10 minutes. I've seen no need to run at WOT for more than a few seconds, and no need to run at 60% loads for more than 10 minutes.


FYI, Tony still recommends spinning up the newer common-rail engines, but not as much or as long.


https://www.sbmar.com/community/topic/running-at-wot/
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Old 02-08-2019, 06:04 PM   #32
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[U][B]

"The prop curve is a simplistic representation of what the power-RPM curve might look like if the propeller were sized exactly for rated power and RPM with no margins. In other words, the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats.

It is a reasonably close approximation for slow speed displacement hulls with conventional propellers. However, it is completely unsuitable if you have:

a. semi-displacment boats, planing hulls or catamarans
b. boats under high thrust conditions, such as tugs or trawlers
c. dynamic loading such as acceleration
d. a propeller sized with a power margin (e.g., 85% MCR)
e. waterjets, surface drives, or highly-cavitating propellers

It's my understanding that a different exponent is used in the calculation for boats that are designed to get beyond hull speed. So the shape of the curve is different at higher speeds. Caterpillar used 3.0 for the above chart. Cummins uses 2.7 and the curve bends upward more steeply at higher speeds. Here's the Cummins 6bt chart. cummins 6bta.pdf
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Old 02-08-2019, 06:06 PM   #33
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This may help. If I loaded things right....
Thanks for the post. Appreciate the quick response.
Ted
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Old 02-08-2019, 06:36 PM   #34
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It's my understanding that a different exponent is used in the calculation for boats that are designed to get beyond hull speed. So the shape of the curve is different at higher speeds. Caterpillar used 3.0 for the above chart. Cummins uses 2.7 and the curve bends upward more steeply at higher speeds. Here's the Cummins 6bt chart. Attachment 85070

No, that's not how I read it. The 'cubic demand curve' is a simple exponential curve that does not work for any hull except a displacement hull. The exponent used was typically 3.0 (hence a "cubic" demand curve), but changing the exponent a little does not solve the problem. Propeller demand becomes a messy and complicated thing as soon as you get away from a pure displacement hull, so MacPherson's whole point is that a simple exponential curve is useless (whether the exponent is 3.0 or 2.7), outside the neat and tidy world of a full-displacement hull. The longer story is in MacPherson's white paper here:



https://hydrocompinc.com/wp-content/...hECEngines.pdf
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Old 02-08-2019, 07:30 PM   #35
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I didn't read any of the links, but I know the prop chart for our semi-planing boat (which is a 2.7 exponent calculation) is shaped like the Cummins curve and tracks real world fuel burn very closely.
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Old 02-08-2019, 07:42 PM   #36
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Tractor engines and engines designed for work versus speed are designed for high torque at lower HP. The torque curve desired is one that hits the torque curve fast and that curve is somewhat flat at the top.

Meaning the torque comes on very soon off idle and stays there as RPM is increased.

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Kind of like this....
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Old 02-08-2019, 08:04 PM   #37
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Yeah that torque curve isn't. It is pretty flat across the 700 ft lb line for the engine RPMs shown. Exactly what one wants in doing work or moving a load.

Notice the torque drops off a little at the 1900 RPM line. Keeping the RPM around 1700 works best for this particular engine.
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Old 02-08-2019, 08:19 PM   #38
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All interesting discussions about engines and ratings. I did a little research and found that at the typical engine in trawler usage like mine might have a "recreational" rating. The manufacturer says that they "allow" it to be run at full rated power for something like 1 hour out of 6. There is usually no guidance as to why they only allow it to be run for the 1 hour. I would bet that it is temperature related, probably oil. They allow it to be operated continuously at 200 rpm below rated rpm, and that would be at full load. Driving a propeller, the load would be significantly less than full load with a 200 rpm reduction. Bottom line is that if one is worried about harming the engine at any normal cruise rpm, don't. The engine is operating at a speed and load far below any possible maximum. And engines don't have a memory - whether it runs continuously at a cruise setting for 2 hours or 40, 800 rpm or 1500, it doesn't care.
I have a question, though. Some say that the engine should be run at high load for a few minutes every few hours. Of what benefit is this? Some say that it "burns off" any carbon buildup within the engine, and I suppose this could be true. However, I wonder if the visible exhaust carbon isn't simply the soot buildup in the muffler that isn't blown out at low engine speed. The white "smoke" then is simply the condensation of the water in the exhaust. After a minute or two the exhaust system heats up and the condensate disappears. I have never "blown the carbon out" and I don't know if there is a need to do that or not. Any comments?
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Old 02-08-2019, 08:20 PM   #39
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Originally Posted by menzies View Post
Final (I think question) for you engine nuts. I have been told that it is healthier to kick an engine onto WOT for a while to "remove carbon deposits" that may have built up when cruising for long times at low RPMs. We spend three months cruising the Bahamas where we very rarely go above the mentioned 1400-1500 RPMs. Should we really kick these engines up for a half hour or so every so often to keep them clear of deposits?
Quick answer, NO. Run it a hull speed for greatest fuel efficiency .......
I have a AT34 with a Cummins 380 electronic. I run it at hull speed, the fuel efficiency is good and sort of follows the builder provided gph. The reserve power and speed..... that's reserved for unusual events such as getting to a port at a particular time or perhaps staying ahead of the weather.
The speed (RPM) vs GPH is created when the boat is running on flat water, has 1/2 of fuel, 1/2 of the water and the fridge running.

I took my AT 34 to the yard.... speed vs RPM and GPH was terrible ..... then, I passed under a relatively narrow bridge. I could actually see the very small waves (3-4 inches approx.) created by an incoming tide, traveling into the wind. The narrowness of the bridge accented the tide and wind effect. I went to the yard for an insurance haul out. Sort of 'out and in' the water, never leaving the slings. Coming back, the GPH was pretty close to the builder provided chart making allowances for a boat filled with all the stuff associated with being a 2 person live aboard 3/4 fuel and full water tank. (300 gal of fuel remaining and 150 gal of water)
My point is, builder supplied chart, the speed vs RPM should be viewed as a suggestion. The best chart/graph is the one you create.
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Old 02-09-2019, 07:05 AM   #40
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"Run it a hull speed for greatest fuel efficiency ."

Hull speed is NEVER the greatest speed for fuel efficiency.

It is the point where the bow wave is so large the boat can not go faster , or climb up on top.

To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".

Hull speed 1.34 is mostly of use for displacement wind sailors with free propulsion to understand when the boat is being pressed enough to consider sail addition as unproductive and perhaps dangerous.
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