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Old 02-03-2019, 11:25 AM   #1
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Fuel Usage Curves

Can someone who knows these things explain the meaning and difference in rated crankshaft fuel consumption and propeller shaft fuel consumption? See below.
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Old 02-03-2019, 11:55 AM   #2
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Your engine is capable of producing the power depicted on the rated power curve. But the propeller only demands a portion of that power until it reaches maximum power...power demand is also referred to as power required. The propeller should be sized so that it is demanding maximum power at the maximum rpm for the engine. With affixed pitch prop, this always keeps the engine and drive train within loading limits. As you can see on the chart there will be a gap between engine power available and propeller power required except where they both meet at the maximum. The gap (excess power available) is what allows the engine to accelerate if you move the power (rpm) lever forward. Beyond maximum propped rpm it will likely start smoking (lugging with a mechanical fuel control), or reach a limit set by an electronic fuel control if so equipped. The engine rated power is measured on a test stand by the manufacturer. The prop chart is typically calculated. There are normally different prop charts associated with the type of hull (displacement, semi-displacement and planing) and the shape will be different for each. I believe engines with ECUs probably have a range of prop charts supplied by the engine manufacturer to fit to different manufacturer's models within a similar hull model line....maybe they have a 57, 59, and 60 range of displacement hulls. But they are still calculated unless the boat builder gathers more precise hull speed/propeller loading data during development testing of a new model. In that instance the hull and drive line would have to be instrumented (torque, for example). Engine/propeller/drive train combinations are gathered for larger aircraft at great expenditure of time and money. I doubt recreational boat builders go to such extremes. The actual fuel burn for the boat/engine/propeller combination should be similar to the prop chart (power required/demand. The prop chart for our boat is very well matched to real world consumption.
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Old 02-03-2019, 12:00 PM   #3
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prop shaft fuel curve is an estimate what you will actually use.

The crank power is often called the max power curve and is measured on a dynamometer in the lab. It is the power produced when the engine is loaded to the point it begins to slow down.

Notice that they converge at the top. that assumes proper prop sizing. If your engine wont reach max RPM then it is overloaded and the curve doesn't apply.
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Old 02-03-2019, 12:57 PM   #4
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If your engine wont reach max RPM then it is overloaded and the curve doesn't apply.
It's overloaded in the vicinity of the rpm where it starts smoking....but it's probably not overloaded at lower rpm/power settings. It will be operating in that gap area between the two curves, and the operator won't know exactly where the over loading begins. Essentially it would be on a new prop curve where the two curves meet at a lower rpm. The problem is that without a new curve or additional instrumentation such as torque or perhaps exhaust gas temperature (EGT) you won't know where the engine enters the danger/damage zone. There are numerous threads in the archives about over propping. There can be longevity advantages to over-propping, especially for large slobbering engines in semi-planing hulls where owners never exceed half of rated power, for example....but you have to know where the damage begins, establish redlines, mark the gauges, and respect the new limits. Engines in aircraft are derated regularly...but the operators are trained and aware that consequences of exceeding a torque limit, for example could be deadly....used in extremis only.
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Old 02-03-2019, 05:11 PM   #5
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Thanks, another question.

Continuous Duty - if I take this at face value it means the RPM which is healthy for the engine if you were running over multiple days, say to Bermuda.

The engines documented WOT is 2400, and the continuous duty is documented as:

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?

Is it because these are based on tractor engines, with very low torque?
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Old 02-03-2019, 05:23 PM   #6
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Is it because these are based on tractor engines, with very low torque?
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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Old 02-03-2019, 05:28 PM   #7
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That Lugger engine is a very low stressed engine even at the High rating (only 25hp/liter). So it does not need to be de-rated a lot for continuous rating (20hp/liter).
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Old 02-03-2019, 05:46 PM   #8
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OK, I am going to admit that you both lost me here. Can you bring it back to a bit more of a laymen's knowledge?

Here is the reason behind my question. When running a long day - say 10 hours, I keep her around 1750. All things being equal that pushes me along at 8 knots. When we arrive in the islands we are only running a few miles each day so I trundle along at 1400-1500, around 6 knots. I find good fuel burn at this level.

Is this healthy for this engine?
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Old 02-03-2019, 06:10 PM   #9
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The Lugger marine engines were based on John Deere engines and are no longer made by Lugger. It might be more instructive to look at John Deere's website for advice on continuous duty engines. I believe that the Lugger LP668T was based on what is now the John Deere 6068T.

That engine comes in three duty ratings: M1, M2, and M3. M1 is rated at 158 hp at 2,400 rpm close to yours. JD says for that rating: load factor >65%, annual useage hours- unrestricted, Typical full power operation: uninterrupted. In other words continuous duty.

The M3 engine is rated at 201 hp at 2,600 rpm and has a typical load factor of <50% and full power operation is allowed in 4 out of 12 hours, in other words a 33% duty cycle which is typical of recreational engines.

I will bet there is little difference among the three models other than ECM settings to limit fueling and rpms. The M1 model is just derated in wot rpm and power production to be able to run continuously.

Your engine is a non common rail design and if it is the continuous duty model which is analogous to M1 then it should be able to run at wot forever.

Here is the link to the JD marine engine lineup. Click on the first 6068 engine in the Data Sheet box for the above data. https://www.deere.com/en/marine-engines/

FWIW the major marine engine manufacturer's all have different approaches to the continuous operation definition. JD is one of the most honest about it, Yanmar not so much.

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Old 02-03-2019, 06:19 PM   #10
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Simple answer is yes.

More complex answer -
In the above graphs torque is not shown. If it were one could state the most efficient way to run the engine. However that may be a theory because these types of curves are usually general to an engine. Each engine installation with different running gear (Transmission & prop) would yield different results. However I digress.

Running an engine in a healthy manner is a Goldilocks thing. Not so hard that it is always under extreme load. (So at the very top end of WOT) And not so light that it is running with almost no load at idle or just off of idle. Especially for a diesel.

Assuming the charts above apply to your application you are running between 1400 and 1750, in the middle of the graph. A very healthy range for the engine.

And know that there is a most efficient range as well. Typically that window is narrower. Many will call this the sweet spot. And many times you can feel or sense it if you are tuned into the engine. A tachometer, flowscan gauge and gps can all be used to find it as well.
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Old 02-03-2019, 07:38 PM   #11
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OK, I am going to admit that you both lost me here. Can you bring it back to a bit more of a laymen's knowledge?

Here is the reason behind my question. When running a long day - say 10 hours, I keep her around 1750. All things being equal that pushes me along at 8 knots. When we arrive in the islands we are only running a few miles each day so I trundle along at 1400-1500, around 6 knots. I find good fuel burn at this level.

Is this healthy for this engine?
Looks like 1400 rpm requires around 50 engine HP (on the continuous curve), and 1750 about 70 HP. Seems reasonable. Helluva a lot better than the slobbering twin 375T's running around at hull speed in many mid 40' size SD boats. We have twin 250s and they're still too big. The engines in your boat would be perfect for getting to 13 KTs on occasion, and poking along at 8KTs the rest of the time.
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Old 02-03-2019, 08:22 PM   #12
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Looks like 1400 rpm requires around 50 engine HP (on the continuous curve), and 1750 about 70 HP. Seems reasonable. Helluva a lot better than the slobbering twin 375T's running around at hull speed in many mid 40' size SD boats. We have twin 250s and they're still too big. The engines in your boat would be perfect for getting to 13 KTs on occasion, and poking along at 8KTs the rest of the time.
Ok the last few posts were really helpful, thanks.

Rufus, to your last point. There is no way I feel I could get to 13kts on this boat. The only way I can get over 10 is if I have a following current and wind!
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Old 02-03-2019, 08:33 PM   #13
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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?
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Old 02-03-2019, 08:45 PM   #14
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Only necessary if you are idling for a long time (e.g. trolling for hours). Even then 30 min of high cruise (not WOT) is all you need.

Your normal cruise rpms look fine. Just make sure your oil temperature gets up to 160-180 deg F to get rid of combustion moisture.
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Old 02-03-2019, 09:21 PM   #15
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If I'm poking along at 6-7 knots on a long leg (6 hours crossing Lake Michigan, for example), the exhaust will start putting out a little white smoke like it's overfueling (might just be water vapor...cool lake water). Anyway, I'll run engines up so slow "semi-plane speed" (about 2/3 power to put some heat in them). The water temp gauges will come up a bit, and then I run them there for about 10 minutes. The exhaust seems to clear up. Repeat every couple of hours. I also run it up just prior to entering the marina.

By the way, I was looking at an ad for a Grand Alaskan with 700 HP Cats and some surveyor had bragged on the fact that the boat had spent it's entire cruising life at 8-9 knots. He was saying that this was a wonderful thing because the engines were well under CAT's recommended loading. I had to chuckle....that boat is a perfect example of a slobbering pig at slow speeds. Yours is decidedly not.
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Old 02-03-2019, 09:31 PM   #16
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If I'm poking along at 6-7 knots on a long leg (6 hours crossing Lake Michigan, for example), the exhaust will start putting out a little white smoke like it's overfueling (might just be water vapor...cool lake water). Anyway, I'll run engines up so slow "semi-plane speed" (about 2/3 power to put some heat in them). The water temp gauges will come up a bit, and then I run them there for about 10 minutes. The exhaust seems to clear up. Repeat every couple of hours. I also run it up just prior to entering the marina.

By the way, I was looking at an ad for a Grand Alaskan with 700 HP Cats and some surveyor had bragged on the fact that the boat had spent it's entire cruising life at 8-9 knots. He was saying that this was a wonderful thing because the engines were well under CAT's recommended loading. I had to chuckle....that boat is a perfect example of a slobbering pig at slow speeds. Yours is decidedly not.
Thanks for that.

Funny thing is, no matter what my RPM, the temps will stay around thermostat 187 or so, and never waver. The only things that change with RPM is oil pressure (expected) and sometimes voltage (I am having the alternators checked during this year's annual service).

1400hp, which GA was it - an 80 footer? Or was it twin 350 cats?
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Old 02-03-2019, 10:07 PM   #17
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This one https://www.theyachtmarket.com/boats_for_sale/1492688/

Lake Michigan is in the 40's in the early spring, so the heat exchangers really pull the heat out of the water jacket coolant.
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Old 02-04-2019, 06:40 AM   #18
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"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?"

The usual rule of thumb is 80% load at 90% RPM. for max cont duty.

Your lower RPM cruise is fine , ideling long term with no load is a big problem but modest loads are not.

The fuel use prop curve is only a rough math approximation of some prop on some boat , probably Not Yours .

If you want for some reason to run your boat hard , just be sure your exhaust has no black smoke and you can advance the throttle 300RPM .

Do idle for 5 min or so after a day at full duty to let the turbo cool down.Usually time spent docking is long enough.
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Old 02-04-2019, 07:55 AM   #19
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David Gerr's book "The Propeller Handbook" has a chapter devoted specifically to engine power, understanding manufactures engine graphs and engine ratings. The characteristics of your engine have a direct correlation to the choice of propeller for a particular boat and it's intended use.
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Old 02-04-2019, 07:58 AM   #20
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This one https://www.theyachtmarket.com/boats_for_sale/1492688/

Lake Michigan is in the 40's in the early spring, so the heat exchangers really pull the heat out of the water jacket coolant.
That's a ton of HP for that boat!
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