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Old 03-26-2016, 12:21 PM   #1
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How To Calculate Loading

I'll start by admitting I'm a diesel novice but trying to absorb and understand as much as possible here at TF.

I have no interest in entering the underloading debate so decided to start a separate thread.

I understand that load IS NOT RPM / WOT RPM but...
Can some of the experts here clarify how to calculate % loading correctly.

Assuming WOT meets Mfg Specs - is % Load... (see attached for Yanmar 6lya - STP Ex's)

A - Prop Power / Crank Power for any RPM

B - Prop Power / Prop Power @ WOT

C - Fuel Consumption @ RPM / Fuel Cons @ WOT

D - Other??
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File Type: pdf Yanmar Performance Curves.pdf (266.9 KB, 17 views)
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Old 03-26-2016, 01:06 PM   #2
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Since AFAIK, there is no ASME or ISO definition for that term. So you can make it anything that you like.


With the advent of electronic engines that measure fuel consumption underway, I have noticed Tony Athens, the Cummins/Yanmar (among others) guru on boatdiesel, starting to talk about propping to 90-95% of wot load, since the convention of propping 100 rpm over rated rpm doesn't work for electronic engines. I think that the instruments on those engines calculate it as fuel measured over wot rated fuel consumption. That is an approximation for hp used over wot rated hp as it is essentially impossible to measure horsepower underway. That approximation works well near wot but gets worse at low rpm loadings.


My definition is hp used (as estimated from my rpm and the engine's prop curve) over maximum rated hp. Others may define it as hp used over rated hp at that rpm. Even some may define it as hp used over prop hp at that rpm.


Take your pick, there is no right or wrong.


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Old 03-26-2016, 02:02 PM   #3
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David

Thanks - No wonder I was confused by all the comments in other threads!!

I thought fuel was a reasonable measure but have seen other descriptions and wondered.
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Old 03-26-2016, 02:29 PM   #4
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For fun, I checked a file for a Cat C32 sea trial sitting on my desk. Compared GPH at cruise to max GPH, then compared that to the %load I wrote down.

Max gph 79.5gph, cruise 66. %load from cruisegph/maxgph= 83%. Math would be similar to cruisehp/maxhp. Would be even higher if it was cruisehp/maxhp@ cruiserpm, but I don't know that number.

Display read 76%.

I have no idea how they calc that number. It does not seem to fit any of the OP's possibilities.

To be fair, Cat has been fiddling with ecm gph calibrations on this particular engine, so maybe they goobered the % load too.
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Old 03-26-2016, 05:49 PM   #5
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you are reading the curve correctly. As said above other than measuring fuel there is no way to actually measure power BUT the prop curve is a mathematical approximation of power used to spin the prop. Depending on the maker it is a simple curve that does use RPM to predict the prop power.
Each maker uses a different exponent in the eqyation (rpm/maxrpm)^X= load fraction.. X is commonly 2.7but can be 2,5 or maybe 3.


Your yanmar data may well give the exponent. If it does do the math and you will reproduce that curve nicely.


Electronic engines are rpm limited so you cant actually prop them to 100 over WOT because they wont get there. other than having a little "pedal' left after wot is reached I don't know how they estimate their recommended over propping.
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Old 03-26-2016, 05:50 PM   #6
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Ski, did you note the % at WOT? If the boat was under propped as David described the motor would not ever get to 100%. That would explain your observed numbers.
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Old 03-26-2016, 05:55 PM   #7
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you are reading the curve correctly. As said above other than measuring fuel there is no way to actually measure power BUT the prop curve is a mathematical approximation of power used to spin the prop. Depending on the maker it is a simple curve that does use RPM to predict the prop power. (rpm/maxrpm)^x= fraction of load. X depends on the maker but 2.7 is common. Look on the yanmar data for the exponent.
Electronic engines wont allow revving over about 50 so the old idea of propping to 100 or 200 over wont work. Other than leaving a little"pedal" at wot I don't know how the guys estimate the underpropping. Of course electronic engine gauges will show say 95% at wot allowing some 5% underpropping.
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Old 03-26-2016, 06:12 PM   #8
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Bacchus, I was trying to figure out the same thing the other day. I still have no idea. My problem, as I finally realized, was that I don't fully understand the concept of what a prop curve is.

I think, that % load is the amount of work that an engine has to do to produce a given rpm with a given prop on a given boat. How to come up with that number is beyond me. I think that % fuel consumption may be the best approximation, but frankly it confuses me.
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Old 03-26-2016, 06:26 PM   #9
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WOW !!!

Had no idea my lack of understanding was in such good company

No wonder there are debates about over / under loading when we can't agree on what we are even debating.

No offense guys I was hoping David & ski would weigh in and there would be a simple straight forward answer... I appreciate the honesty.

I don't feel as uneducated as I did earlier - still don't Understand but recognize I'm in good company
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Old 03-26-2016, 06:32 PM   #10
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Quote:
Originally Posted by Jeff F View Post
Ski, did you note the % at WOT? If the boat was under propped as David described the motor would not ever get to 100%. That would explain your observed numbers.
Full power: 2358rpm, 78gph. 98% load. Figured gov was holding off fuel so estimated full load burn 2% higher, 79.5gph. Did not dive into this in a serious way, just goofing around with numbers.

Maybe they are comparing current conditions to max available TORQUE at that rpm.
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Old 03-26-2016, 08:17 PM   #11
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Quote:
Originally Posted by Ski in NC View Post
Full power: 2358rpm, 78gph. 98% load. Figured gov was holding off fuel so estimated full load burn 2% higher, 79.5gph. Did not dive into this in a serious way, just goofing around with numbers.

Maybe they are comparing current conditions to max available TORQUE at that rpm.
Actually some of the very complex formulas go back to percentage of torque as you suspect. The simplest way to get a good approximation is fuel usage. Electronic engines deal with some elaborate calculations and openings and other things far beyond me. However, our manual calculations based on fuel consumption are not far from the electronically generated load percentages we're receiving.
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Old 03-26-2016, 08:47 PM   #12
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JD defines engine load by the fuel used:

Quote:
Load factor is the actual fuel burned over a period of time divided by the full-power fuel consumption for the same period of time. For example, if an engine burns 160 liters of fuel during an eight-hour run, and the full-power fuel consumption is 60 liters per hour, the load factor is 160 liters / (60 liters per hour x 8 hours) = 33.3 percent.
Later,
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