Not sure this language in Athen's article agrees with that.
"With the 380 rating you’d be at least 15% over propped at a minimum
Look at the attached graphs to see where you are now.. It should be obvious that you are already asking more from the engine that Cummins recommends and you are on a short road to a short lived engine from overloading it beyond what it is designed for."
As a practical matter, most diesels last longer than the interest of the owners to maintain them. Over propping is, according to Athens and every other expert I know of, bad for the engine. So maybe it reduces the life from 20,000 hours to 15,000 for the guy who owns it for 5,000 hours. He's likely the one insisting lugging the engine is just fine. But Athens is unequivocal in all of his comments I have read on the undesirability of over propping.
This whole debate reminds me of the arguments against fuel polishing or bypass filtration, which are arguments against cleaner fuel and cleaner oil. Doesn't matter, frequently, for most owners but that doesn't change the reality that cleaner fuel and oil and a prop pitched per the manufacturers requirements for warranty is desirable.
These were the paragraphs from the Tony Athens article that made me think that way....I wasn't thinking as high as 15%, maybe half that...
This was his example....
"You have a new 50,000 lb 45 Ft “East Coaster” with a new QSM11 Cummins rated at 450HP at 2100 RPM and the vessel uses 10 GPH at 12 K’s at 1600 RPM & 800 RPM propeller speed (2:1 gear ratio) – A tad better than 1:1. You are one Happy Camper because you got all the performance you wanted and at 12 K’s, your engine is only loading to 45%, just loafing along.
Now, to make my point, I go down to the vessel and reprogram the ECM for a 300HP at 1800 QSM Fuel Curve Rating. The owner takes the boat out the next day and brings it up to 1800 RPM and looks at his gauges. “WOW” he says – All of a sudden he sees a 70+% load on the engine yet the RPM, GPH & vessel speed is exactly the same. What happened? Actually nothing that matters has happened at all. We are still extracting the EXACT same amount of power from the engine – the “circumstances” for rotating that prop at that prop RPM has not changed so the vessel is performing at that RPM exactly as before. What has changed is, now we are asking the engine to deliver 70+ % of it’s AVAILABLE POWER. So the load % on the engine in relation to that AVAILABLE POWER is much higher. Does that mean we are adding more “wear & tear” to the engine and the engine may not last as long running it at 70+% load versus well under 50% ?? Think about it (answer at the end)."
At the end of the article here was his answer....
"The engine is this case will not have any additional wear & tear taking place.
All I did was, when re-programming the engine, put an “electronic cork” under the throttle pedal, so to say. In fact, with the software/electronic governor and engine set this way, the engine would usually have a much longer life, as now the operator does not have the option to run hard even with the “pedal to the metal” and the engine at 100% loading for hours on end. In other words the “electronics” is limiting the HP of the engine to 15 GPH or about 300HP max.
And I need to mention something else related to this discussion that chaps my hide way too often. It’s often said typical dock talk) that you must run a diesel at 70-80% load for max life/efficiency. I say total Horse Manure and the answer is right in this article. Just look at the QSM11 for a perfect example."
"Put the 300HP version of this engine on a dyno producing 215 HP @ 1600 RPM burning 10.6 GPH just like on the attached graph below, and leave it there “forever”. The 300HP version will be running at approx 72% load. Next to it on another dyno, use the 715 HP version and set it up to produce approx 215HP at the same RPM burning 10.6 GPH. The 715 HP version will be at about 30% load. Both engines would be on the “factory prop curve” at the same RPM ( how could that be?) . Outside of the maintenance that each engine would require when “running forever” , (and, I will take this to the bank) ———– Both engines will last the same as to “wear & tear” and both would have no measureable efficiency differences between them that matters. And, how long might that be? I’d say not less than 30,000 hours or 300,000 gallons of fuel burned at that load & RPM with all of the right maintenance done applicable to the engine"
