Diesel RPM?

[Pandor]

I just wanted to thank Lee Jerry and the others. The concept of the propeller load curve and its relationship to fuel efficiency isn’t an easy one to grasp. It is easy to slip into the misconception that pulling back on the throttle reduces fuel to the engine and rpms, therefore leading to the thought that an over-propped motor with a lower rpm will save fuel as well. Because by over-propping, you actually shift the prop load curve, this means that the lower rpms you see for any particular speed will equate to more power used and therefore, fuel consumption is not directly reduced by the lower rpm. What is clear, however, is that you are losing a good part of the top end of hp range by over-propping. And potentially not doing your engine any favors, either.

[malbert73]

Originally Posted by Lee Jerry

That's an unusual definition for loading. It seems more like the definition of "efficiency." The former would generally entail a reference to the maximum capability of the engine. Common metrics are power, torque and RPM. A quick internet search shows that one type of load gauge measures air flow and compares to max flow to indicate the load %.



Inefficient yes; underloaded no (or not necessarily). Those two are generally (always?) opposites - the less efficient the more stuff (power, torque, whatever...) needs to be produced which will be higher loading; more efficient means less power and loading.



I don't think we are ready or qualified to discuss the intricacies of engine design.



Yeah, it's almost as if over-propping doesn't really change the engine loading at all, which is determined by the resistance.



So I looked at the Dashew reference you gave above. It consists of ~1/2 page where he states the same as you (or more likely, you as him) with no real discussion or justification (which is probably why you don't either).

Here are some extracts of what he says:

"The key is to be sure to always run at no more than 80 percent of the engine’s available power."

"Here’s how this works. You want to momentarily find the point at which the engine begins to “lug,” or becomes overloaded. {snip} Once this point is established, back off 20 percent on rpm, and you’ll be at about the 80 percent point on available power."

So, starting with the gross error in the latter: for a displacement boat, a 20% reduction in RPM is NOT a 20% reduction in power. A 20% reduction in RPM from max would be ~45% reduction in power. You can see this in N Calder's "brilliant" prop curve demonstrations. A 20% reduction in power would be 7-8% reduction in RPM.

Now, following his advice, even the 20% power reduction (and not 20% RPM reduction), that seems like a pretty big hit to me. That would mean your Beta 50 instead of losing just a couple HP by over-propping, you are now losing ~10 hp.

That doesn't seem like "no downside" to me. I'd say your "experts" are 0 for 2.

And with that I am done. You give little indication that you have even read my posts let alone understood them, so there's no point in continuing further.

Yawn, I’ll go with Calder, Harries (and the engineers who contributed to his articles which are the most robust explanations I’ve seen), and Dashew. Not to mention my own experience.

Stay with your dogma

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[Rockinar]

Every truck stop in America is full of diesel trucks idling all night, every night, 365 days a year for decades. And most have over a million miles on them.

"Ahhh man my engnine blew up. I let it idle too much" said no truck driver ever.


The "Diesel engines dont like to idle and need to be loaded up" is 100% myth.