Thread: Under propped?
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Old 03-15-2014, 01:13 PM   #24
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City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 5,152
Originally Posted by markpierce View Post
Are we sure we're properly interpreting JD "M" ratings? My JD4045DFM70 is also rated "M2." So, full power operation is allowed 16 hours during a 24-hour period; however the stated load factor is equal to or less than 65% (actual fuel consumption divided by full-power fuel consumption). Sixty-five percent for my engine is about 2100 RPM out of a maximum of 2400 WOT. Thus, in my mind, anything between 2101 and 2400 RPM is "full throttle" limited to 16 out of 24 hours, a 299 RPM spread, not a mere 100 you cite for the Nordhavn's JD. (I usually operate at 1800 RPM, or a load factor of 43%)
Well, I don't know. But looking at the specs more closely, and calculating out what 65% load factor would be, I think you are right. It's too bad the various vendors aren't clearer about this stuff.

First, I though I saw a note about cruise RPM being 100 less than max, but I must be mistaken about that. I do see a note saying it's 200 RPM less.

But reducing the engine RPM by 200 doesn't drop the fuel burn to 65% of full power, so now I'm really confused since I have two conflicting specs. Which do I follow?

If we assume (a bad thing) that we can't exceed either limit, then it means we can't run over 2000 RPM for more than 16/24 hrs AND we can't burn more than 65% of full power burn. That means when we are running below 2000, we need to get in a bunch of time well below 2000 to get the overall burn down. For reference, the 65% burn rate happens at about 1850 RPM, which suggests that's the continuous load point.

Now, just when I think I understand it, I look at the M1 continuous rating and it allows for 2100 RPM non-stop, 24x7, which is well over 65% of the M2 fuel burn. OK, so which is the real continuous rated RPM? 2100 or 1850?

So we have 3 seemingly conflicting specs.

But, like you said, it's really moot since nobody is likely to run that hard anyway. I sure won't, so I'm not going to worry about it too much.
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