Thread: Under propped?
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Old 03-15-2014, 01:13 PM   #24
twistedtree
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Quote:
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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