MapisM
Well-known member
By the by, the rough rule of 1 litre for every 5hp of output is actually valid not only at WOT, but through the whole normal range.
The problem is that it's difficult to find the corresponding numbers, because the output in hp along the prop demand curve is normally not included in the manufacturer's specs (with the notable exception of Caterpillar).
But we can cross check if the rule is valid also based on the above MAN table, with some patience.
And also with no patience at all, for those who already invested 15 mins or so in preparing a simple spreadsheet.
Let's take 1800rpm for instance, at which the above table tells us that the engine can produce up to 714hp at full load.
But the fuel burn of 82 l/h is related to the output at prop demand, hence at a much lower load.
What we don't know if how much power the engine actually produces, at that rpm and burning that amount of fuel.
Otoh, they also declare the specific fuel consumption (aka BSFC) at 1800rpm, i.e. 216 g/kWh.
Incidentally, this already tell us that the engine, when producing X hp (X being still unknown for the moment) at 1800rpm, is slightly more efficient than at WOT, where the BSFC is 230 g/kWh.
But I digress.
Back to the point, are these numbers sufficient to understand how many hp the engine produces at 1800 while burning 82 l/h?
The answer is yes, and what we must do is:
1) convert g/kWh in l/kWh. Using a factor of 0.85 for the specific weight of diesel, 216 g/kWh translates into 0.254 l/kWh.
2) divide 82 l/h by 0.254. The result is 323, and this is the output measured in kW.
3) make a kW to hp conversion, and 323 becomes 433hp.
Now, dividing 433 by 82, we get a factor of 5.28, which is a bit better than the 4.94 at WOT, but yet again in the usual 5hp/litre ballpark.
Q.E.D.
As a side note, it's interesting to see that with these engines the fuel burn at 1800rpm could be anywhere between 82 l/h with an output of 433hp, and up to 133 l/h with a max output of 714hp at full load.
The first assumes a boat whose hull requires exactly as much power as predicted by the theoretical prop demand curve, while the latter indicates either an overpropped boat, or a fouled hull, or whatever.
And it's in any case a condition better avoided, obviously: high EGT, and all that!
But it might well be that at the same rpm, one boat burns 82 l/h and another 90, 100, or even more - nothing wrong with that.
It just means that their REAL prop demand curves (as opposed to the manufacturer one, which is theoretical) are different.
At the opposite extreme, the engine could also burn much LESS than 82 l/h of course, if lightly loaded.
Up to the absolute minimum of running out of gear, and burning only as much as required to keep all the internal iron bits spinning.
But actually, I've yet to come across a P boat which burns less than what the engine manufacturer's prop demand curve suggests.
Fwiw, when compared to Cat prop demand curves, my old lady was right on the money at 9 kts, burned even less at 8 kts and below, and progressively more above 9 kts.
But D boats are another kettle of fish, obviously.
All that said, I'll have some sleep.
These lockdown night habits aren't healthy...
...But tomorrow, boating time, yippee!
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