RPM/Knots Per Hour/GPH Burn Rate/MPG/Total Range

It s all wrong.

there are two speeds that make sense on a power boat. Hull speed or on plane. Hull speed is 1.3 x sq root of LWL... which on your boat will be 9.5 knots.

either you run at or just below that speed or you get on plane which likely will be about 17/18 kts. I ran a friends 59 Marquis a few years ago and it was a decent running boat.

Go just one knot over hull speed and fuel burn will double. Let me say this again... 10% more speed for 100% more fuel, does it make sense ?

look at your own numbers... your best speed for economy will be 1100 rpm Not 1167. At 1100 you should Burn about 10/12 GPH or just under 1 nmpg.

Anything in between and you ll be pushing a big wake, burning a ton more fuel and loading up your engines. 1300 is just about the worst possible speed.

Now looking at your numbers you have a serious problem there if your max RPM is 2206. Your engines should turn 2300 / 2325 at WOT. Being short 100rpm is a big deal that will affect the longevity of your engines.

i don’t recall the exact numbers of that 59 I ran, but at 1900 or so, the MTUs where running 70% load or so and 21 knots. Something is wrong there. Either your boat is dirty, or you have another issue but this kind of overload is bad.

for comparison I just got back from Staniel Cay Exumas to Miami yesterday, burning just under 500 gallons to cover 240 nm at 11 kts. With a 110 footer running a pair of 16V2000 MTUs. Basically the same as your 1300 rpm speed in a boat half the size. Think about it.

Below is my 2c, related to a boat which is slightly smaller than yours (56'), and powered by 800hp MAN engines.
The problem with old school mechanical engines like these is that neither the real time fuel burn nor the load are available.
Actually, additional fuel flow measurement instruments like Floscan or Maretron could monitor the former, but I don't have them, so there is necessarily a degree of estimation in my table.
On the other hand, I could check the numbers below over several years by now, and I'm confident that they are reliable enough.

Now, I believe that when you are talking of 825hp MTUs, you are referring to their Series 60 engines, which as I recall were driven by electronic DDEC governor, but I can't recall which numbers are available on their instruments.
I suppose you would have shown the load in your table if available, because the RPM % is meaningless, as others already mentioned.
Though it's interesting the 96% associated to 2209 RPM, because it suggests you are aware that the engines are actually rated for 2300.
Now, if the boat does reach 2300, but you just didn't include it in the table, fine.
If not, the previous comments on the missing 100 RPM remain valid.
Anyway, I added the two last columns on the right to my usual table, just to show you how useless the RPM % as an indicator of engine load.



PS: ref. your question about NMPG flattening out once the boat is on the plane, generally speaking that is normal for most planing boats.
In some boats - usually those with waterjet or surface drive transmissions - the fuel burn per Nm can even slightly improve with speed.
So, the small inconsistencies in your numbers can well be just a matter of roundings, but in principle I don't think you should worry about them.

Fully agreed.
In the OP boots, based on his numbers, I would either cruise under 1100 or anywhere between 1800 and 2000.

I just realized I had sea trial data from the 84 I used to run. I didn’t bother writing data between 1000 and 1500 rpm as it was useless at these speed

this was in calm water and fairly shallow (10/12’). Planning performance would be about 5-10% lower offshore.

Again, hull speed is 1.3 the sq. root of LWL. NOT the typical inflated LOA... check the specs for your boat but I doubt your waterline length is longer than 52 or 53’

At 53’ LWL your hull speed will be 9.5 which is very different from the 10.6 kts you re getting at 1300. As in TWICE THE FUEL different...

You need to understand hull speed is like a wall. Once you hit it, you have to jump over it...

I would add a couple of things to what Pascal correctly pointed out.

First, as I recall the Marquis 59 has an overhanging swim platform, which adds nothing to the waterline length.
So, I suspect the actual LWL might be even less than what Pascal envisaged - a 50' ballpark would be my estimate.
But as he said, check the builder's specs (if available - not all builders declare LWL).

Second, the theoretical rule for hull speed is just that, theoretical.
I'm not saying it's wrong, but hull speed can actually be stretched or reduced by other factors, for any given LWL, depending on the specific hull shape.
Typically, hull specifically designed for displacement cruising are slightly more efficient (i.e. faster) than planing hulls of the same LWL.
And the only way to estimate that (other than fuel burn) is look behind you, and see your own wake.

As an example, I cruised for many years with a 53' pure displacement trawler, whose LWL was just a few inches short of 50'. Which by coincidence, is almost exactly the same of my current (planing) 56 footer.
Now, the first left behind her a significant wake already at 8.5 kts.
She could cruise all day at 9.5 anyway, which was her "theoretical" hull speed, but I very rarely did that, because I saw no point in trading a 12% time saving for a 50+% worse NMPG.
This might lead you to think that she wasn't very efficient, but my current 56' planing boat is actually worse in this respect, because at 8 kts, her wake is already more substantial compared to my previous trawler at 8.5 kts.
Not that I care, because cruising at D speed with a planing boat is a PITA anyway, and in fact now I rarely cruise under 20 kts.

Bottom line, if (as you said in your OP) slow cruising speed is fast enough for you, I wouldn't even bother calculating more accurately what your theoretical hull speed is, but I'd rather look behind the boat and see how the wave changes every half a knot, because that's the simpler and most immediate indicator of the speed each boat is happy to cruise at effortlessly.
You might be surprised.

Yes, it is very close, and I didn't say it isn't.
But what the rule defines is the maximum hull speed, and my point was that the most efficient (meant as best NMPG) speed is always a bit under the theoretical hull speed.
And for any given LWL, this most efficient speed can be slightly different for each boat, depending mostly on the hull type.
In fact, I was talking of half a knot differences, which is not much, but it can and does make a difference in NMPG, hence range.

Indeed the formula is general and there will be variations depending on exact hull shape. That it was always spot on for everybody i ve run with flow meters

an extra half a knot above HS usually costs at least 25% fuel economy