Updated: Jun 25
Please excuse the delay in posting this.
At least two of the emails that came in after I recently put up the blog announcing Purchase Method Two had to do with cruising speed and fuel burn. This has come up before and there exists misleadingly simple answers and an answer that more precisely conveys your burn rate. [One of the 4 (fired) brokers struggled tremendously with anything other than a simple single number.... "what's the REAL fuel burn?" over and over. Did not appear to understand the definition of real.] There are several statements/graphs from NT and Cummins to help with planning how you operate the boat. I found the simple table shown above from NT to be plenty good when trying to ball park your operation. Please note... not a simple single digit number. No offense.
So I'll take a swing at the hybrid of the simple and the complex answers to the "what is the burn rate at cruising speed?" question. First, there is no fixed or rated cruising speed. And in fact, cruising speed may be driven entirely by each operator's feelings about burn rate... aka a "vacuous truth". You pick the speed at which you wish to operate and your burn rate will be somewhere between 0 gph (tied up at the dock) and 25 gph, throttle pinned. I'll provide 2 fixed operating points from the continuum which defined the entirety of my own operating range (other than the last 10 min, below). If you're trying to max out your range or just minimize your fuel burn, let me suggest running 1000 rpm for 6.2 kt and a burn rate of 1.6 gph.... like... free. I'd say I did the vast majority of my operating life on CW had me moving at 8.5 - 9.5 kt... ~1400 - 1800 rpm respectively and 4.5 - 6.5 gph also respectively. A fair assumed burn rate is 5 gph.
At the end of any sortie, I'd run the engine at max rpm for 10 min per the factory recommendation. 24 gph. I don't care what that table says about the upper end of the operating range. Unless you have an airplane big enough to drop the boat from, I consider that purely a marketing number. My recollection is I could semi-count on seeing 14 kt if conditions allowed... smoother surface... no big head wind... somewhat reduced water/fuel tanks. Plenty of times I had to be happy with 12.5kt and for this dizzying speed, I'd be burning 15-16 gph. Almost always, you got nothing for pushing the throttle more forward.. every drop of fuel just goes into building a bigger bow wave. You want to go really fast in NT terms? Here's the recipe... minimum fuel and water in the tanks and hit the head before you board the boat to depart the marina.. maybe you hit 14.5. Personally, I never operated at that pointless operating condition. For the vast bulk of my ownership, I kept the water and fuel tanks topped and this pushes what is my choice of operating speed from that table lower because the added weight is forcing me to make a bigger bow wave at a non-linearly punitive efficiency cost.
Summary? Someplace from 1 to 5 gph for all but the last 10 min of any normal sortie.