IF you can make that speed OVER THE GROUND subject to wind and current so the hours AND the gph are correct your math is good. 15% reserve is not a lot though. I'd be happier with 25-30%.
Furthermore you will need to do some "point of go-no go" calculations. IE you proceed to a point where either the numbers jive and you press on, or you return with the numbers as they are. This calculation will be updated hour by hour as you progress. In fact most missions will have several of these decision points.
In other words, don't paint yourself in a corner. The fact that you come here and reduce a complex question to an over simplified one is of some concern as to your experience level versus the level required to do such a mission.
+1 One of the first things I make sure to do with the boat is accurately measure fuel burn -vs- distance -vs- time to start amassing knowledge of how the boat (and I) perform over distance. There are two figures for fuel consumption, like Oscar and psneeld point out, the important one is not how many gallons per hour the engines CAN burn, but the final equation of current/wind/weather and your own use of the boat systems and how much the boat DOES burn.
I've measured my fuel burn at 6.5 knot speed settings at just a shade under 2 gph.
BUT. But I rarely see 6.5 kts at that throttle setting due to current. I try to plan so I see that or more than that and sometimes do, but also do not like to get up at 4 am to make the flood. Any cruise that has you running all day in the PNW where I live has you hitting at least one ebb and flood cycle so it will tend to even out at best.
For a trip, my equation of time/speed/distance/fuel burn would look something like:
3000 miles (+distance to jog in weather + distance spent chasing course + distance spent avoiding ships or obstacles + distance spent seeking shelter not in chart plan) / 7 kts (+ current in my favor - current against me + wind in my favor - wind against me + speed slow down for weather or obstacles) * 2 gph (+ any time spent at higher RPM due to anything + generator fuel burn + diesel heater fuel burn)
For my local inshore cruising the above seems to be the factors that influence me the most - albeit the wind influence tends to affect more WHEN I decide to move or whether I intentionally slow own or jog into slop rather than purely slowing me down pushing on the boat.
Our 2 week cruise this summer showed average speed made good (I didn't compute this - it was vacation) of around 5 1/2 kts, and our total average fuel burn was 2.41 gph with the accessories contributing. Quite a difference from the "manufacturers suggested"!
Depending on how I look at it, my 400 gal total fuel capacity could get me 1300 miles (2 gph @ 6.5 kts) or 900 miles (2.41 gph @ 5.5 kts) based on my real world factoring. leaving myself a 25% reserve (300 gal / 2.41 gph x 5.5 kts) eats that down to 684 miles, roughly half of what my initial thumbnail of 1300 miles would have been...
As we use NWD more, I will continue to track our fuel use and speed made good as a whole, so our trip planning will be more spot on.
(now, when the solar is in place and I can stop running the genset so much, how far is Ketchikan from Anacortes again?!
