I am trying to determine my fuel burn. I have downloaded charts from Caterpillar, prop demand and Max power data. Which do I use?
The prop demand curve is going to be closer than the max power curve. That said, these curves are used to compare engines, since engine makers tend to all use the same ASTM/ ISO testing procedure. But know that these are mathematical "best guesses."
On a real boat, not a theoretical one, you are going to have different props than another boat powered by the same engine. Also differences in performance one might seek. For instance, is the mfr test assuming a 3-bladed wheel or a 5-bladed wheel? With how much pitch, diameter, DAR, rake, skew, etc......
What boat and engine exactly are you trying to reference. We may have data that will help you. Boat builders and designers will all tell you that actual sea trial data trumps formulaic assumption.
Click this link, and scroll through the boat tests to find a boat/engine combo similar to the one you are investigating: