Modern diesels 4 produce about 20 hp for each gallon of fuel
per hour. For technical details look up BSFC, brake specific fuel consumption
. That figure works fairly well over the load range. The difficulty folks have in estimating fuel use comes from a misunderstanding of engine
makers fuel curves. All diesel makers show max load fuel curve but for boats that curve has no relevance except at WOT. Boats consume power via the prop and are loaded along a propeller
load curve not max load. The prop curve is always lower than the max load curve at all points except WOT if the boat is propped properly. The prop curve is shown on data sheets
for many marine
diesels. For the technically minded it is usually a cubic or 2.5 exponent curve. Estimating fuel use is quite possible from the prop curve IF the engine is loaded to reach full wot.
Some people over prop the boat so that the engine will not reach rated WOT. In that case the fuel at the actual max rpm
will be a point on the max fuel curve and below that point will follow the same exponent curve.