Find out, all engines are not the same... I can tell you how if you like?
first, find a speed you like say 5 knots, and a comfortable rpm where the engine is not at max rpm" a good balance point speed vrs not over stressing the engine."
Now fill the boat all the way up with fuel to the top of the filler neck but not spilling any in the water.
While at dock and a filled to the top boat tie her stern to the dock " while in your slip, put her in forward and set her engine speed to your comfort rpm and pull for an hour.
At an hour shut it all down and measure the amount of fuel to fill it up to the top again..
TA-DA you now have a full load fuel consumption # now it will be a bit high of a reading because you were " under load " IE " the dock did not allow you to ever not be under a load.
This would be similar to a load in a storm under bad conditions but is a great tool to calculate fuel consumption on a trip or so.
I hope this helps you all.. And yes i work on Diesel's for a living.