I had a 32 foot.. 10,500 pound Sailboat over 25 years ago. I sailed it to Bermuda in 1981..but that is irrelevant. It had a 15HP Yanmar diesel. It burned 1/5 GPH. Yup..I'm not making this up....real world experience. That's ONE gallon every FIVE hours....BTW at 6 Knots.
I am an engineer and as such am obsessed with keeping records.
NB