I'm curious to know what the fuel consumption is on a 1983 O'day 30 is based on my guess 2000 rpm on an Univversal M20 (16 HP)? I understand there are some variables in there, but if anyone has some infor, I'd appreciate knowing.
You are right... Too many variables to give you a reliable number. I woud suggest you fill the tank go for a run and then refill the tank recording how long you powered at 2000 rpm and how much fuel was used.
As a genral consideration, I would start - assume - you use 1 gallon of fuel per hour. You are likely to find that it is better than that. Towards .5 gallons per hour. But if you have growth on your boat bottom, the prop is not the most efficient, etc your numbers will vary.
Keep a log and you will soon know your numbers.
Now attched is a link to your engine manual. Page 57 (i think) has your engine data for the M20. The recommended cruise range is 2000 to 2400 rpm. Diesels like to work. Depending on the prop you have you may find that 2000 will be too slow for the engine to be working at it’s best. Baby the engine and it will coke up and break down. Make it work and you will have a happy long life together. Try 2400. She will tell you what works best. Listen to the sound and the vibration of the boat and find your boat’s sweet spot.