I just purchased a 42' aluminum crew boat. With 36" wheels it runs wide open at 25 knots, there are no tacks but I figured I would slow them a little and run 20 knots. I know it's just a guess but I'm curious if anyone has something similar. We have to run 300 plus miles and I'm trying to figure estimated fuel needed. My 671 with a blower on a 26' aluminum crew boat sips fuel even wide open. I know it's a guess but that's all I'm trying to do is make a more educated guess.
Without HP, Injector and turbo size, and RPM. You're just pissing in the wind. Probably around 25 GPH at 20 knots per engine. I've run a few crew boats and have seen the detroits setup every which way. Detuned with smaller injectors and able to run them to the pins all of the time, govenors set to run less rpms at full rack, and somewhere in between. Many of them wouldn't turn past 1800 rpms at WOT.
According to the J&T chart a 6-71 485 HP engine burns 24 gallons per engine at WOT, 2550 rpm. Mine burn close to 35 GPH at 1900 RPM @ 18 kt pushing 40000 lbs. I don't have numbers for your engines but Capt J's numbers should work.