Look at 8 knots and see how many MPG you get, which is also 8 Nm traveled in one hour.
If for instance you get 1.75 mpg, just divide the 8 miles by the 1.75 mpg and you get 4.6 gallons used.
If you want to double check your figures, multiply the 4.6 gals by the 1.75 mpg and that will give you 8 miles.
I personally never count gals/hour. I always think in miles. I check my gas tanks and I see 50 gals minus 10 gals from empty and I have 40 usable gallons left. At 7-8 kts, I know I that at 1.75Mpg I can go 70 miles on 40 gals before running on fumes.
All of this is assuming no real adverse current or wind. I use a dipstick I made to accurately measure gas. I mean more accurate than any gauge, which I never trust.
When I buy my next load of fuel in a mont or so, I will give you the updated chart.
Feel free to call anytime between 8AM and 8PM EST- I am retired, so no problem.
Cruising the Eastern U.S. Inland Waterways
Presently on the Tennessee River - Joe Wheeler State Park - Rogersville, Al.