Crunched a few more numbers. Say that it is decided to run the John Deere 6048TFM50 engines at 45% of the maximum continuous power rating of 154 horsepower because this is more conservative than 40% to prevent slobbering. This means the boat now goes 8.12 knots at 1830 RPM, uses 7.6 gallons per hour and gets 1.07 miles per gallon.
Say the boat is low on fuel so lets go 5 knots at 1100 RPM, use 2.2 gallons per hour and get 2.27 miles per gallon and worry about running at 75% power for an hour when there is plenty of fuel.
The speed length formulas from which I take these numbers are for an average boat. These numbers may vary depending on the actual boat. If you have a long narrow beam boat such as this:
http://www.gerrmarine.com/power_70.html and look at Iron Heart boat, the speed length ratio can go beyond 1.34 as the maximum hull speed.
For any particular boat, it would be wise to do actual fuel consumption and speed under no wind and no current conditions, making a run in either direction to average out just in case there is a little bit of wind and/or current. Make sure the bottom of the boat and the propeller are clean
The 6068TFM50 John Deere engine can be pushed beyond the 154 horsepower to 225 horsepower for 5% of the time, say if you have a storm trying to push you onto the breakers. Well in that case, I guess forget the 5% and just get out of there. Anyway that extra horsepower is available under really adverse conditions so use it if needed.