Fuel consumption per hour for mercury 800 80hp outboard?

I just took my 1983 Mercury 80 hp Blue Band out yesterday on a 5 meter half cabin Australian Swift Craft. I kept it at approx 20 knots (backed off from full throttle which would be about 38 knots) for approx 80 minutes and used 27 Liters of Fuel. I was shocked that it used so much fuel.... was thinking sail boat. Or at least always cruising with the wind at my back. I noticed the huge difference in consumption coming back against the wind. It was my first time to measure fuel consumption. I'm still playing with cruising speeds.... Guess we are back to the sixties.