Glad to see you too are puzled by your engine (Ford 2712 I remember) consumtion.
I came to a comparable situation, running at 1600 rmp gives less than 6 liter per hour, at around 6 knots, under normal circumstances.
I usually warm up the engine for 15 minutes and upon arrival I let the engine run for another 5 or 10 minutes. Even though the clutch is engaged at 1000 rpm I expect 1 liter per hour during this warm-up and warm-down period.
When I deduct those “warming” minutes and mere liters for figures on the hourmeter and the fuel pump I find – over an entire season – actual effective miles to “cost” slightly higher, namely some 5,6 to 5,8 liter per effective hour, or 0,8 to 0,9 liter to the nautical mile.
I took the opportunity of careful refillings to correlate the tank level needle with actual gasoil capacity. I have included a picture that helps me. 1/16 of the capacity being some 16 liters or 2 to 2,5 hours. This is correct enough for me and the paralax