How many rpm do you mantain in cruise and wath is your fuel cosumption?
The D2 55 is mounted from 38 to 45 ft I suppose.
1800rpm give me 7kt on my 40 with about 2.5 liters per hour
The 40 is overpowered with the D2 55 so you do not need to run it hard. Its rated maximum rpm is 3000 and the rule of thumb is you should run continuously at a minimum of 60% maximum - that is 1800rpm.
As you see from the link this gives a fuel consumption of just under 3l/h
www.darthaven.co.uk/userfiles/products/files/Technical%20data%20D2-55.pdf
I wouldn't recommend running it continuously at less than 2000prm. Otherwise you may have exhaust elbow problems as the engine isn't being allowed to get up to full working temperature.