Theoretical hull speed is (square root of (water line length) * 1.44 which works out to about 6.5 knots for you. My 240 spec is very similar and I can motor at over 6 knots but fuel consumption goes to about 2 litres per hour. At about 5.5 knots I use just over a litre per hour with much less vibration and noise at about one third throttle. You will need to find a combination that works for your boat and motor irrespective of theoretical hull speed.
Cheers
