Antares 710 under propped ?

I have recently purchased a 2005 Antares 710 (numerous bumps on head) and I am convinced it is under propped. I am getting 1 Mile per 2 litres Diesel and 19 kts flat out Max RPM in gear is 2500 regardless of load, speed falls 1 kt with 4 extra adults but still no drop in RPM. The engine is a 155 Nanni. I have been told that the previous model with the volvo 150 hp engine does 25 kts and uses 1 litre per mile. Does anyone have similar problems or info to share?