prop

[robbie]

hi can anybody help. friend just bought a boat,23 birchwood,2ltr ford disel.came orig with petrol engine,but had disel put in by old boaty who knew his stuff,boat never been in water with disel engine,had prob with balance sorted with some balash up front,boat now level,the prob is with the prop at full revs the boat only does 8 knts at 35000 rpm,prop brand new recomended by shop think its 15/17 seems like it cavertates alot,also tried 17/21 which was also no good/anybody with a simular boat or any idears i would be most gratefull,tried prop cal the boat also has a volvo 270 leg not sure of the gear ratio,engine runs fine,just cant get the prop size right ,boater going no where fast rob