If you have barny's on the prop your bottom is probably not as clean as you think.
Boat length and engine size is not the only variables - boat shape and weight also come into play as do current and wind strength/direction.
Entire books have been written on
propeller selection etc.. (David Gerr's for one) - the bottom
line is ask your perkins dealer or C&C forum users what they are going with. You do need to find out more about your prop - being 3 bladed isn't enough to work with - you need both pitch and diameter to work the calculations.