That's a hard question to ansewer. Different hulls will run different speeds with the same motor. My 17 Wareagle with just me and heavy loaded will get 31 if conditions are right with a 50 four stroke. Thinking 40 is way optimistic. Maybe in low 30's. Do you have a tach? What is max RPM for your motor? What size is the prop your running now? And if you have a tach how many RPM's is it turning?