I have a 1984 Bayliner capri cuddy with a 4 cylynder volvo inboard and 270 outdrive.It runs great but will only get up to 17 miles per hour at 3800 rpm,and thats with just me in the boat.With 4 or 5 people it drops to about 12 miles per hour at 3000 rpm.Im not sure on the pitch of the prop but it looks in good condition.Whats wrong? Is it the outdrive or prop?