Our houseboat (Waterhouse 14' x 44') has 1996 75hp mercury 2 stroke. It is turning a 14" x 10" prop. But cannot reach my WOT @ ~5000rpm nor theoretical boat speed (8 knots). We are looking for the most effective powerplant/prop combination to have optimum fuel efficiency, has well as meet hull speed. I am considering increasing Hp to 90 or 115, 4 stroke. This unfortunately will not increase my propeller diameter just raw horsepower. Any thoughts on this would be very appreciated. Thanks in advance.