Sorry to burst your bubble, but those numbers just dont add up. A 21 pitch prop at 5000 rpm will only push any boat, no mater what the hull or weight to 58.3 mph. The faster the speed, the less accurate the speedodometer. One thing you should consider is, if at 5k rpm do you still have more throttle? If not the next size prop (22" 4 blade) will only lug your motor down and give a slower time to plane. If at 5k rpm you still have a little more throttle, then a 22" 4 blade should help with overall performance and a little more top speed. A 22" at 5k will push a boat 61.1 mph, figuring 12% prop slip.