Re: What can I do to make my boat go faster?
first, the factory prop is probably the right one. On a motor that small with a heavy load, the prop differences will be negligable and not worth the money.
All the other fine-tune suggestions aren't going to solve an 8 mph issue.
The only two things that will make a worth-while difference are HP and hull attitude. The first is solved with money; the second with weight distribution and motor angle, each of which adjusts hull attitude.
The goal is (a) to run flat and (b) to be on a plane. Some boats, you drop the bow to get up on a plane--the water pressure from digging in raises the bow. Others you drop the stern and let the water push the stern up. There are other tricks like running over shallow water to lift the boat.
for many boats getting on the plane (hole shot) and then trimming to stay on, or run best for conditions, requires adjustments by the PTT during the process; that's not an option for you, so you have to set the motor for best all-around, then adjust the hull attitude during the process by moving weight (people) around.
We used to have a 16' starcraft BR with a 55, no PTT so to get a skiier up once we got older, we'd send everyone to the bow on take-off then back once we were up. I have a 19' flat bottom skiff with a 50 and when loaded it can have a 3 minute hole shot but once it's up on a plane it slides along like a hockey puck. Takes all the tricks to get there.
I also have a jon boat like yours; the 15 will cure your problems. BTDT.
