The prop is a 10" diameter by 7" pitch four bladed model. Since the boat isn't equipped with a tachometer I don't know how many RPM it's turning so I don't know if I should try more or less pitch. I can get a smaller diameter prop as well. But trying different props can get expensive. The Mercury Pro Kicker is actually designed to be a "kicker" motor for a sailboat. I know a two stroke motor would do better. I have a lot of experience with two strokers being a motorcycle mechanic. These four stroke engines just don't have what it takes and I don't like 'em. They're twice the size, twice the weight, and half the horsepower. But, they are cleaner.

I have the same Mercury 9.9 Kicker on my 14' jon boat. It came with a 4-blade propeller, but I changed it for a 9.5 x 8 three-blade prop. I go just under 15 mph when by myself, but I sit in the back of the boat with a tiller. My battery and gas tank are also at the back. I weigh about 155 lbs. When my 180 lb son is in the front seat, I slow down considerably. I have the same issue as you - I had a 20 hp two-stroke on this boat, and it went fast as I want to go, but the closest lake to my house only allows 10 HP, so I switched out to the 9.9 four stroke.