I built a 6.5 hp longtail from the ebay plans. Has a 7x4 backwater prop, what they suggested for the 6.5hp engine. Top speed i can get on my 1416 flatbottom (not a heavy boat), with just me and a battery in it, is 6mph. I was thinking about going up a size on props, as the motor does not even grunt turning the 7x4 (i did a couple of mods to the engine, but nothing major). Figured it would run a little faster than 6mph... What kinds of speeds are other people getting with similar setups and what should I expect?

I get about 10 mph with me(185 lbs), lab(75) and 5 dozen deeks in a 1448 aluminum jon boat.
Swamp Ratt mud motor, 7 hp Kohler and 7x4 Ron Hill prop.

Ive got a cheap clp 6.5 longtail from tiwan and get about 10mph on a 14x36 weldbuilt crawdad with 2 ppl in the boat.

yesterday by myself and light hunting load 11mph on a 14x36 jon with a hill 7x5 prop with the cup ground off of it

switched to a Backwater 8x4 and it made all the difference in the world. Huge increase in speed and torque, now running 11-12mph and enough torque to get through and over most obstacles.