Stockamant?
It's simple; place a Heavy Reactor of the latest type on your ship, that'll provide it with enough supplies to last a good while. What I usually do, is put on the command/control systems, THEN the engines, and THEN I start worrying about other things - weapons and the like. That way you can choose whether or not it's a fast or slow ship, and if it's range is short or long, before worrying about other stuff. In a Medium tech start game, I've currently got two types of Ark ships:
1. A type with 3 movement and 620 cargo space, for inner-system colonization of my home system.
2. A type with 7 movement and 20 cargo space, for colonization further away.
Both have 1 Heavy Reactor and 2 normal Reactors (to fill up space). The first type essentially has a longer range, but does not use it because of the fact that it's so slow. The second type, on the other hand, has a shorter range, but gets to where it's going much quicker, albeit with less population on board.