Step 8: Rib shape
As you proceed toward the bow, the hull will get narrower and taller so that the ribs start becoming more arched in the center.
As you move toward the stern of the boat, the hull becomes flatter so the ribs tend to stay flat in the middle with the bends concentrated near the edges until the very end.
In general, the flatter the ribs are in the middle, the flatter the bottom of the kayak will be and the more stable it will feel.
Ribs that are more uniformly arched throughout will give you a more rounded hull that is less stable but slightly faster and livelier than a flat bottomed hull.
If you are building your first kayak or don't have much paddling experience go for the more flat hull.