It would make sense that a cylinder is the easiest shape to roll, but it is not neccessarily true. It would be true if the center of mass was aligned with the center of the cylinder. A wide, flat boat does have greater initial stability and is harder to roll. But you were talking about making a low volume boat taller without changing the width. In this case the boat with the lower deck in the area of the cockpit will be easier to roll.

Increasing the volume generally makes the boat harder to roll whether you do it by making the boat wider or taller.

: I supose that you can look at it as a cylinder is the easiest shape to roll -
: thus log rolling. Once you add a hump, it's like having a branch sticking
: out of that log. It'll push water. A wide, flat boat should have the best
: initial stability, but that stability works upside down as well, making it
: harder to roll.