Rocker-reducing Background

Most (but not all) traditional wooden driftboats
were designed around 4x8 plywood. To make side
panels you would scarf two sheets together
to make a 4x16 and cut out the side panels.

The bottom chine edge often had a straight line:
the edge of the plywood sheet. That pattern holds
true for most molded fiberglass boats (that are
built around a plug, that starts off as plywood)
and aluminum too, which comes as square sheet stock.

There are some important historical exceptions to this
pattern. The traditional Rogue River boats had a curved
chine edge to their side panels (rather than straight).
But most boats still revolve around a straight chine edge.

That straight edge is what creates the rocker.
Think of it this way: if you stand a side panel up on
its edge, the straight chine is flat and flush to shop floor.
If you tilt that panel out sideways (like that side of the
boat) and then bend the ends in (they way the ends would bend,
in order to attach to a stem and transom) then the ends of the
side panel rise up from the floor, as you bend in the ends of
the tilted panel. That's where the rocker comes from.