I may be all wet here (no pun intended) but in my experience and studies with
the Merchant Marine and later the U. S. Navy, I was always taught that a ship
that hogged was higher in the center than at either end. Conversely, a
sagging ship would be lower in the center. In constructing long, rather
hollow ships intended for the transport of heavy cargo or liquid (tankers),
it was usually desirable to build a slight hog into the ship. This can be
likened to an arch or a preload on a truss. When the ship is properly
loaded, the hog will virtually disappear and the strain of the load is less
than if the ship were built perfectly level and then loaded.
Also, in extremely high seas with long swells, hogging and sagging can occur
as the waves travel from one end of the ship to the other. I have
experienced seas where the ship literally climbed up one side of a swell
(wave) and then pitched over and raced down the other side. Quite a
vibration is set up when the prop(s) momentarily leave the water.
In reviewing numerous photographs and drawings of sternwheelers that operated
in Western Florida and the adjoining rivers of Alabama and Georgia, it
appears that all have truss supports built into them. These are in the from
of large beams (poles?) that originate in the hull and below the waterline
and extend upward at an approximate 30 degree angle from the vertical usually
protruding through the top deck. Where they can be see above the upper deck,
they are tied together longitudinally with what appear to be taut cables.
Some are even tied athwartship. What type of load distributing structure
that is internal can only be guessed at. However, it seems a good guess that
theses beams and cables are there for the express purpose of strengthening
the ship's structure much like a truss bridge across a creek (that's a small
river way down South).
Harry