Old Boat, Reverse Sheer

I know what you're thinking. That boat was warped on account of that concrete pillar coming out of the cockpit. But not so. The concrete pillar coming out of the cockpit is an accident of framing.  I could tell when I was taking the picture that maybe my framing was not that good, but I was not in the mood to reposition the boat just for a picture.
Anyway, what I am trying to show here, and it may be more obvious if you click on the photo to see a larger version of it, is that the sheer line has gone into reverse, that is, the sheer is higher in the middle than at the ends. And the bottom of the boat has flattened out to the point of almost no rocker. 
The sheer line for the non-nautical is the line made by the tops of the gunwales, that is, the two boards that form the edge of the deck.
This was not a boat I built, but I am sure that when it was built, it had more sheer and rocker and that the downward movement of the bow and stern with respect to the middle of the boat was the result of the nylon skin tightening up over time. 
The other thing I noticed when I build boats is that since both the keelson and the stringers are bent into place over the ribs, they exert a downward force on the ends of the kayak that combined with the force of a shrinking skin will tend to make the keel line of a skin on frame boat more straight over time.
I am theorizing here, but the Unangan (Aleut) builders always built their keelsons in two or three parts and the parts were carved into a curved shape, not bent into place to avoid this kind of downward pressure on the ends of the boat.