I'm trying to glue up a 24x36 panel out of 1x8s that have been ripped
down to 6 inches wide. When I assemble the boards there is a small but
significant gap in the middle of each seam (wide enough that I couldn't
over come it with clamps). I layed the edge of each board on a flat
surface and sure enought, each of the boards is slightly hour glass
shaped.

When I ripped the boards I took 1/4 inch off of one side, flipped the
board then ripped it to width. The blade is a WWII and the fence and
blade were just trued up. The 1x8s were pine (S4S) and reasonably
straight and unbowed. I can't figure out what in my setup or technique
would cause this "hourglassing".

But wait, there's more . . . a this will probably sound even stranger.
Not having a planer and only having 4 boards that needed to be trued, I
figured I could set up the router and a straight bit to accomplish the
same thing. I clamped down the subject board and used a 8ft Tru-Grip as
my straight edge. I took .030 off of the edge of the boards, then layed
out the boards again. Now the gap between the center 2 boards
(previously about .040) was now double that! The Tru-Grip is not bowed
and I kept the point of contact between the router base and the
Tru-Grip at the same position for the entire cut. Also the gap can be
seen from top and bottom.

The first issue confused me . . . the 2nd is shaking my faith in
geometry.

Anyone have an idea why this is happening?

Thanks,
Mike