Please look at the bottom of the attached hyperlink.
You will observe the highest stresses are just away from the fixed support, when displacement is added.
http://web.mit.edu/calculix_v2.0/CalculiX/ccx_2.0/doc/ccx/node6.html

Oversimplifying the hull in front of the main beam acting as a cantilever, from statics the highest shear, bending moment, and torsion occurs at the main beam,
with the least rate of displacement due to bulkhead/reinforcement.

Traveling just a little ways from the main beam there is not only high bending, shear & torsion, but now add rate of displacement.
Cyclic section distortion (sans bulkhead) as the hull displaces under load (in,out,& twist) compress & uncompress the foam, as the fiberglass inner and outer skin is 10x stiffer than the foam.
The inner & outer layer move at different rates to accommodate hull distorsion, the foam eventually looses the battle.
The '70s Tornados added a bulkhead in front of the main beam for reduction of stress & deflection.