Dear Peter, boundary conditions and loads are shown in pictures below (they are referred to the 2nd picture above). Boundary conditions are established by a regulatory body in ship structrural certification therefore I am not sure about the background to require straight edges but usually is because the panel is sorrounded by many other panels, therefore edges are assumed ideally straight.

I tried using simetric conditions in edges B3 a B4 but, in this case, edges can not be loaded. I hope this helps. Thank you in advance.