Hull Rigidity

I am midway through the construction of a 15' Jon boat and have discovered the boat is quite flimsy if you twist it end to end. It appears to be very rigid everywhere else. It is stitch & glue construction with triple taped seams both sides and six coats of epoxy on the outer hull, none yet on the inner hull. It has double shear stringers, single side and bottom stringers with the seat boxes as forms.
The boat appears to be well constructed and strong so far with the exception of the end to end twist. PLEASE HELP!!

 

If you want to avoid twisting you need closed profiles. Think of a peace of paper. It is really easy to twist. But if you make a tube out of it it becomes quite rigit.

 

Add vertical knees from seat box to sheer clamp, add horizontal knees from sheer clamp to transom and sheer clamp to bow transom and from seat box to topsides stringer (if fitted), add a small deck at the bow and stern. This type of boat will never be really rigid, but these additions should stiffen it up considerably. Good luck.