Re: boat rigidity
If you have all tubes inflated to its factory working recommendation pressure which is usually 3.5 PSI will accomplish a rigid sib, bear in mind that with defeleted tubes lower floor fabric will rock along floorboards and seat. Correct pressure on tubes and keel gives floor fabric rigidity.
OTOH seat's end has 2 attached side hooks, which inserts on side tube's seat patches, this gives a floating like situation as opposed to a solid transom which holds tubes tight in place, so seat will rock accordingly and will not provide side tube stiffness as one would imagine specially if underinflated. In order to achieve a well tensioning seat on hooks, side tubes must be fully inflated to their working PSI.
Happy Boating