Sure does seem to be a theme developing, namely that the most likely explanation is incorrect inflation, particularly of the keel. Pretty typical newbie mistake, right?
It was precisely this type of helpful feedback I had hoped for from my post, so I thank everyone for their considered input. I will be checking the inflation and re-testing the boat this weekend, so will post my results.
Jyasaki's description of the boat is the same as the one I have - 2 stringers per side, both equal length, with floor pieces 1 and 2 not attached to stringers and held in place by the side of the boat only.
Thanks RIB (SIB) net