411A said....
"Hughes mentioned to Donald Douglas (Sr) later on after the one and only flight, that the tail vibrated terribly on liftoff, with a violent twisting motion."
That explains [to me anyway] why Hughes remained close to the water! To have violent twisting on take off was not a good start to a maiden flight!
He had had enough injury from aeroplanes not to want to push his luck and he wasn't flying solo was he? 29 SOB.
Looking at the newsreel footage [angles are not great] it is hard to see any tail twisting.
Mind you 8x 3000 HP plus heavy spray etc and a lowish tailplane must have collected some turbulence!
The Martin Mars has quite a large dihedral on its tailplane doesn't it?