Hull Structural for the Type IXC, IXC40, IXD2, XB and XXI.

The cylindrical pressure hull had a diameter of 4400mm (14.42') and was made of 18mm (,73") steel with inside bulb tee frames 200X11 (7.88" web depth X .43" web thickness) on 700mm (27.56") centers. The plating is gradually reduced throughout the tapered section fore and aft to 16mm (.63"), and the frames are correspondingly reduced to 130 X 9 (5.12" X .35"). Frame spacing remains unchanged in the tapered sections.

The specification for the pressure hull plating and frames calls for a tensile strength of 74,000 psi with a yoeld point of 51,300 psi. The steel was known as No. 52; the specification for plates is KM 9104, and for the frames is KM 9103.