Prior to the Balao class, U.S. submarine hulls were built of mild steel. The thickness of hull plating until about 1943 was specified in terms of the weight of a square foot of plate rather than the actual thickness, and this was gradually increased.
With the Balao class, the material of the pressure hull was changed to high-tensile steel (HTS).
A particularly good example of weight-saving through careful hull design was Andrew I. McKee's treatment of the single-hull sections (forward and aft torpedo rooms) of the boats for which he prepared detailed designs at the Portsmouth facility. This feature became one of the distinguishing differences between the Portsmouth and the Electric Boat hulls. Very few operating submariners were even aware of it.