The single most significant technical achievement in the development of Dolphin is the pressure hull itself. It is a constant diameter cylinder, closed at its ends with hemispherical heads, and utilizes deep frames instead of bulkheads. The entire design of the pressure hull has been kept as simple as possible to facilitate its use in structural experiments and trials. Hull openings have been minimized for structural strength and minimum hull weight, in addition to eliminating possible sources for flooding casualties. The submarine has no snorkel mast; her one hatch must be open while her diesels are running.