The United States Navy Salmon-class submarines were an important developmental step in the design of the "fleet submarine" concept during the 1930s. An incremental improvement over the previous Porpoise class, they were the first US submarine class to achieve 21 knots with a reliable propulsion plant, allowing them to operate with the Standard-type battleships of the surface fleet. Also, their unrefueled range would allow them to operate in Japanese home waters. These rugged and dependable boats provided yeoman service during World War II, along with their immediate successors, the similar . In some references, the Salmons and Sargos are called the "New S Class", 1st and 2nd Groups. Design Authorized under the Fiscal Year 1936 provision of the Vinson-Trammell Act, two distinct, but very similar, designs were developed, to be built by three different constructors. The Electric Boat Company of Groton, Connecticut designed and built , , and (SS-182 to 184). The Navy's lead submarine design entity, the Portsmouth Naval Shipyard of Kittery, Maine submitted a design for the Government group, which became and (SS-185 & 186). Using the Portsmouth plans and acting as a follow yard, the Mare Island Naval Shipyard of Vallejo, California built . The two designs differed in minor details such as the locations of the access hatches for the forward engine room and crew's quarters, the shape of the horizontal conning tower cylinder, and, most significantly, the closure of the main induction valve. This difference led to casualties in Snapper and Sturgeon, and to the loss of Squalus. Larger than the design of the Porpoise-class, the conning tower installed by Electric Boat had two concave spherical ends, The Portsmouth design had a concave end aft and a convex one forward. Portsmouth and Mare Island ran into production difficulties with their conning towers, discovering cracks that caused the cylinder to fail the required pressure test. The problem was successfully fixed, but the experience caused the government yards to adopt the double concave design for the next several years. Externally, there were minor differences in the shape of the upper edge of the aft end of the conning tower fairwater. The Electric Boat design had a gradual downward taper to this bulwark, the Government design was slightly higher and straighter. Also, as built the Electric Boat trio had two 34 foot periscopes. This resulted in a fairly small periscope shear support structure above the fairwater. The three