As early as 1934 it was realized that the existing R and S-class submarines would need to be replaced at some point. A smaller submarine had value in some areas of the Pacific, around Australia, New Guinea, and the Solomons area where transit distances would be greatly lessened. While the big fleet boats were out operating with the fleet, some type of submarine was needed for defensive patrols near the U.S. coast and the Panama Canal. Admiral Thomas Hart of the Navy's General Board became an ardent supporter of the concept, and worked hard to convince the fleet boat minded operating forces that a smaller submarine was needed. Indeed, most of the force was in favor of a S-boat replacement, but only if they could be built without affecting the production of the big fleet boats.
Hart's efforts resulted in two 800 ton boats being inserted into the Fiscal Year 1939 appropriations. They were essentially improved, but slightly smaller, versions of the earlier Cachalot. Unfortunately for Hart's program, the opening of hostilities and the shocking early Japanese victories immediately swept away any thought of building any more of the smaller patrol submarines. The Navy had committed itself to the long-range fleet boat concept and history has proven that to be a wise choice. Mackerel (built at EB) and Marlin (Portsmouth) were well designed and built and in general were successful boats. They remained stateside for the entirety of the war conducting valuable yet somewhat unglamorous training missions and anti U-boat patrols. If the Mackerel class had been started earlier and built in reasonably large numbers they would have made fine replacements for the obsolete (by 1941 standards) S-boats. See this table for their specifications.
Both boats would have starring roles in the 20th Century Fox 1943 movie Crash Dive.
Mackerel (SS-204)
USN photo # 19-N-23722 from the Bureau of Ships Collection in the U.S. National Archives.
Mackerel running builder's trials in Long Island Sound, March 22, 1941. In general, she was visually similar to the Salmon/Sargo class, just smaller. At the tip of the bow is the round towing fairlead, also known as a "bullnose". Aft of the bullnose is a double height hydraulic capstan, used to assist in line handling while tying up to a pier. Her bow planes are folded up alongside the superstructure, and she has been equipped with a bulky conning tower fairwater similar to the fleet boats.
