 Complete the following phrase:  The Mackerels stemmed from design studies ordered by the Navys General Board beginning in 1936, when Admiral Thomas C. Hart joined the Board and spearheaded the small submarine effort. At the time it was felt that mass production of large fleet submarines would be impractical. It was also felt necessary to replace the aging S, R, and O-class submarines, to provide area defense for submarine bases and operate in restricted waters. For the Mackerels, two somewhat different designs were produced by the Electric Boat Company and Portsmouth Navy Yard. As prototypes, Mackerel and Marlin were near-sisters rather than sisters. References agree that Mackerel used a direct drive propulsion arrangement, but differ as to whether Marlin had direct drive or diesel-electric drive. Both had engine designs not used in other US Navy submarines; Mackerel an in-house Electric Boat design, Marlin an ALCO locomotive design. By late 1941 it became apparent that fleet submarines could be mass-produced, and interest in small submarines began to wane. Interest in them was revived with the examination of the German U-570, a Type VII U-boat captured by the British and loaned to the US. Also, Admiral Hart returned to the General Board in late 1942, following a courageous part in the ultimately futile defense of the 