The two submarines of the S-7 Class were ordered to replace the S-1s, which had been laid up in reserve to function as school boats. They were bigger than the previous S-3 Class, and carried four 18" torpedo tubes instead of two. They were also subdivided into three compartments with watertight bulkheads for increased survivability - engine, control room, and torpedo room. Meant to be squadron leaders, they could be seen with two of the previous S-3s nearby on exercises or excursions into the Gulf for target practice.
Still designed for coast defense, their range was limited by fuel load and provisions (limited to dry goods and whatever the crew could load aboard for a few days).
These would be the last submarines designed with gasoline engines. Electric Boat would begin building safer, less noxious diesel boats instead. The Submarine Force would have no choice but to follow the change in technology.