The Searay class was born out of wartime need. Starfleet needed a fast scout to act as they eyes and ears of its advance battle fleets, as such a ship that was faster than the Heston and Constitution classes was required.
The Searay added a pair of nacelles to the basic Hermes class scout design, the resulting ship was immensely fast for short periods, but notoriously short ranged. Given the mission profile intended for the class, this trade off was deemed acceptable.
The 12 ships built performed well, but were not retained postwar in operational service as the short range handicapped their performance in most peacetime roles