The Siva class single engine destroyer was an adaptation of the Saladin class building upon feedback from the active Service of that class. The Siva used the FWC warp drive rather than the FWD of the Saladin. This provided a potent mix of speed and maneuverability as well as additional power compared to it’s older cousin (however the balance returned to the Saladin in 2255 when the FWD-2 equipped Saladin Mk2 entered service). The internal arrangements of the Siva were considerably different to those of the Saladin, hence the classification as a new class as opposed to a new mark.
Unusually the prototype of the class was the USS Siva from the Siva class of 2243, damaged beyond sensible repair she was massively refitted to this more standard configuration as opposed to the missile armed configuration of the previous design. Upon the recommissioning of the Siva in this new configuration the former class was renamed the William Howe class.