Rothesay Class Frigate - Modification

Modification

Increasing submarine performance in the 1960s demanded detection and engagement of targets at a greater distance from the fleet. Detection was improved with new sonar designs such as the Type 177 search and Type 199 Variable depth. To attack targets at a greater range, the Royal Navy adopted the MATCH (Medium-range Anti-submarine Torpedo Carrying Helicopter) system. MATCH was essentially the Westland Wasp HAS.1, a lightweight navalised helicopter small enough to operate from a small hangar and flight deck that could be fitted to contemporary frigate designs yet large enough to carry a pair of anti-submarine homing torpedoes (US Mark 44 or 46 types), allowing engagement of underwater targets at some distance from the parent vessel, outside the range of the shipboard Limbo anti-submarine mortars. To allow MATCH to be carried, all of the Type 12I class were modified and modernised, beginning with Rothesay from 1966 and finishing in 1972.

The after superstructure was removed, along with the foremost Limbo mortar, with the well being plated over to create a small flight deck. A small hangar was constructed in front of this, on top of which the GWS-20 Sea Cat missile and director was (finally) shipped. The mainmast was replaced by am enclosed design, carrying the Type 1010 IFF antennas, with the funnel height was increased to carry the hot exhaust gasses over the taller superstructure. The electronics fit was also upgraded from the World War II era sets fitted in the Whitbys. A large, enclosed foremast replaced the short lattice, carrying the distinctive "half cheese" antennas associated with the Type 993 target indicator. The Mark 6M director was replaced with the MRS3 system carrying radar Type 903, allowing the removal of the Type 277Q height finder. Additionally, Knebworth/Corvus 3-inch countermeasures launchers were fitted on either side of the bridge, as were a pair of World War II vintage 20 mm Oerlikon guns for "policing" work (and strictly limited anti-aircraft defence).

The extensive modifications of the Rothesays brought their fighting capabilities into line with the that of the original Leander-class vessels. Later in the early 1980s, Rothesay, Yarmouth and Plymouth had an extensive refit, which included upgrading their main radar and target indicator to the 994 radar which was essentially a version of the Plessy AWS1 in the old antenna, giving in particular faster updating, to about one second delay on the screens in the Rothesay's operations room. This recent refit and marginally better radar made them somewhat more suitable for use in the Falklands War and patrols in the South Atlantic, in the immediate aftermath than many of the other Type 12's. In the late 1970's their also serious plans to refit 4 other Rothesays as Towed Array frigates, but Leanders were used instead after the 1981 defence review and Faklands war reassessment.