To meet a post war requirement for fast target craft, the RAF authorised the conversion of four 68ft HSLs to Remote Controlled Target Launches (LTRCs). The four selected craft were all re-allocated to Vosper Ltd, Portsmouth in 1948, the first HSL 2631 transfered on the 2nd February with the remainder on the 16th April. The conversion involved removing a substantial portion of the superstructure aft of the bridge, including the guns and turrets. The resultant large open aft deck was then covered with armour plating to withstand vertical attack from 25lb break-up bombs dropped from a height of 25,000 feet. A large radar reflector was installed, composed of three of three square intersecting plates over which was fixed a large horizontal "T" with lights. The remote control box, developed by inventor P.F Parfitt was simplistic. Eight orders could be given to the launch through the radio equipment, five push buttons mounted on an eight-inches square mobile control unit, enabled the operator to start and stop engines, open and close the throttles and turn on the launches lights. A sixth button provided a "spare" command. A further control switch geared to a compass card enabled the launch to be steerd. The normal controls were still available, the changeover from normal to automatic (remote) control was possible within two minutes.
After conversion by Vosper Ltd, three of the LTRCs were put back into RAF service in October 1949. LTRC 2631 was allocated to serve with 1110 MCU out of Immingham whereas LTRC 2678 went to 1103 MCU Felixstowe. LTRC 2690 went to 1101 MCU and operated out of Fowey.
LTRC 2688 remained with Vosper Ltd for trials, initially for a six month period, however on the 30th November 1950, the trial period was extended to last until 23rd January 1951. Whilst with Vosper Ltd, the craft was demonstrated to many foreign services to show off the advantages of the remote control system. During this period the craft was painted overall in yellow, thereby gaining the nickname the "Yellow Pig" The LTRC 2678 operating out of Felixstowe was also painted yellow for some of the time she was remotely controlled. The other pair may well have been similarly re-painted, however photographic evidence available only shows the pair painted in traditional RAF home livery.
The LTRCs were not successful and the three in RAF service were subsequently handed over to Groves & Guttridge for full conversion to RTTL MK.I standard. The first pair were handed over in June 1951, with LTRC 2690 following a month later. LTRC 2688 remained with Vosper Ltd, who carried out a full conversion to RTTL Mk.I standard after completion of the LTRC trials and demonstrations. LTRC 2688 came out of Vosper Ltd with a new mast arrangement, which was very similar to that fitted to later RTTL Mk.IA craft.
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