Design
The Lochs were based upon the hull of the preceding Riverclass with increased sheer and flare to improve seakeeping and modified to suit it to mass pre-fabrication, with sections riveted or welded together at the shipyard. Accordingly, as many curves as possible were eliminated, producing a noticeable kink in the main deck where the increased sheer forwards met the level abreast the bridge.
The fighting capability of the Loch class was a great jump forward, being based around the new Squid ahead-throwing A/S mortar. Previously, escorts had attacked with depth charges, requiring the attacking vessel to pass over the submerged submarine and drop warheads over the stern. Consequently, the ahead-looking ASDIC (sonar) set lost contact at the vital moment before the weapons were launched, allowing a skilled submarine commander to take successful evasive action. Squid, however, lobbed three 12-inch (300 mm), 390 pound warheads up to 600 yards (550 m) ahead of the launching vessel falling in a triangular pattern with sides of 40 yards (37 m). As the clockwork fuses detonated the warheads at a depth determined by the ships' ASDIC set, the Squid therefore attacked in full ASDIC contact for greater accuracy and lethality. Two mortars were carried, with the bombs set to explode at different depths in order to create a convergent pressure wave to crush the hull of a submarine. The first successful "kill" was by Loch Killin on 31 July 1944, sinking the German submarine U-333. Such was the utility of Squid that depth charge carriage was reduced to only 15, with a single rack and a pair of throwers being carried at the stern, resulting in a clear quarterdeck compared to the eight throwers, two racks and stowage for 100 plus charges in the Rivers.
The weapons outfit was completed with a single QF 4-inch (100 mm) Mark V gun forwards. An oversight in the River design had been to place the gun in the favourable position forward on the shelter deck, with the ahead throwing Hedgehog on the main deck, exposed to the worst of the seas breaking over the fo'c'sle, consequently this was reversed in the Lochs; the Squids being on the shelter deck behind protective bulwarks. For anti-aircraft (A/A) defence, a quadruple mounting Mark VII QF 2-pounder was shipped aft along with up to 12 20 mm Oerlikon guns, in 2 twin powered mounts Mark V in the bridge wings and 8 single pedestal mounts Mark III. Loch Arkaig, Loch Craggie, Loch Eck, Loch Glendhu, Loch Tralaig and the South African Navy ships HMSAS Good Hope, Natal and Transvaal carried single Bofors 40 mm Automatic Gun L/60 mounts in lieu of the twin Oerlikons.
In addition to the new weaponry, the Lochs also carried new sensors, in the form of Radar Type 277. This set utilised the cavity magnetron to transmit on centimetric wavelengths for target indication purposes, excelling at picking out small targets such as a submarine periscope or snorkel from the surface clutter. The increased weight of the stabilised antenna array and the carriage of HF/DF at the masthead meant that a lattice mast was stepped in lieu of the traditional tripod or pole. Some vessels completed with Radar Type 271 or 272 and the associated "lantern" radome until Type 277 became available. ASDIC Type 144 was carried for search and attack functions with Type 147B used for depth finding.
As with the previous wartime escort designs, mercantile machinery was adopted to speed construction, with a pair of 4-cylinder vertical triple-expansion reciprocating engines. Loch Arkaig and Loch Tralaig were fitted with Parsons single reduction steam turbines to establish the feasibility of such an installation, but it was not possible to provide enough sets of turbines for all vessels. One advantage of the use of mercantile machinery was that it was instantly familiar to the mainly reservist and volunteer (RNR and RNVR) crews who manned these ships.
Service
Of the one hundred and ten vessels ordered, twenty-eight were built as frigates, entering service from 1944. Another two – Loch Assynt and Loch Torridon – were converted while building and completed as Coastal Forces Depot Ships, being renamed HMS Derby Haven and HMS Woodbridge Haven. Due to a need in 1944 for a version fitted as anti-aircraft vessels with the British Pacific Fleet, twenty-six units were authorised for completion to a modified design labelled the Bay-classfrigate and were renamed. A further fifty-four Loch-class vessels were cancelled in 1945.
Of the twenty-eight Loch-class frigates completed as such, Loch Achanalt, Loch Alvie and Loch Morlich were transferred to Canada in 1943 but retained their Royal Navy names and were returned after the war; a fourth ship – Loch Fionn – was also earmarked for transfer but was retained by the Royal Navy as a Bay-class frigate. Loch Ard, Loch Boisdale and Loch Cree were transferred to South Africa as Transvaal, Good Hope and Natal respectively on completion. In 1948, six vessels, including two of the ex-Royal Canadian Navy trio, were refitted from reserve and transferred to New Zealand. During the Korean War, the Royal Navy reactivated several vessels and transferred them to the Mediterranean where they released Ch-class destroyers for war duties. In 1964 Loch Insh was transferred to Malaysia.
Modifications
Loch Assynt and Loch Torridon were modified whilst under construction to depot ships for coastal forces, armed with a twin QF 4-inch (100 mm) Mark XVI on a single mounting Mark XIX forward and six single 20 mm Oerlikons. With the war in the Atlantic won by 1944, and a need forecast for additional fleet A/A escorts for the Royal Navy's increased Far Eastern commitments, twenty-six Loch class were redesigned and renamed (of which nineteen were completed) as Bay-class anti-aircraft frigates.
In 1953, seven vessels were modernised; Loch Alvie, Loch Fada, Loch Fyne, Loch Insh, Loch Killisport, Loch Lomond and Loch Ruthven. The single 4-inch (100 mm) gun was replaced by the ubiquitous twin 4-inch (100 mm) mounting HA/LA Mark XIX and the A/A weaponry was standardised as a single twin mounting Mark V and four single mounting Mark VII for the 40 mm Bofors gun, with a Simple Tachymetric Director shipped for the Mark V. Radar Type 277 was fitted with the new ANU antenna array. Loch Killisport sported a gunshield manufactured from GRP to test this material for suitability of constructing the housing of the new 4.5-inch gun Mark 8. Loch Fada paid off in 1967 and was used to test an early vertical launch variant of the new Sea Wolf missile.
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