Castle Class Corvette (Frigate) Association

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Castle Class Corvette

In its preparations for the second World war, the Admiralty had assumed that the requirements for the protection of merchant ships and the war supplies that they would carry would be very similar to that of the First World War. Then most of the anti U-boat action occurred in the Western Approaches to the British Isles where an extensive convoy system had proved effective. Convoys made it more difficult for U-boats to find their targets. Whilst the concentration of escorts made it more difficult for the U-boats to attack.

Between the wars the Admiralty, through its Anti Submarine Detection and Indication Committee had been developing an underwater sound detection system which came to be generally known as The Asdic. This was extensively fitted to H. M. Ships, especially destroyers as well as to the Flower Class Corvettes. The range at which a submarine could be detected varied considerably depending on the weather, sea temperature and the skill of the operator. The average value was about 1,300 yards with a maximum of 2,500 yards.

In terms of the number of ships that can be protected per escort, large convoys increase the effectiveness of the escort screen. Even so, the short range of the Asdic required a large numbers of escorts. Because of the limited resources available, the escorts had to be simple and easy to build; preferably in small yards. The Flower Class corvettes fitted these requirements. They were originally developed and designed by the Smith Dock Company of Middlesborough. They were based on an existing design of theirs for a whale catcher. Although designed for coastal escort work, for want of anything else they were used as ocean escorts.

In spite of their deficiencies they and their crews did a remarkable job. They bore the brunt of the Battle of the Atlantic under extremely arduous conditions. The fall of France gave the German navy new bases on the Atlantic seaboard and allowed the battle to spread across the Atlantic. It was realised that more ships would be needed with a longer endurance and with better sea keeping qualities and better weapons.

Based on the ocean experience of the Flower Class, the Smith's Dock Company came up with proposals for two new types of vessels. The single screw corvette, to become the Castle Class and a twin screw corvette, later to be designated as a frigate that became the River Class. The Board of Admiralty approved the Legend and Design of the new single screw corvette on 7th May 1943. The estimated cost, excluding guns and ammunition was £174,000 per vessel. The design called for an overall length of 252 feet, 47 feet longer than the Flower Class with a displacement of 1010 tons. The freeboard forward was increased by four feet and by three feet aft as compared to the Flowers. The ship was designed to carry 480 tons of fuel oil and have an endurance at 15 knots of 6,200 miles; 2,200 miles further than the Flower Class. With these dimensions and a beam of 36.5 ft the ships could be built by the smaller shipyards and keep the building capacity fully extended.

Much of the superstructure was designed to be pre-fabricated. The specification for the Class was :-
Length between perpendiculars 225'
Length on waterline 234'
Length overall 252'
Beam amidships 36' 6"
Displacement (tons) 1010
Mean draught, deep condition 13' 2"
Engines (1) 4 cyl triple expansion developing 2,980 IHP
Propeller Shaft 1
Speed in standard condition 17 knots
Speed in deep condition 16.5 knots
Fuel capacity (tons) 480
Endurance at 15 knots , clean bottom 6,200 miles
   Port Steam 1 60 KW
   Starboard Steam 1 30 KW
   Auxiliary Diesel 1 15 KW
Compliment Officers and Men 100-110
Armament Squid single MK 1 1
Squid projectiles 81
4" Mk XVI gun on MK XXIV mounting; 300 rounds fixed shells
Rocket Flares 64 MKII w
Single Oerlikons 2
Twin Oerlikons 2
Depth Charges 15 carried on deck only
Depth Charge Throwers 2 (MK iv)
Depth Charge Rails 1

Forty ships were commissioned; Twelve of these were transferred to the Royal Canadian Navy and one was operated by the Royal Norwegian Navy. The remaining 27 were commissioned by the Royal Navy.

The Protection of Convoys

The ships were designed primarily to protect merchant ships from attacks by U-boats, and to provide a certain amount of protection from close range air attack. In particular the ships were designed with the expectation that their main sphere of operation would be in the North Atlantic. In this they were particularly successful. During the period 11th December 1943 to 6th June 1945 the Castle Class Corvettes escorted 237 ocean convoys comprising just over 12,000 ships for a loss of less than 10 merchant ships by enemy action. During that time they were associated with the destruction of 7 U-boats for the loss of three corvettes. As the ships were commissioned they were placed under the command of the Commander -in- Chief Western Approaches (CinC W.A.) and allocated to Escort Groups

Escort Groups

Convoy escorts were organised into Escort Groups. Each Group comprising some six ships. The Senior Officer of the Group was usually accommodated in a Frigate - larger and faster than the corvettes that formed the bulk of each Group. At the time when the Castle Class Corvettes began to come into service the Flower Class Corvettes formed the main body of the Escort Groups escorting convoys. At this time specialised Support Groups were being formed. These comprised of Groups of Frigates which moved from convoy to convoy in response to the tactical situation and concentration of U-boats. They provided additional anti-submarine detection and fire power to individual convoys as they passed through areas of heightened danger.

The one Royal Norwegian Navy Castle Class Corvette and the Royal Navy Castle Class Corvettes were distributed between the B1 to B7 Escort Groups, B 21, B 22 and B 23 Escort Groups. Castle Class Corvettes were also included in the 7th and 8th Escort Groups for Russian Convoy duties. Two Escort Groups, the 30th and 31st Escort Groups, were formed wholly from Castle Class Corvettes . These two Groups were allocated patrol and convoy support roles in the western and northern coastal waters of the United Kingdom.

The Royal Canadian Navy Castle Class Corvettes were distributed between the C 1 to C 8 Escort Groups. These groups never included more than 2 Castle Class Corvettes . The Canadian Escort Groups were wholly occupied in escorting trans-Atlantic convoys, whereas the British Groups were also involved in escorting convoys between the UK and Gibraltar.

The hull design provided a very dry ship and it was rare for the bridge personnel to be drenched by spray and adequate shelter was provided for the Squid and Gun crews on watch. There was only one really bad design fault. Under certain conditions of rough weather the quarter deck would ship green seas thus becoming a hazardous place to work.

The excellent sea keeping qualities of these ships is illustrated by the performance of the RCN "ORANGEVILLE" when she encountered hurricane force winds in the general position of 46°N 26°W in February 1945 whilst forming part of the escort for convoy ONS 41. During the afternoon and until shortly after midnight the wind was blowing from the SW strength 11 with the sea and swell recorded as 77. The convoy was hove to. During the night "ORANGEVILLE" realised that she had overtaken part of the convoy and although the wind had moderated to force 9 the sea and swell were 67, "ORANGEVILLE" altered course 180° to regain contact with a bunch of straggling ships. Two hours later she again altered course 180° with the sea and swell still recorded as 67. And wind force 9. The ship's deck log does not record that any damage was incurred by these manoeuvres.