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
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
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
Length overall 252'
Beam amidships 36' 6"
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
Endurance at 15 knots , clean bottom 6,200 miles
Port Steam 1 60 KW
1 30 KW
Auxiliary Diesel 1 15 KW
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
Flares 64 MKII w
Single Oerlikons 2
Twin Oerlikons 2
15 carried on deck only
Depth Charge Throwers 2 (MK iv)
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 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
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.
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
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