Convoy escort vessels are designed to protect convoys of merchant or other types of ship from attack.
The Royal Navy initiated a building programme for convoy escort vessels prior to 1939. Yet when war broke out on 3 September 1939 they were still desperately short of such specialised vessels.
In the absence of specialised escort vessels, Royal Navy destroyers were employed on convoy escort duty, particularly older destroyers from the First World War.
However they could only fulfil this role effectively after significant modification, which usually removed their ability to do the job they were originally designed for – attacking the enemy.
As German U-boats took an increasing toll on British merchant shipping, it became glaringly obvious to the Admiralty that the number of escort vessels had to increase, and quickly.
1. Bridgewater, Hastings and Grimsby class sloop
Apart from older vessels dating from the First World War, the Royal Navy’s stock of escort ships already in service in 1939 consisted of small sloops, principally of the Bridgewater and Grimsby classes, and the larger, more capable sloops of the Black Swan class.
These smaller vessels displaced just over 1000 tons and had a maximum speed of 16 knots. All carried an outfit of depth charges and mounted a pair of 4” guns and light anti-aircraft (AA) weapons. The Grimsby class carried an additional 4’’ gun.
As more modern vessels became available, these older sloops were generally re-deployed to less intensive areas of operation. Nonetheless they played an important role in combating U-boats in the early years of the war.
2. Black Swan class sloop
The Black Swan class were the best escort vessels available to the Royal Navy in September 1939.
Displacing some 1300 tons, with a speed of 19 knots, they mounted a heavy armament of 4’’ AA guns and were well equipped to defend convoys against both aircraft and submarine attack.
However their cost and quality of build mitigated against speedy construction. Additionally, it wasn’t easy to modify the design to carry more radar and anti-submarine equipment without sacrificing some of the firepower that made the class so valuable in the anti-aircraft role.
Black Swan class sloops played a vital role in the Battle of the Atlantic. The famous 2nd Support Group, which operated under the command of the anti-submarine ‘ace’ Captain Frederic “Johnnie” Walker, was initially composed entirely of the Black Swan class.
3. Flower-class corvette
It was vital that the Royal Navy found an effective escort that could be produced quickly. They went to Smiths Dock of Middlesbrough, who designed a small escort ship based on their whaling ship ‘Southern Pride’.
The design could be built rapidly and in large numbers by commercial rather than naval shipyards. The result was the famous Flower-class corvette.
Originally intended for escort work in coastal waters, the growing U-boat threat forced their wider deployment in the wilder waters of the Atlantic.
The Flower-class were small, displacing just 950 tons, with a single reciprocating engine driving a single screw to give them a maximum speed of 16 knots. Armament was limited to depth charges, a single 4” gun, and some light AA weapons.
The basic dimensions of the vessels limited modification. The crew complement originally numbered 85 but as extra equipment such as radar and high-frequency direction finding sets (Huff-Duff) was added, the crew expanded to more than 100. This put extra strain on the already cramped crew accommodation.
The most famous vessel of the class was in fact fictional. The HMS Compass Rose was the heroine of ‘The Cruel Sea’, the supreme novel of the Atlantic War written by Nicholas Monsarrat.
4. River-class frigate
The Flower-class were not ideal escorts. They were too small to add new weapons systems as the war progressed. The Admiralty therefore began work on a new larger design to incorporate all the war-time lessons learned about what made an effective convoy escort ship. The result, entering service in 1942, was the River-class frigate.
The River design increased the inadequate dimensions of the Flower-class to 1400 tons, with twin screws and machinery to give them a speed of 20 knots.
Armament comprised a pair of 4’’ guns and light AA weapons, along with an extensive fit of depth charges and a new ahead-throwing anti-submarine mortar code-named Hedgehog.
The larger dimensions gave the River-class scope for subsequent additions in radar equipment and armament.
5. Castle-class corvette
Though a more successful design, the River-class came with its own drawbacks. Smaller shipyards could not accommodate their production. To solve this problem, a modified corvette design was also produced, called the Castle-class.
The Castle-class were only slightly larger than the Flower-class and displaced just over 1000 tons. Like the Flowers they had a single screw reciprocating engine for a speed of 16 knots and carried a similar gun armament.
Where they were superior to the Flower- class was in anti-submarine equipment. They mounted a Hedgehog mortar as well as carrying a larger number of depth charges.
6. Loch/Bay-class frigate
The Bay-class frigate was the ultimate development of the River design, modified to aid mass production.
They displaced a little over 1400 tons. Their gun armament was similar to the River but they mounted a new design of ahead-throwing mortar named Squid.
Instead of the small contact fused bombs employed by the Hedgehog mortar, Squid fired a trio of conventional depth charges and was a more effective weapon.
The Bay-class were modified to serve as AA escorts, sacrificing some anti-submarine capability to mount two twin 4’’ gun turrets and a heavier outfit of automatic AA weapons.
7. Captain and Colony-class frigate
Under the Lend-Lease agreement of 1941, the United States moved away from its neutral position in the war and began supplying the Allies with materiel.
Among the supplies delivered to Great Britain were almost 100 destroyer escort vessels of the Captain and Colony classes.
They displaced 1300 tons and differed only in propulsion, with the Captain-class powered by turbines and capable of 26 knots, and the Colony-class powered by reciprocating engines producing 18 knots.
To maximise their anti-submarine effectiveness, most were retrofitted with Hedgehog mortars.