From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 107-110:
The British response to the U-boat threat was to establish a convoy system. Though convoys had been used by maritime powers to protect trade since the Age of Sail, the Admiralty had been initially reluctant to embrace the concept during the First World War. After all, a convoy conveniently clustered all the merchant ships together, thus creating a target-rich environment for a stalking U-boat. Then, too, convoys necessarily had to proceed at the speed of the slowest vessel. Despite these apparent defects, however, the events of 1917–18 had proved that convoys were by far the most effective countermeasure to a U-boat threat, and in 1939 the British established a convoy system even before the war began.
From the start, each convoy was identified by a code that indicated its origin, destination, and numerical sequence. The first outbound convoy from Liverpool, for example, was OB-1. Eventually, regular convoys were established for routes from Gibraltar (HG), Jamaica (KJ), Freetown, Sierra Leone (SL), and scores of other places, though the busiest and most important route was the transatlantic one between Halifax, Nova Scotia, and either Liverpool or the Firth of Clyde (Glasgow) in Scotland. Eastbound convoys from Canada to Britain were designated as HX convoys (homebound from Halifax), and westbound convoys were ON convoys (outbound to North America). Typically they consisted of twenty to forty merchant ships organized into seven to ten columns of four or five ships each. To avoid collision in rough seas or heavy fog, the ships in each column steamed at intervals of four hundred to six hundred yards, and the columns themselves were a thousand yards apart. As a result, a forty-ship convoy filled a rectangle of ocean five miles wide and two or three miles long, an area as large as fifteen square miles.
The merchant ships were under the supervision of a convoy commander, a civilian who was usually a retired Royal Navy officer and who rode one of the merchant ships as commodore. His job was to maintain order within the convoy and issue the periodic course changes by flag hoist or blinker light that kept it zigzagging across the sea, a protocol designed to throw potential attackers off their stroke. Maintaining order in a convoy was often difficult since civilian merchant captains were unused to making the precise tactical maneuvers required to reorient forty ships simultaneously on a new course. The commodores necessarily had to adjust their expectations of instantaneous execution when ordering a course change.
In the van and on the flanks of this large rectangle of ships, and often maneuvering independently as well, were the armed escorts. If Dönitz was frustratingly short of operational U-boats, the British were equally deficient in the number of available escorts. Destroyers were the most effective convoy escorts, but destroyers were needed everywhere, and the heavy losses during the Norway campaign and especially off Dunkirk meant that the Royal Navy had a severe shortage of these critical workhorse warships. To make up the shortfall, all sorts of vessels were called into service for escort duty.
Among them was a new type of small warship called a corvette. Because the first generation of corvettes were all named for flowers, they were known as Flower-class ships and they bore such unwarlike names as Azalea, Begonia, Bluebell, and Buttercup. At only 940 tons each, they were tiny and carried only a single 4-inch gun on their foredeck plus twin .50-caliber machine guns; against virtually any conventional warship they were all but helpless. They were not only small, they were also slow. With a maximum speed of sixteen knots, corvettes were no faster than a surfaced U-boat. They were nearly as uncomfortable as well, especially in the volatile North Atlantic, where even in a moderate sea they bounced around like so much flotsam. A crewman on the Rhododendron recalled that being on a corvette “was like a terrier shaking a bit of rag. The old ship [would] corkscrew up on top of a wave and you’d be up and you’d look down into this trough and you’d think crikey, and the next thing you’d be down in there and a bloomin’ great wave’d come over the top.” That, plus the fact that a crew of fifty men was crammed into a 190-foot hull made service in a Flower-class corvette a challenge to one’s constitution and endurance. The novelist Nicholas Monsarrat, who served three years in corvettes, vividly recalled the challenge of simply eating a meal: “When you drink, the liquid rises toward you and slops over: at meals the food spills off your plate, the cutlery will not stay in place. Things roll about and bang, and slide away crazily.” Standing topside watch was an ordeal. “Every night for seventeen nights on end,” Monsarrat wrote, “you’re woken up at ten to four by the bosun’s mate, and you stare at the deck-head and think: My God, I can’t go up there again in the dark and filthy rain, and stand another four hours of it. But you can of course.”
On the plus side, the corvettes were inexpensive, could be built quickly, and had both Asdic [early sonar] and depth charges. Churchill extolled them as the “Cheap and Nasties,” meaning that they were cheap to build and nasty to the enemy. Fifty-six of them were laid down prior to September 3, 1939, and forty-one more soon after the war began. Eventually, Britain and Canada built 269 of them, including 130 for the Canadian Navy. Despite their floral names, minimal armament, and cramped quarters, they played a crucial role in sustaining Britain’s maritime lifeline to the outside world.