The value of convoys to protect merchant ships from submarine attacks was well known in WWI. Why was the lesson forgotten in early WWII?

Convoy was not forgotten, and was implemented from the very start of WWII. However, convoys were poorly, or even not, escorted during the early years of the war. This stemmed partly from a lack of ocean-going escorts, but also from a misapplication of the lessons of WWI to the new war.

In the interwar period, it was generally understood within the Royal Navy that should Germany return to a policy of unrestricted submarine warfare, then convoy should be reintroduced. Convoys were a frequent topic of discussion even before German rearmament really took hold. In 1934, a document entitled Merchant Convoy Instructions was published by the Admiralty. This outlined the ways in which convoys should be implemented, and attempted to predict ways in which submarines might attack convoys. In January 1937, a Shipping Defence Advisory Committee was set up by Admiral William James. This brought together civilian analysts, naval officers, technical experts and merchant owners to discuss the defence of trade in a forthcoming war. One of its seven subcommittees specifically discussed the organisation and command of convoys. In July of the same year, Admiral James specified Admiralty policy with regards to convoy: 'the convoy system is considered by the Admiralty to be the most effective form of protection against surface, submarine or air attack’. A paper published in the same year by the Committee for Imperial Defence stated that 'In the case of unrestricted warfare, convoy will be adopted and all A/S vessels available will probably be required as escorts'.

Meanwhile, the RN was building a considerable number of ASW escorts. Most of the ships produced or designed were intended as coastal escorts. These were to protect what was thought to be the most vulnerable convoys, those along the East Coast of the UK, which German subs could most easily reach. This included a considerable number of ASW trawlers, plus 22 sloops ordered between 1927 and 1934. In 1938, it became clear that these ships would not provide sufficient escorts for the convoys needed, so design work began on what would become the Flower class corvettes. The Hunt class of destroyer escorts was also begun to provide primarily anti-aircraft defence for these convoys. For open ocean work, focus was mainly on destroyers, which could also be used for escorting warships and for naval combat. From 1934, work also began on a series of long range sloops, designed primarily as ASW escorts. Finally, the Flower and Hunt classes could be pressed into use for this role, though they were short-ranged thanks to their coastal origins.

Following the beginning of WWII, the Admiralty interpreted the sinking of the liner Athenia on 3 September 1939 as the start of unrestricted submarine warfare. As such, convoy was officially implemented on the 7 September. However, an unofficial convoy had sailed on the 2 September; eight tankers heading to the Persian Gulf had been held at Gibraltar until it became clear whether or not Italy would join the war. These ships sailed together, taking the long way round Africa, with an initial cruiser escort. On the 7 September, two convoys sailed from the UK to Canada; OA 1 from the South Downs, and OB 1 from Liverpool. On the 16th, the first convoy sailed from Canada, with HX 1 leaving Halifax with 15 ships. Convoys heading from the UK received a small escort, usually just a destroyer or sloop. This would remain with the convoy until it reached 15^o West, when the escort would depart to join a convoy sailing from Canada. From the start of 1941, this moved to 20^o West. The convoy would proceed to its destination unescorted. Similarly, fast ships could sail independently, as it was believed they would sail fast enough to escape attack. In this case, the lower limit on speed was 15 knots, dropped to 13 in November 1940.

Meanwhile, much of the RN's ASW resources were being focused on hunting U-boats. It is key to remember that WWI experience showed that, while convoy was good at protecting merchant ships, it was bad at killing U-boats. WWI-era convoys were escorted by two-three destroyers or sloops, which were primarily tasked with driving submarines away from the convoy. Killing submarines was more the preserve of hunting groups, or of roving decoys. The interwar period did little to dispel this idea, that hunting killed submarines, not convoys. The idea was encouraged by the views of people like Winston Churchill, First Lord of the Admiralty, who saw convoy as defensive, while the more offensive hunting tactics were more appropriate for the RN. The view that hunting was better at killing submarines was arguably correct - later in the war, when sufficient escorts were present for convoys, hunting groups, directed by radar, aircraft, and signals intelligence, proved to be very capable at destroying U-boats. However, in the early war, when few of these advantages were present, hunting groups served solely to divert scarce resources away from where they were needed, with the convoys. Fortunately, hunting groups were depreciated following the loss of Courageous on the 17 September 1939, though it took another year before the practice was fully ended.

The Fall of France in June 1940 brought new challenges to the convoy system. In 1939 and the first half of 1940, convoys in the Mid-Atlantic were generally safe from the depredations of U-boats. The long trip around the UK depleted the endurance of U-boats sufficiently to prevent them acting with impunity against these unescorted convoys. The longer trip also meant that fewer submarines were on station at any one time. However, the Fall of France gave the U-boats access to bases on France's Atlantic coast. This allowed them to range further into the Atlantic, and truly threaten the unescorted convoys in the Mid-Atlantic. Meanwhile, many of the best long-range escorts, the destroyers, were pinned down on the coast of the UK to protect against the threat of invasion. However, to compensate, coastal escorts such as the Flower-class were pressed into service. The fifty American destroyers from the Destroyers for Bases Agreement also helped bolster the British escort stable. Finally, the expansion of the British building program in 1939 began to bear fruit. This allowed convoys to have considerably heavier escorts, and for those escorts to travel further west. Assistance also came from the USN, which from 1941, helped escort convoys to Iceland under the auspices of the Neutrality Patrol.