The Royal Navy had a large force of older destroyers available to it in 1939. There were the last few WWI-era destroyers of the R and S classes, the W and V classes, built in the last two years of the war, and the 1930s 'standard' destroyers of the A-I classes. In addition to these destroyers, there were the various classes of sloops, specialist escort vessels. From 1940, these numbers would be added to, both with American ships transferred through Lend Lease or the Destroyers-for-Bases agreement, and with home-built British ships like the 'Flower' class corvettes. Given this plethora of escorts, why did the RN struggle to find escorts in 1939-41?

One answer that is traditionally given is that the RN diverted too many ships away from escorting convoys and towards 'hunting groups'. Hunting groups were small squadrons of destroyers which would patrol an area of the sea to hunt down U-boats. These groups were a common feature of the First Battle of the Atlantic in WWI, and had featured significantly in inter-war exercises and training. The classic argument is that the RN saw hunting groups as a more aggressive, offensive strategy, that it preferred to the more passive, defensive strategy of convoy. As such, convoys were neglected and hunting encouraged. However, more recent work suggests that this is untrue. Malcolm Llewellyn-Jones has shown that the RN saw hunting groups as being only useful against an enemy that followed prize rules. Against an enemy that carried out unrestricted submarine warfare, it intended to switch to convoys as soon as possible. Hunting groups were formed at the start of the war, and were common in the first few months, but were only intended as a stop-gap measure until the convoy system was fully set up. British planning assumed that it would take six weeks to fully set up the convoy system. This delay was due to the time needed to put convoy commodores and naval officers into place, to control shipping, as well as the time needed to clear the sea of ships that had departed individually before the war started. These ships needed protection, but could not easily be formed into convoys. Instead, hunting groups were formed to protect them. These hunting groups were fairly effective, sinking three of the seven U-boats sunk in the first two months of the war. However, they soon withdrew from prominence. The last sinking by a hunting group was the sinking of U-35 on the 29th November 1939. In January 1940, Admiral Talbot called for hunting groups to be formed in the North Sea, to attack subs located and forced under by aircraft; the only ships that were spared for this were trawlers, too slow, short-ranged and unseaworthy for the Atlantic convoy route.

While ships were not diverted to form hunting groups, they were diverted to other tasks. The so-called 'First Happy Time' came in the summer and autumn of 1940. During this time, U-boats found it easy to attack poorly escorted convoys in the Western Approaches. It was no coincidence that this coincided with the height of the invasion scare in Britain. Fears of an invasion were ignited by the German invasion of France; from the French coast, it was a relatively short crossing to the UK, while Britain's east coast was practically undefended. To prevent any invasion, a large force had to be maintained in the threatened areas. Destroyers were removed from the convoy routes, and moved to the south and east coast. The Nore Command (covering the area running from Flamborough Head to Beachy Head), which contained the areas seen as the most likely German targets, would be assigned 32 destroyers and five corvettes. More destroyers were retained elsewhere on the south coast, ready to pounce on any invasion convoys. Destroyers on anti-invasion duties could not protect convoys. Ships were also diverted for more immediate, short-term actions, which could lead to losses, or leave them needing significant repairs. Dunkirk is a good example of this. Of the 41 destroyers committed to the invasion, six were sunk and 23 damaged. Finally, the Atlantic convoys were not the only ones that needed protecting. Britain relied on coastal shipping, especially along the east coast, and coastal convoys needed protection. Fifteen of the V and W class ships (plus the larger leader Wallace) had been converted into specialist anti-aircraft escorts, or WAIRs, for this purpose; these ships were no longer useful for the Atlantic route. Other ships were also assigned to protect the coastal convoys, albeit without receiving the specialised modifications that the WAIRs did. Following Italy's entry to the war, ships were needed to protect convoys to Malta, as well as those running round Africa to Alexandria. This further stretched Britain's stock of escorts in 1940-41.

Another factor was the relatively short range of British escorts. British destroyers, built mainly to operate with the fleet, were designed for high speeds. Their hulls were optimised to reduce resistance at high speeds, but increased drag at low speeds. This reduced their endurance at convoy speeds. Their machinery was old-fashioned and their steam pipes often leaked, reducing their endurance further. Finally, their anti-fouling paint was not up to modern standards, leading to additional drag from biofouling (seaweed, barnacles and the like). Few ships had the range to cross the Atlantic with a convoy. Even newer escorts suffered from range issues. Pre-war planning had envisaged that the commerce war would be fought close to the UK, mainly in the North Sea and the closer parts of the Western Approaches. New escorts, like the 'Hunt' class of destroyer escorts, would only need a short range, as they would only be operating close to the UK. This failed to foresee the Fall of France. By allowing the Germans to base on France's west coast, this pushed the U-boat hunting grounds out further into the Atlantic, into areas that escorts could not easily reach. The typical convoy route was ~3,000 nautical miles, though an escort would typically travel much further than this, through zig-zagging and time spent at high speeds to hunt down a contact. A V & W class destroyer had a range of just 2,180 nm at convoy speeds. More modern destroyers had longer ranges - the 'H' class could travel 4,000 miles at convoy speeds - but these were more scarce. Underway refuelling could enable these short-range ships to cross the Atlantic with a convoy. However, the RN was relatively inexperienced with it, and the RFA (Royal Fleet Auxiliary, the civilian service which provides logistical support for the RN) had few tankers capable of it. The provision of tankers to support convoy escorts began on a trial basis in 1941, but it only really came into use fully in 1942. Until then, only the modern destroyers and sloops could protect a convoy throughout its journey.

Finally, we must consider the need to repair and refit ships. The Atlantic route was tough on ships. Bad weather could heavily damage a ship, but even lighter impacts could do serious damage over time. The destroyers had a tendency to 'slam' in relatively light seas (slamming is when the bow raises fully out of the water, and then rapidly and violently re-enters it). This could shake loose rivets, damage sonars, and cause leaks in fuel and water tanks. Waves flowing over the deck could cause heavy damage, especially on older and more lightly built ships. The old 'S' class and the ex-American 'Town' class often suffered damage to their bridges and funnels from waves; the captain of one of the 'S' class ships was killed when a wave collapsed the bridge on his sea cabin. Ships could also be damaged in combat. A torpedo would often sink an escort, but those that survived would require a long period of repairs. While U-boats rarely did damage with their deck guns, many British captains chose to ram submarines on the surface. This was a more reliable way to ensure a kill than gunfire (as the thick, cylindrical plating needed to withstand high pressure was also good against the small shells fired by the guns of a destroyer), but could do significant damage to the ramming ship. This often put the rammer out of action for several months. Finally, there was a need for regular maintenance. As an example, the boilers on British ships needed cleaning every 750 hours; this could not easily be done when the ship was operating at sea. Ships needed time to refit and to receive new equipment like radar, new sonars and more depth charges. These would increase their capabilities, but put them out of action for several weeks. Across the whole war, 22% of escorts would be unavailable for action for any reason; in the summer and autumn of 1940, it was 25%.