Introduction

In the Battle of the Atlantic, Allied convoy survival depended on more than escorts and firepower—it also depended on being difficult to find. U-boats had to locate a convoy before they could attack, and one of the Allies’ most flexible defences was to adjust when and where convoys sailed. As the campaign intensified—and as intelligence, air cover, and U-boat tactics evolved—convoy routes did not remain fixed. This post asks a straightforward question: did North Atlantic convoy routing change in systematic or predictable ways as the battle unfolded?

Background

Some time ago I wrote about the Canadiana collection of World War II convoy maps and the launch of the World War II Convoy Atlas. In the seven or eight months since then, I added 368 convoy routes to the Atlas. Early on I decided to focus on the North Atlantic for two reasons: it was often the main stage in the convoy war with the U-boats, and the Canadiana collection includes a substantial run of North Atlantic route maps.

The dataset used here covers four convoy series: HX and SC (eastbound to the UK) and ON and ONS (westbound to North America). Each series is mapped from about October 1941 (the earliest routes available) through to the end of July 1943. That end date sits roughly halfway between the start of the Canadiana run and the end of the war in May 1945—an appropriate point to pause and look for spatial and temporal patterns.

Convoys were introduced shortly after the outbreak of war in 1939 as a defensive measure against U-boat attacks, and they expanded across the Atlantic as U-boats pushed farther from their European bases. The North Atlantic convoys ran between Halifax, Sydney and—later—New York City and the UK, most often to Liverpool but sometimes to Loch Ewe in the north of Scotland. They carried civilian and military supplies eastbound and returned westbound most often in ballast. From 1 November 1941 to 31 July 1943, 361 North Atlantic convoys sailed; 342 of these were mapped here. The unmapped convoys were either missing from the collection or illegible.

	Number of North Atlantic Convoys arriving at destination between 1 November 1941 & 31 July 1943	Number North Atlantic Convoys Mapped
HX Series	94	94
SC Series	88	84
Eastbound Convoys Total	182	178
ON Series	166	155
ONS Series	13	9
Westbound Convoys Total	179	164
All North Atlantic Convoys	361	342

Map 1 shows all 342 mapped convoy routes that arrived between 1 November 1941 and 31 July 1943. The red line indicates the shortest route – the Great Circle route – between Cape Spear, Newfoundland, and the North West Approaches leading to Liverpool.

As the map suggests, few convoys took the shortest path. Routes range from the Azores in the south to Greenland in the north, with most tracks lying north of the Great Circle. On its own, however, Map 1 doesn’t reveal clear structure. To see patterns, we need to add time.

Mapping by Time

Snapshot 1: November 1941 – March 1942

There were 89 convoys that arrived between 1 November 1941 and 15 March 1942, all of which are mapped here. Routes follow a broad, roughly 1,500 km-wide band between North America and the UK, with Halifax as the main North American port. UK arrivals and departures are split between Liverpool and Loch Ewe (see Map 2). Westbound convoys (ON, shown in blue) generally sailed to the south—or to starboard—of their eastbound counterparts (HX in red; SC in orange).

Across these 89 convoys, 3,544 ships sailed and 3,176 (89.6%) arrived safely. Of the 368 that did not, 41 were sunk by U-boats; most of the remainder turned back—usually because of mechanical problems—or were lost to accidents or weather. Average passage time during this period was 16.3 days.

Snapshot 2: March – June 1942

By mid-March, the convoys were sailing relatively undisturbed across the Atlantic. The mapped convoy routes for this three-month period reflect that (Map 3 below). Westbound convoys (in blue) continued to sail to the south of their eastbound counterparts, the slower SC convoys (in orange) to the north of the HX convoys (in red).

During this 3 month period, 57 North Atlantic convoys sailed with 1,971 ships, 95.3% of which arrived safely at their destination. U-boats sank 11 of these ships. The average convoy travel time dropped to 15 days.

Snapshot 3: June – September 1942

The summer of 1942 marks the start of a year in which convoy routing became increasingly dispersed. In June–September 1942 (Map 4), convoys still took relatively direct tracks that broadly followed the Great Circle route. Average passage time across all series was 14.1 days, and the safe-arrival rate (95.4%) remained high for this slice of the campaign.

Snapshot 4: September – December 1942

From mid-September to mid-December 1942 (Map 5), routes are noticeably more dispersed than in earlier months and shift north of the Great Circle. HX and SC convoys still broadly track together, while westbound convoys appear more scattered. Travel times increased by about three days to an average of 17.4 days. The safe-arrival rate dropped to 92.9%, and U-boats sank 66 ships sailing with the North Atlantic convoys during this period.

Snapshot 5: December 1942 – March 1943

From 15 December 1942 to 15 March 1943 (Map 6), routes appear more widely dispersed, with many convoys pushing much farther north during the crossing. Average passage time rose to 19.6 days—a 38% increase compared with June–September 1942. The safe-arrival rate fell to 86.8%, and U-boats sank 68 vessels in these convoys. No surprisingly, these were the worst months for the Allies in the Battle of the Atlantic

Snapshot 6: March – June 1943

This dispersal continued into spring 1943, but Allied fortunes were already improving (Map 7). Routes were still spread across the North Atlantic, yet average passage time had dropped back to 17.4 days. The safe-arrival rate rose to 93.5%, while U-boats sank 55 ships during this period.

Snapshot 7: June – July 1943

By the end of May 1943, the crisis for the North Atlantic convoys had passed. Germany lost 43 U-boats from all causes during May—22 of them in the North Atlantic—prompting a “temporary” withdrawal from the main convoy routes. From 15 June through the end of July 1943 (Map 8), no Allied ships sailing in the North Atlantic convoys were sunk as a result of U-boat action. Average passage time fell to 14.8 days and, from 22 June on, each convoy series had its own set of standard routes, “and the most suitable one, in the light of U-boat and seasonal conditions, was designated in the pre-sailing telegram.”¹ The safe-arrival rate rose to 97.7%.

Sailing Time

Measuring and comparing the length of each mapped route directly would be time-consuming, especially because many routes are not shown in full from port to port. Sailing time can serve as a useful proxy for distance. Using departure and arrival dates published on Convoyweb, I charted changes in passage time over the period. Convoys that dispersed before reaching port (all between November 1941 and August 1942) were removed from the dataset.²

From November 1941 to September 1942, average convoy passage times were between 11 and 18 days. In September and October 1942 there was a sharp jump, and from then through July 1943 averages range between about 14 and 21 days. Early in the period, the slower SC convoys were clearly distinct from the faster HX convoys. By June 1943—when North Atlantic convoys were sailing largely unbothered by U-boat attacks—that gap nearly disappeared, with all series typically taking 14–15 days to cross.

Choosing the Route

The first map in this post shows a dense tangle of tracks across the North Atlantic. It can look random at first glance, but each line reflects a deliberate routing decision. Convoys sailed with a planned course—set using the best available intelligence on U-boat and surface threats—and that plan could change at sea. Many Canadiana charts show both intended and actual tracks, with earlier routes crossed out as new information arrived. In other words, what you see on these maps is not just where convoys went, but how often they were steered away from danger in real time.³

As the war progressed, expanded air cover, intelligence and better detection—radar and direction-finding in particular—made it easier to identify where U-boats were operating. That advantage gave Allied planners more room to manoeuvre: routes could be shifted away from reported contacts, and traffic could be dispersed when concentration itself became a liability. By early summer 1943, when passage times stabilized and the U-boat threat eased, routing looked less like evasive zig-zagging and more like a return to efficiency.

If the North Atlantic battlefield had a front line, convoy routing sketched it. From spring 1942 through the end of that year, the safest water gradually migrated toward the margins of the ocean—especially to the north and the west. Later, the routes fanned out across much of the Atlantic, suggesting both a heightened threat and an Allied ability to use intelligence to pick paths through it.

Seen in sequence, these tracks are a record of adaptation rather than fixed shipping lanes. When risk was lower, routes tended to tighten toward quicker crossings. When danger increased—or when uncertainty about U-boat positions forced caution—tracks widened, shifted, and sometimes changed mid-voyage. By June–July 1943 the pattern reversed: shorter, more consistent passage times point to a convoy system no longer shaped primarily by evasion. In that sense, the geography of these routes doubles as narrative—an evolving front line traced by ships trying to get through.

You can see the route changes for yourself in this short animated map video:

Further Reading