The first imperative was to staunch the losses, and convoy was the principal instrument. To Admiral William Sims, who commanded American naval forces in European waters, a convoy ‘seemed to be a rather limping, halting procession. The speed of a convoy was the speed of its slowest ship … The whole mass was sprawled over the sea in most ungainly fashion: twenty or thirty ships, with spaces of nine hundred or a thousand yards stretching between them, took up not far from ten square miles of the ocean’s surface’. Yet on to the maelstrom of Allied shipping movements convoy imposed an order that Sims likened to the trans-continental regularity of the US railroads. The system’s nerve centre lay in London, where an enormous map, accessed by ladders, covered one wall of the Admiralty’s convoy room: a paper ship marked each convoy, and circles the suspected positions of the U-boats. As merchantmen converged on Britain from across the globe, they formed up at assembly points. Those from the Mediterranean gathered at Gibraltar, which was the busiest point of the entire network: on one day in November 1917 117 ocean-going vessels lying at anchor. Steamers from the Cape, East Asia, and Australasia met at Dakar; those from Panama, the Gulf of Mexico, and the southern USA at Hampton Roads; and those from the middle Atlantic American States and Canada at New York, whence they proceeded to Sydney (Cape Breton Island) or Halifax (Nova Scotia). Convoys normally left New York at eight-day intervals, cruisers or old pre-Dreadnought battleships accompanying them to the edge of the danger zone around the British Isles, where a destroyer escort took over for the final leg. Faster and slower vessels followed different routes, and the Admiralty planned for the North Atlantic convoys’ destination to alternate between Britain’s west-coast and east-coast ports, the latter reached via the Channel. In the reverse direction, vessels from the Channel and east coast assembled off Devonport or Falmouth; those from the west coast at Milford Haven, Lamlash on the Isle of Arran, or Queenstown (Cobh) in Ireland. Originally convoys averaged twenty vessels, but as experience grew so did the numbers: thirty became commonplace, and convoy HN73 leaving New York in June 1918 numbered forty-seven.
Convoy emptied the seas. Before its introduction, merchant ships funnelling in singly through the thinly patrolled approach cones to Britain’s western coasts had been easier to detect. In contrast, in Churchill’s words:
the sea is so vast that the difference between the size of a convoy and the size of a single ship shrinks in comparison almost to insignificance. There was in fact very nearly as good a chance of a convoy of forty ships in close order slipping unperceived between the patrolling U-Boats as there was for a single ship; and each time this happened forty ships escaped instead of one.
Of 219 Atlantic convoys sailing between October and December 1917 only thirty-nine were even sighted: and this not only because of the ocean’s immensity, but also because shipping could now be better managed. The German Admiralty’s espionage network in Britain failed to ascertain the routes and schedules, and by mid-1918 British counterintelligence had broken it up. Few merchant ships yet had powerful wireless apparatus, but most warships did, and the Admiralty communicated with the convoy commanders to re-route their charges away from danger. Its convoy room was the clearing house for reports of U-boats, gathered from sightings by ships and – increasingly – aircraft, but above all from the surprisingly frequent moments when the submarines broke their isolation by radioing each other or their bases. The Allies’ direction-finding equipment rarely gave a precise enough location for the submarine to be attacked, but it did indicate vicinities to avoid, and frequently the messages – which might include a U-boat’s bearings – could be decrypted. Hence the Admiralty’s Anti-Submarine Division had plentiful information for its daily briefings, and redirecting convoys may have done as much as anything to save cargoes and lives.
Even if a U-boat did locate a convoy, the chances of a successful strike were small. The submarines were still an infant technology. Their radius of action was short, only a handful in 1918 being able to operate much beyond the British Isles. Rather than submarines proper they were submersibles. They could remain below for only a few hours before resurfacing to recharge their batteries, unless they could halt and sit on the bottom, which was possible only in shallower waters such as the Channel and the Irish Sea – in the Atlantic the pressure would crush them. They had two sets of engines: diesel for travel (at up to 15 mph) on the surface, and electric for travel (at up to 8 mph) beneath. When cruising to and from their killing grounds they were generally surfaced, and when surfaced they were most vulnerable, especially in fine weather, when visibility was unhindered. Once they were sighted, a single shot could pierce their hulls. Whereas a contest between a submarine and an isolated merchant ship was stacked against the latter, against a convoy escorted by destroyers the odds were reversed.
For the final stage into harbour a convoy zigzagged in formation, arranged in columns with the destroyers on its flanks, steaming at the highest speed it could muster. At night its lights were extinguished. Normally the destroyers parted company with an outward convoy at the edge of the danger zone, only promptly to rendezvous with an inward one, thus operating continuously though sorely trying their captains, several of whom broke under the strain. Yet the destroyers needed to be worked hard, as they were the vital anti-submarine weapon. Shallow in draught so that torpedoes would pass under them, they were fast (up to fifty knots) and could accelerate quickly. As well as 4- to 5-inch guns, they carried depth charges: canisters packed with high explosive that water pressure detonated. If one exploded within a hundred feet of a U-boat it was likely to destroy it or force it to surface. The Royal Navy pioneered depth charges during the war, and whereas in 1917 destroyers typically carried three or four each, by summer 1918 they carried thirty or more, as well as hoists that would throw them well out from the ship. In 1916 the British used 100 depth charges a month; in 1917, 200; and in 1918, 500 (although in 1942, by comparison, 1,700).
One consequence of convoy was that submarines attempted many fewer sinkings by gunfire, which had been their preferred method.16 Losses to submarine-laid mines also diminished; partly because the Admiralty re-routed convoys; partly because casualties among German mine-laying submarines had been very high indeed, no fewer than thirty-six being destroyed in 1917; and partly because minesweeping improved. In addition, from late 1917 all larger British merchant ships were fitted with the ‘Otter’ – a mine cable cutting device akin to the ‘paravane’ used by warships – although their captains did not always use it. This left sinking by torpedo, but torpedoes were bulky and expensive and U-boats typically carried only eight to twelve, their captains being advised to limit themselves to one hit per victim. Moreover, before firing successfully a submarine commander had first to take up position and aim, preferably from as close as 300 feet in order to inflict most damage by targeting the engine room. As a submarine’s torpedo tubes were located at its bow and stern, it had to fire from a point at right angles to the merchantman, whose high speed and direction changes obscured its likely ‘mean’ forward course to the point where it and the torpedo might make contact. Aiming was a highly skilled art, practised in the most stressful of circumstances, and the torpedo’s wake – a broad and distinctive foaming white line – would betray its originator’s position. Hence a U-boat could risk only one or two shots against a convoy, or might prefer simply to track it in the hope of picking off stragglers. Convoy losses proved extremely low, and the Admiralty’s statistician (a seconded railwayman, George Beharrel) quantified them. In August to October 1917 the loss rate among convoyed ships was 0.58 per cent, compared with a non-convoyed rate of 7.37 per cent. Even if a convoyed merchant ship were hit, the accompanying vessels might well salvage it or at least take off the crew, and inhibit U-boat practices such as taking the master and chief engineer captive. Against a form of warfare that relied on terror, convoy offered psychological reinforcement.
By early 1918, 90 per cent of the vessels sailing the Atlantic were convoyed, and more than half of Britain’s total overseas trade. Yet a major reason why sinkings continued high was that the system remained incomplete and the U-boats found the chinks in it, 85 per cent of the losses being independents. One weak point was the mid-Atlantic islands – Madeira, the Azores, and the Cape Verdes – where new longer-range ‘U-cruisers’ could operate, and the Germans extended their sink-without-warning regime in winter 1917/18. The prime targets were grain ships from the river Plate, and in response the British introduced a convoy service direct from Rio. A second weak point, the Mediterranean, was more difficult to deal with. In early 1918 losses there were rising, and the British First Sea Lord regarded conditions as ‘very critical’. The Mediterranean seaways were vital for Allied operations in Palestine, the Balkans, and Italy; for France to ferry troops from Africa; for Italy to import cereals and coal; and for Britain to bring supplies from Australasia and Asia without going via the Cape of Good Hope. None the less, the Cape route had become the norm and the shipping ministry estimated that restoring through-voyages from Suez to Gibraltar could free up the equivalent of forty steamers. Yet the Mediterranean’s narrowness and shallowness made it a happy hunting ground for the U-boats. In October 1917 fourteen Austro-Hungarian and thirty-two German submarines were stationed in the Austrian bases at Pola and Cattaro on the Adriatic, and four more German submarines at Constantinople. During the whole of 1917, only two Mediterranean U-boats were destroyed, and during 1918 Mediterranean sinkings averaged a quarter to a third of total Allied shipping losses. Through-convoys ran between Gibraltar and Egypt from October 1917, but the heaviest traffic plied the Western Mediterranean, and key routes such as those from Gibraltar to Genoa and to Bizerta had to wait until the spring to be organized. Yet even once the system was operating, Mediterranean loss rates averaged more than twice those for convoys as a whole, and in early 1918 shipping was so short that many units at Salonika were receiving only half their needs. Part of the problem was that the Mediterranean was a poor relation, its convoys having fewer and older escort vessels than those in northern waters and including auxiliaries such as converted trawlers. Moreover, the multiplicity of ports of call on both shores required a complex, criss-cross pattern of routes. Whereas Britain (with increasing American assistance) managed the Atlantic convoys, in the Mediterranean much greater international co-operation was called for, but not always forthcoming, Italy’s navy conducting almost no escort duty outside its coastal waters. Conversely, fourteen Japanese destroyers that had arrived in 1917 (in return for secret Allied pledges to support Tokyo’s claims to Germany’s island colonies in the North Pacific) proved indispensable as troopship escorts.
Even larger losses than in the Mediterranean occurred round Britain’s coasts. In response to the convoys’ introduction the U-boats shifted their attention from the Atlantic towards the Irish Sea and English Channel, and between May to July 1917 and November 1917 to January 1918 the proportions of sinkings in European waters occurring within ten miles of land rose from 17.7 to 62.8 per cent. The change had advantages for the Allies. Ships attacked inshore were easier to salvage and their crews more likely to be rescued. Moreover, the vessels were smaller: the German Admiralty advising U-boats that none the less these targets were worthwhile, as every loss would accentuate the Allies’ overall stringency. Trout being absent, minnows would do. Yet the coastal seas presented a formidable organizational challenge, as they contained not only ocean-going steamers but also shoals of short-haul vessels, and the British Isles were caught in a pincer between the German High Seas Fleet submarines operating from North Sea bases in the Heligoland Bight and the Flanders U-boats stationed at Bruges and exiting via canals to Ostend and Zeebrugge. The U-boats and U-cruisers used in Flanders were smaller, cheaper, and quicker to build, and they sank more tonnage each than did their High Seas Fleet counterparts.
Convoy was the essential element in the package that would frustrate the U-boats in these new theatres, but it competed for resources with other methods, some of dubious effectiveness. ‘Dazzle painting’, for example, the brainchild of Lt-Cdr Norman Wilkinson, entailed decorating ships in bewildering geometrical patterns. By October 1918, 2,719 British merchant ships had been painted, as well as 251 warships, and other Allied merchant fleets followed suit. Merchant skippers found it reassuring, but whether it really confused the U-boats’ aim remained inconclusive, and captured submarine crews said they failed to see its purpose. Other measures included the Jarrow smoke-box, which created an obscuring cloud by expelling boiler smoke at sea level rather than through the funnel; and a scheme of masthead look-outs whereby larger British merchantmen would train and pay bonuses to crew members who kept watch from the crow’s nest. More ambitious and expensive was DAMS (defensive arming of merchant ships), which entailed fitting cannon on to merchantmen and training their crews to use them (the American merchant navy, in contrast, placed service personnel on merchant vessels). DAMS ranked low among the Admiralty’s priorities, but the numbers of armed ships more than doubled, from 1,749 in September 1916 to 4,203 by November 1918. Yet although cannon were effective against surfaced U-boats, they were useless against submerged torpedo attacks, and altogether 1,784 armed British merchantmen went down.
The Admiralty’s answer to the U-boats, however, was not purely defensive. Indeed, part of the reason for the admirals’ lack of enthusiasm for convoy (at least in certain quarters) was the equivalent of the supposed offensive bias among the generals on land: the opinion that the navy’s proper function was to go out and take aggressive action. American naval thinking was similar. Allied strategy in 1918 therefore combined convoy with more pro-active measures, the latter facilitated by convoy’s success in easing the pressure. This approach achieved results, although concentrating all available vessels as convoy escorts would have done so more quickly and cheaply.
In the Mediterranean the centrepiece of the offensive strategy was an attempt to block the 45-mile exit from the Adriatic via the Straits of Otranto. Initially the Otranto ‘barrage’ had consisted of a courageous line of fishing vessels with nets, inadequately supported by naval patrols. On 15 May 1917 the Austrians attacked it, destroying fourteen drifters out of forty-seven. In 1918 the Allies tried to build a fixed barrier using special steel wire from Britain, but the wire arrived slowly and the new barrage was completed only in September. On the other hand, the Austrians attempted on 9–10 June to repeat their previous raid, but they withdrew after an Italian torpedo boat sank one of their most modern battleships, the Szent István. This success helped the Allies to station an enormous force of some 30 destroyers, 15 submarines, 18 trawlers with hydrophones, and over 1,000 drifters, as well as 36 American ‘submarine chasers’ – 110-feet-long wooden vessels equipped with new listening devices and depth charges. Between them the nets and barrage ships destroyed two submarines, and frightened and disheartened the crews of others. And yet, according to the Allies’ estimates, passages through the Straits fell by only 30 per cent. What really turned the corner in the Mediterranean was a reorganization and extension of the convoys in June, allowing more vessels to be escorted along a total of eighteen routes. Sinkings and tonnage losses fell permanently to about half the previous average, and of the fourteen U-boats destroyed in the Mediterranean during 1918, eight fell victim to convoy escorts. The Otranto barrage diverted forces from a convoy system that was losing 2 cent of the vessels it protected to a scheme that swallowed up resources to little purpose. Reviewing the situation in October 1918, Commodore George Baird, the Malta-based Director of Shipping Movements, considered that the escort vessels’ increased strength had destroyed more submarines and deterred attacks on convoys, especially when supported by air patrols. Better intelligence about the U-boats and convoy re-routing had also helped, as (and probably most significantly) had worsening morale among submarine crews. Yet although the methods that had succeeded in the Atlantic were now prevailing in the Mediterranean, he warned that the diversion of resources to the barrage was cutting convoy escorts to the bone, so that even at this stage in the war it would ‘really be a matter of luck and successful dodging if sinkings remain at the present low level’.
In British home waters too, Allied strategy included more aggressive elements. Overrunning the Flanders U-boat bases had been one goal of Haig’s 1917 Third Ypres offensive, and one reason why the Germans had so tenaciously resisted it. In 1918 the Bruges–Zeebrugge–Ostend ‘Flanders triangle’ remained a British target, but now via naval and air action. Most spectacular was the raid against Zeebrugge and Ostend on the night of 22/23 April 1918, undertaken on St George’s Day, blazoned in the press after weeks of bad news, and silencing murmurs that the navy had lost its Nelsonian daring. Another attack on Ostend followed on 9–10 May. Yet although both operations aimed to close the U-boats’ exit channels by sinking blockships, and although they seemed at first to have succeeded, neither matched up to expectations. Three blockships were sunk at Zeebrugge, but the Germans dug fresh channels round them. The sinking of the Vindictive closed only one third of the channel at Ostend. Channel losses fell afterwards, but mainly owing to a concatenation of other measures. Almost daily air attacks during the summer damaged the Zeebrugge lock gates, but they were quickly repaired, and coastal minelaying failed to close the sea exits. Second World War-style concrete pens shielded the U-boats, and eventually the Germans withdrew them because of land operations: the Allied advances in October obliged Wilhelm to order the Flanders submarines back to the homeland. But by now the Flanders flotillas were in decline anyway. In the final months they fell from twenty-five to sixteen vessels, and only one in seven of those returning through the Dover Straits in August and September did so without difficulty. The perils simply of getting to and from their harbours added further stresses to a nerve-racking existence.
Tighter Dover Straits security resulted from improved mines. For most of the war the Germans’ mines had been much superior and they had strewn them round the British Isles. Against this threat the Royal Navy and the civilians of its Auxiliary Patrol assembled an unsung armada that included sloops, trawlers, drifters, and motor launches. During the war no fewer than 3,685 auxiliary vessels enrolled in the Royal Navy’s service, and 445 were destroyed: 2,304 of their officers and men lost their lives. None the less, by late 1917 minelaying was diminishing and minesweeping becoming more effective: German mines sank 128 British merchant ships in 1917 but ten in 1918. Moreover, in autumn 1917 the British began mass production of the Mark H2, a ‘horned’ mine patterned on its German counterpart, which was far more reliable than its predecessors. By June 1918 the Royal Navy had 32,500 in store, was producing 6,000–7,000 per month, and was laying about the same number.
The ‘H’ mines could be moored at greater depths, and in autumn 1917 a new deep field was laid across the Channel between Cape Gris Nez and the Varne. Radio intercepts had revealed that U-boats were passing freely via the Dover Straits, and the previous barrage was useless. But whereas Rear-Admiral Roger Keyes, the head of the Admiralty’s Plans Division, wanted to prevent submarines from passing over the barrier at night by stationing trawlers along it with searchlights and flares, Vice-Admiral Sir Roger Bacon, the commander at Dover, refused to place ships in such an exposed position. Keyes was moved to Dover to supplant him, and he set the trawlers in place. On the night of 14/15 February German destroyers then raided the barrage, sinking eight fishing vessels and killing seventy-six men from their crews. Yet they never repeated the operation and the new barrage did make passage of the Straits much harder, forcing more U-boats to take the far longer alternative route round the north of the British Isles. Some 14–16 U-boats were lost in the Straits during 1918, and in May they were ordered not to use them until the weather became more suitable, the Flanders bases thereby losing much of their value. The new barrier did achieve something, if later than the British realized.