Sinking of the Linda Blanche out of Liverpool by Willy Stöwer

Beginning in 1915, when shipping losses to U-boats began to climb significantly, the Admiralty diverted a substantial portion of its existing resources to antisubmarine warfare (A/S in Britain, ASW in America) and asked scientists, engineers, academics, and others to help develop ways to destroy U-boats. In the belief that the best defense was a strong offense, the chief ASW weapons to emerge in World War I were these:

• SURFACE HUNTERS. The Admiralty sent scores, then hundreds, then thousands of surface ships out offensively scouring the oceans for U-boats. These vessels included destroyers, frigates, sloops, trawlers, yachts, and heavily armed raiders (Q-ships) disguised as tramp steamers. Some vessels were fitted with crude hydrophones—passive underwater listening devices—which could detect the engine noise of a surfaced U-boat, but only if the hunting vessels were not moving.

In 1916 many of these offensive ASW ships were armed with a new weapon called the depth charge. The best of these underwater bombs, derived from mines, contained 300 pounds of TNT or Amatol and were fitted with hydrostatic fuses which could be set to detonate the charges at 40 and 80 feet, and later 50 to 200 feet. Since early depth charges were rolled from stern tracks (or racks) and exploded at shallow depth, the attacking vessel had to put on maximum speed or risk severe damage to its stern. Therefore, slower vessels could not use the 300-pound depth charges until fuses with deeper settings had been developed. In all of 1916, British naval forces sank only two U-boats by depth charge. In 1917 and 1918, when depth charges had been improved and were much more plentiful, the kill rate by this weapon increased significantly.

• AIRBORNE HUNTERS. When the war commenced, the aviation age was merely a dozen years old. The Royal Navy had acquired about fifty seaplanes and seven nonrigid airships, called “blimps,” to scout for enemy naval forces. Some of these aircraft were diverted to U-boat hunting but, owing to the unreliability of engines, slow speed, limited fuel capacity, tiny bomb loads, and other factors, they were useless against U-boats. It became apparent, however, that when an aircraft appeared near a U-boat, it dived and became essentially immobile. Hence air patrols were useful for forcing U-boats under, thus enabling ships to skirt the danger area and avoid attack. In 1915 the Royal Navy acquired much improved seaplanes (the American-designed Curtiss American) and blimps in greater numbers. These were armed with impact-fused 100- or 520-pound bombs or 230-pound ASW bombs with delayed-action fuses that exploded at a water depth of seventy feet, but the U-boat kill rate by aircraft remained essentially zero.

• SUBMERGED HUNTERS. On the theory that it was wise to “send a thief to catch a thief,” the Royal Navy saturated German home waters with submarines equipped with hydrophones. The early patrols produced no confirmed kills, but the presence of British submarines in German waters, including the Baltic Sea, where German submariners trained, caused great anxiety and disrupted routines. Beginning in 1915, British submarines began to torpedo U-boats in significant numbers. The Admiralty designed and produced a small submarine (R class) specifically for U-boat hunting but it came too late. Had British torpedoes been more reliable, the submarines doubtless would have sunk many more U-boats.

• MINES. From the first days of the war both sides employed moored contact mines, planted in shallow water, usually defensively but often offensively. Defensive minefields were sown to prevent enemy forces from penetrating one’s coastal waters for shore bombardment, interdiction of shipping, or invasion. Such minefields were charted and planted with great care, leaving secret safe lanes for friendly shipping and naval forces. In order to attack British shipping, U-boats often had to negotiate the periphery or heart of defensive minefields, a hazardous undertaking. Many U-boats strayed into British minefields or hit live mines that had drifted their moorings or had broken loose. Offensive mining was more complicated and often hit-or-miss. Surface vessels, operating under cover of darkness in great haste, planted mines in likely spots such as sea-lanes or sometimes even in the safe lanes of the defensive minefields, to catch opposing naval vessels or merchant ships by surprise. Later in the war, both sides employed submarines for minelaying, combining two much-feared naval weapons.

To prevent U-boats from reaching the Atlantic via the English Channel, the British sowed lines of mines across it from Dover, England, to Cape Gris-Nez, France. However, in 1915 and 1916, British contact mines were defective, and not until the Admiralty copied and mass-produced the standard German contact mine could the Dover “field” be depended upon to block the passage of U-boats. When the Dover field was finally effective, it forced U-boats destined for the Atlantic to go northabout Scotland, adding about 1,400 miles (and about seven days) to the voyage.

After the United States entered the war and offered the Royal Navy a secret mine with a magnetic fuse, the Allies put in motion a grandiose scheme to plant 200,000 such mines across the top of the North Sea from the Orkney Islands to Norway. Although American and British forces planted about 80,000 mines in this so-called Northern Barrage, most of these mines were also defective and, other than frayed nerves, caused the Germans small harm. Even so, Allied mines in all areas ranked high as U-boat killers.

• RADIO INTELLIGENCE. When the war began, radio transmission or wireless telegraphy (W/T) was a new military technology at which the British excelled. Taking advantage of a lucky capture of German naval codebooks, as well as an appalling lack of sophistication in German radio procedures and security, the British thoroughly penetrated German naval communications. The British first perfected Radio Direction Finding (RDF) to pinpoint and identify German shore- and sea-based transmitters. Utilizing the captured codebooks, they “read” on a current basis most German naval transmissions. This priceless intelligence enabled the Admiralty’s secret signals-intelligence branch (known as Room 40) to track U-boat operations to a remarkable extent. A British historian wrote that by “early 1915, Room 40 knew the total strength of the U-boat fleet, the rate at which it was growing … the composition of each flotilla … the number of U-boats at sea or in port, and when and if it put to sea … losses, as evidenced by the failure of a U-boat to return, and in most cases, the size of the [U-boat] threat in any particular area.”

Still, these many and varied ASW measures were absurdly inadequate. In all of 1915 the Germans lost merely nineteen U-boats while adding fifty-two boats to the force. In 1916 the Germans lost twenty-two boats while adding 108 boats. Notwithstanding a massive British antisubmarine effort, during the first four months of 1917, the Germans lost only eleven U-boats. To then, the average monthly U-boat loss rate had been only 1.7, a continuing losing battle for Britain because the Germans were producing seven or eight new boats per month.

In the wake of the spectacular shipping losses in April 1917, Britain’s new Prime Minister, David Lloyd George, urged the Admiralty to organize British shipping into convoys, escorted by destroyers, frigates, sloops, and other ASW craft. This was hardly a new idea; defense of sea commerce by convoy was as old as the sail and, as the British naval historian John Winton put it, “as natural and as obvious a tactic as, say, gaining and keeping the weather gauge.”

The Royal Navy had opposed the formation of convoys for numerous reasons. The principal reason, Winton wrote, was that Royal Navy officers had forgotten their history—that the main purpose of the Royal Navy was to protect Britain’s sea trade. Imbued with the aggressive doctrines of the American naval theorist Alfred Thayer Mahan (and kindred souls), who postulated that control of the seas could most effectively be insured by husbanding naval assets for a single, decisive, offensive naval battle with the enemy, they opposed the diversion of naval resources to convoying, which they viewed as mundane and defensive and which, if adopted, would be an admission that Britain had, in effect, lost control of the seas to an inferior naval power.

There were other reasons. First, notwithstanding huge losses of merchant ships on their very doorstep, the Royal Navy continued to grossly underestimate the overall effectiveness of the U-boat campaign on British maritime assets. Second, the admirals insisted convoys were enormously inefficient, compelling faster ships to reduce speeds to those of slower ships, overwhelming seaport facilities during loading and unloading periods, and posing difficult organizational problems in distant, neutral ports. Third, the Admiralty doubted the ability or desire of merchant-ship captains to accept or to follow orders or to station-keep in the required tight zigzagging formations at night or in inclement weather. Fourth, the admirals held, the concentration of merchant ships into a single large body presented U-boat skippers with richer targets, which they were not likely to miss, even with poorly aimed or errant torpedoes.

With the assistance of American naval power, the Admiralty finally—and reluctantly—agreed to a test of inbound convoying in the Atlantic. The first convoy, consisting of sixteen ships, sailed from Gibraltar to the British Isles on May 10, 1917; the second of twelve ships from Norfolk, Virginia,* on May 24. The Gibraltar convoy arrived in good time without the loss of a ship. The Norfolk convoy, escorted by the British cruiser Roxburgh and six American destroyers, ran into minor difficulties. Two of the dozen ships could not maintain the convoy’s 9-knot average speed and fell out. One of these was torpedoed going into Halifax, Nova Scotia. However, the other ten ships crossed the Atlantic in foggy weather, maintaining tight formation, zigzagging all the way, and arrived safely in the British Isles.

With the results of these tests and other data in hand, in August 1917—the beginning of the fourth year of the war—the Admiralty finally adopted the convoy system. It was a smashing success. By October over 1,500 merchant ships in about 100 convoys had reached the British Isles. Only ten ships were lost to U-boats while sailing in these convoys: one ship out of 150. By comparison, the loss rate for ships sailing independently (inbound and otherwise) was one in ten. By the end of 1917, almost all of the blue-water traffic was convoyed. These convoys had been instituted in the nick of time; U-boats sank nearly 3,000 ships for 6.2 million tons in 1917, most of them sailing independently. The historian Winton wrote: “Convoying did not win the war in 1917. But it did prevent the war from being lost in 1917.”

A U-boat skipper remembered the impact of convoying on the German submarine force. Convoying, he wrote, “robbed it of its opportunity to become a decisive factor.” He continued: “The oceans at once became bare and empty; for long periods at a time the U-boats, operating individually, would see nothing at all; and then suddenly up would loom a huge concourse of ships, thirty or fifty or more of them, surrounded by a strong escort of warships of all types.” The solitary U-boat, he went on, which “had most probably sighted the convoy purely by chance,” would attempt to attack again and again, “if the commander had strong nerves” and stamina. “The lone U-boat might sink one or two of the ships,” he concluded, “or even several; but that was a poor percentage of the whole. The convoy would steam on.”

During the final twelve months of the war, convoying became the rule rather than the exception. The British and American navies established large organizations to administer convoys and provided surface and, where feasible (close to land), aircraft escorts, armed with new and improved aerial bombs. In many instances, intelligence from Room 40, accurately identifying U-boat positions, enabled the authorities to divert convoys away from U-boats. After the full convoy system was in place (outbound from the British Isles as well as inbound) in 1918, total shipping losses fell by two-thirds from 1917: 1,133 sunk. Of these, 999 sailed independently. In the ten months of naval war in 1918, only 134 ships were lost in convoy.