Although primitive submarines had been used in action as early as the American War of Independence (1775-1783), it was not until around 1900 that technology had advanced sufficiently to make them a real threat to major vessels.
But even by August 1914, the Imperial German Navy had only 33 operational U-boats, and these were regarded as very much secondary to the battleships of the High Seas Fleet. Their low priority was reflected in the fact that the pre-war German naval construction programme included provision for a total of only 72 U-boats by 1920. (At the outbreak of war, on the other hand, the Royal Navy had 74 operational submarines.)
In contrast to German doubts about the viability of submarine warfare, a few senior British naval officers clearly saw the potential threat to the Empire’s trade routes. In July 1914, Admiral Sir Percy Scott wrote a letter to The Times stating that:
Our most vulnerable point is our food and oil supply. The submarine has introduced a new method of attacking these supplies. Will feelings of humanity restrain our enemy from using it?
After a relatively slow start, the U-boat campaign against Allied merchant shipping began in earnest in February 1915, with the German declaration of unrestricted submarine warfare – the abandonment of the requirement imposed by international law that U-boats should only sink merchant vessels after allowing their passengers and crews to take to the lifeboats.
All the waters surrounding Great Britain and Ireland, including the whole of the English Channel, are hereby declared a war zone. From February 18 onwards, every enemy vessel found within this war zone will be destroyed without it always being possible to avoid danger to the crew and passengers. Neutral ships will also be exposed to danger in the war zone…
This first unrestricted U-boat offensive sank about 750,000 tons of Allied shipping, but failed to achieve decisive results as there were never more than 30 U-boats available (and 16 of these had been lost by the time the campaign was abandoned in September 1915). This was due to vehement protests from neutral countries, especially the United States, where public opinion was outraged at the deaths of 128 Americans when the liner Lusitania was torpedoed by U-20 on 7 May 1915.
Despite the reimposition of restrictions on attacks on merchant shipping, the U-boats still inflicted serious losses during the remainder of 1915 and throughout 1916. But the greatest crisis came with the German resumption of unrestricted submarine warfare in February 1917.
Allied vessels totalling almost 3,000,000 tons were sunk in the first five months of the campaign. The effect was to come close to forcing Britain to make peace, but, crucially, also contributed substantially to the US decision to declare war on Germany in April 1917.
Despite the introduction of the convoy system and increasingly sophisticated anti-submarine weapons, the U-boat menace was not fully under control until mid-1918.
Nets, Ramming, and Gunfire
There were few anti-submarine devices in widespread use at the outbreak of war. Steel anti-torpedo nets had been carried by most battleships since the 1870s, but, although these nets were reasonably effective against the small torpedoes of the late 19th century, trials showed that they gave little protection against the larger, more powerful torpedoes that entered service between 1900 and 1914. Nets were still fitted to most battleships in 1914, but they were removed during wartime refits, and the last of them had gone by 1918. (Enormous steel nets were also used with rather greater success to protect harbours and fleet anchorages against U-boat attacks.)
The first week of the war seemed to indicate that the U-boat could be dealt relatively easily. On 9 August, the light cruiser HMS Birmingham rammed and sank U-15 in the North Sea.
But on 3 September, U-21 sank the light cruiser HMS Pathfinder off the Firth of Forth. Then, on 22 September, Lieutenant Otto Weddigen’s U-9 sank the old British armoured cruisers Aboukir, Hogue, and Cressy off the Dutch coast, with the loss of almost 1,500 men.
If a U-boat could be caught on the surface, it was vulnerable to gunfire as well as ramming. This was exploited by more than 300 decoy vessels, known as Q-ships, which were commissioned during the war.
These were small, elderly merchantmen fitted with well-concealed guns. They gave the appearance of easy targets, tempting a U-boat to the surface to sink them with its deck gun, in order to conserve its limited number of torpedoes.
If a submarine took the bait, a specially trained ‘panic party’ would hastily abandon ship, before the Q-ship’s guns were unmasked to smother the U-boat with close-range fire.
These unarmoured vessels were vulnerable to the U-boats’ 88mm or 105mm deck guns, and had their holds filled with balsa wood or cork to keep them afloat even when badly holed.
In a total of 150 engagements, 27 British Q-ships were sunk, but they destroyed 14 U-boats and damaged a further 60.
The naval mine eventually proved to be an effective anti-submarine weapon. In August 1914, roughly 4,000 of the rather unreliable British ‘Navy Spherical Mine’ were available.
It was soon found that these were totally insufficient, as minefields had to be far larger than had been anticipated in pre-war planning. As an emergency measure, 7,500 former Russian mines were bought from Japan. These had been captured at Port Arthur at the end of the Russo-Japanese War and were significantly better than their British counterparts.
British and Allied mine-laying grew to an enormous scale by the end of the war. Huge minefields or ‘barrages’ were laid in the Channel, Irish Sea, and North Sea, totalling roughly 128,000 mines.
Conventional minefields were usually laid at depths of about 3 to 4.6 metres (10 to 15ft). These were effective enough against ordinary shipping and surfaced U-boats, but in order to catch submerged U-boats, it was found that a proportion of mines needed to be much deeper. By 1918, mines were routinely laid at anything up to 47.5 metres (150ft).
Allied minefields greatly inhibited German operations, especially the Irish Sea Barrage, which sank ten U-boats. By the end of the war, British mines had sunk a total of 150 enemy naval vessels, including 35 U-boats.
For much of the war, it was impossible to reliably detect submerged U-boats. Development of the first hydrophones had begun in 1912, but performance of the early models was poor, and it was not until mid-1917 that more effective versions began to enter service.
In the meantime, an amazing variety of improvised weapons were tried. Some anchorages were ‘protected’ by small patrol boats carrying blacksmiths’ hammers that were intended to smash U-boat periscopes. These patrols also carried a number of 18.5lb (8.4kg) guncotton bombs, which were dropped by hand on suspected submarines.
A slightly more sophisticated weapon was the Lance Bomb, which comprised a 16-18kg (35-40lb) cone-shaped warhead on a 1.5 metre (5ft) shaft, intended to be hand-thrown at a surfaced submarine. It seems likely that only one U-boat (the small coastal submarine UB-13) was disabled by one of these weapons, which were phased out of service as depth charges became more widely available.
The concept of the depth charge originated with a 1913 RN Torpedo School report describing a device intended for counter-mining, which was described as a ‘dropping mine’. At Admiral John Jellicoe’s request, the standard Mark I and II mines were redesigned at HMS Vernon, the Royal Navy’s Torpedo and Mine School. Their shallow depth settings and large explosive charges posed a real hazard to the ships using these weapons, but the basic principle was sound.
The first effective depth charges, the Type D, became available in January 1916. These were barrel-like casings containing high explosive, which evolved into several distinct models.
As production could not keep up with demand, anti-submarine vessels initially carried only two depth charges, which were dropped from a chute at the stern of the ship.
The first confirmed success for the weapon came on 22 March 1916, when the Q-ship Farnborough sank U-68 in the Western Approaches. The Germans became aware of the depth charge following unsuccessful attacks on U-67 and U-69 in April 1916. UC-19 and UB-29 were the only other submarines sunk by depth charges during 1916.
As more depth charges became available, the number carried per ship increased to four in June 1917, to six in August, and 30-50 by 1918. Smaller vessels suffered from stability problems due to the weight of this growing number of charges and their racks; this forced guns and torpedo tubes to be landed to compensate.
The introduction of the Thornycroft thrower, a projector which could fire a Type D depth charge to a range of 27 metres (40 yards) allowed attacks using ‘patterns’ of anything up to ten charges to increase the chances of destroying the target.
Improved fusing allowed greater depth settings in 15 metre (50ft) increments, from 15 to 61 metres (50 to 200ft). Any ship could safely use the Type D Mark III at speeds over 19km/h (10 knots) provided that it was set to detonate below 30 metres (100ft).
Monthly use of depth charges increased from 100 to 300 during 1917, finally averaging 1,745 during the last six months of the war.
By November 1918, 74,441 depth charges had been issued to British vessels, of which 16,451 were used operationally, scoring 38 kills in all, and aiding in 140 more.
British aircraft and airships flew increasing numbers of anti-submarine patrols. By 1918, the Royal Naval Air Service (RNAS) had a total of 2,949 aircraft, 103 airships, and 126 coastal stations.
The C and C* depth charges were ineffective, but the heavier conventional bombs of the period, such as the 50.8kg (112lb) and 235.8kg (520lb) bombs carried by the airships and larger flying boats of the RNAS could be fatal to a surfaced U-boat. However, these bombs had to make direct hits, which were all too rare against such small targets, given the primitive bombsights of the period.
Although only one U-boat (the coastal submarine UB-32) was sunk by bombing, air patrols forced many more to crash dive and probably prevented hundreds of attacks. By the end of the war, the Germans were so concerned about the air threat that all U-boats were being fitted with air-search periscopes.
Despite the limitations of contemporary sensors, it seems likely that British submarines sank a total of 17 U-boats during the war. By 1917, their potential in this role was recognised by the development of the R Class as what would later be termed ‘hunter-killer’ submarines.
Ordered in December 1917, the streamlined R Class were designed to be faster underwater than on the surface, achieving a submerged speed of 26 km/h (14 knots) and a surfaced speed of 17 km/h (9 knots). The bulbous bow contained five sensitive hydrophones and six forward-firing 457mm (18-inch) torpedo tubes.
Only the lead boat, R1, was commissioned before the end of the war. She attacked a U-boat in October 1918, firing a full salvo of six torpedoes, one of which hit, but failed to explode. •
David Porter worked at the Ministry of Defence for 30 years and is the author of nine Second World War books, as well as numerous magazine articles.
British Depth Charge Data
Cruiser Mine Marks I and II
Design date: 1914
In-service date: 1915
Total weight: 521.6kg (1,150lb)
Weight of explosive: 111-113kg (245-250lb) wet guncotton
Sink rate: 2-2.5m/sec (7-8ft/sec)
Depth settings: 14m (45ft)
Type A and Type B
Design date: 1914
In-service date: 1915
Total weight: 22.7kg (50lb)
Weight of explosive: 14.7kg (32.5lb) wet guncotton
Sink rate: n/k
Type A: 12m (40ft)
Type B: 12m (40ft) or 24m (80ft)
Type C and Type C* (aircraft depth charges)
Design date: 1914
In-service date: 1915
Total weight: 40.8kg (90lb)
Weight of explosive: 15.9kg (35lb) amatol
Sink rate: 3m/sec (10ft/sec)
Type C: 12m (40ft) or 24m (80ft)
Type C*: 15m (50ft)
Design date: 1915
In-service date: 1916
Total weight: 191kg (420lb)
Weight of explosive: 136kg (300lb) TNT
Sink rate: 2.1 m/sec (7ft/sec)
Depth settings: 12m (40ft) or 24 metres (80ft)
Type D Mark III
Design date: 1916
In-service date: 1917
Total weight: 191kg (420lb)
Weight of explosive: 136kg (300lb) TNT (later production batches were filled with amatol due to shortages of TNT)
Sink rate: 2.1m/sec (7ft/sec)
Depth settings: up to 91m (300ft)
All images: WIPL.