In response to the urgent need for ways to attack dived
submarines, British shipbuilders John I. Thornycroft designed the depth-charge
thrower in 1917.
Soon after the outbreak of World War I a number of
destroyers were fitted with the 'modified sweep', a 60.96m (200ft) loop of wire
fitted with explosive charges. The upper leg of the loop was kept buoyant by
wooden floats, while a 'kite' depressed the lower leg of the loop. It was
intended to be streamed when a submarine had submerged after being sighted, and
could not be towed at more than 10 knots.
An electric indicator showed if any obstruction fouled the
sweep, allowing the operator to detonate the charges. As it took some 20
minutes to deploy and because it restricted the ship's maneuverability, the
sweep was heartily disliked by its operators, but it has now gone down in
history as the first dedicated anti-submarine warfare weapon, and it is credited
with dung U. 8 in March 1915 and UC. 19 in December 1916.
World War I marked the first earnest conflict involving
significant use of submarines and consequently marked the beginning of major
efforts to counter that threat. In particular, the United Kingdom was desperate
to defeat the German U-Boat threat against British merchant shipping. When the
bombs that it employed were found to be ineffective it began equipping its
destroyers with simple depth charges that could be dropped into the water
around a suspected submarine's location. During this period it was found that
explosions of these charges were more efficient if the charges were set to
explode below or above the submarine. However many other techniques were used,
including minefields, barrages and Q-ships and the use of cryptanalysis against
intercepted radio messages. The airship ("blimp") was used to drop bombs
but fixed-wing aircraft were mostly used for reconnaissance. However the most
effective countermeasure was the convoy. In 1918 U-boat losses became
unbearably high. During the war a total of 178 U-boats were sunk, by the
following causes:
Mines 58, Depth charges 30, Gunfire 20, Submarine torpedoes
20, Ramming 19, Unknown 19, Accidents 7, Other (including bombs) 2
British submarines operated in the Baltic, North Sea and
Atlantic as well as the Mediterranean and Black Sea. Most of the losses were
due to mines but two were torpedoed. French, Italian and Russian submarines
were also destroyed.
Before the war ended, the need for forward-throwing weapons
had been recognized by the British and trials began. Hydrophones had been
developed and were becoming effective as detection and location devices. Also,
aircraft and airships had flown with depth bombs (aerial depth charges), albeit
quite small ones with poor explosives. In addition, the specialist
hunter-killer submarine had appeared, HMS R-1.