Anti-submarine developments, along with developments in submarines themselves, occurred throughout World War One. At the start of the war, submarines went to the Baltic; eventually they operated in the North Sea, the Dardanelles and remained in the Baltic. At the start of the war, the only things they had to fear were mines and being rammed while on the surface. As the war progressed, so the weapons used against submarines became more advanced and scientific.
The most famous anti-submarine weapon was the depth charge. This weapon was only invented in 1916 and so between 1914 and 1916, submarines on both sides had relative freedom to move about freely. After 1916, this was not the case.
The depth charge was an underwater bomb that exploded at a pre-set depth. This was governed by a hydro-static valve. The depth charge could only be effective if a submarine had been detected. After 1915, many destroyers from Britain and Germany were fitted with hydroplanes which allowed them to listen out for submarines while they were underwater. If destroyers hunted in groups of four to six, hydroplanes could be an accurate way of detecting a submarine as cross-bearings could be taken and a submerged submarine could be clearly detected. Once it had been detected. depth charges were used to attack it.
Later in the war, some destroyers were fitted with ASDIC - a more sophisticated underwater listening device.
electric current when passed through a quartz plate, caused it to
oscillate very rapidly. The oscillations, when in contact with sea water
sent out a supersonic beam which, if it came into contact with a solid
surface (such as a submarine) was reflected back to give an echo. By
measuring the time interval between the beam being sent out and the echo
being received, it was a simple matter to calculate the range of the
submarine and the direction of the beam gave the bearing."
P Kemp "H M Submarines"
ASDIC came in too late in World War One to have an impact, but it made its mark in World War Two.
Nets proved to be a very effective anti-submarine device. A huge series of nets was strung out across the English Channel making it impossible for German U-boats to get through. If one was trapped in a net, it had no option but to surface. U-boats had to use the route round Scotland to get to the Atlantic and a number were lost doing so. Where nets did not exist in the Channel, mines were used. This barrier proved so effective that the Allies could supply their men in France by boat with little problems from German submarines.
Another way of trapping a submarine was for destroyers to set up a series of nets at sea if a submarine was believed to be in a certain area. Connected to each other, the destroyers could easily outpace a submarine and trap it. Buoys were attached to the top of the nets - if a submarine did sail into one, these buoys would give away its position as they would be depressed in the water. Again, once caught, a submarine had no choice but to surface.
Merchants ships were seen as fair game when the Germans introduced unrestricted submarine warfare. To counter this, Britain introduced the convoy system in April 1917. However, the Admiralty protested about the misuse of destroyers. Convoys were only introduced as a result of the insistence of Sir Maurice Hankey, Secretary to the Cabinet. As a result of Hankey's insistence, the loss rate of merchant ships fell from 25% to 0.24% in the month that convoys were introduced.
German submarines were also picked off by Q boats. These were merchant ships but armed in the same manner as destroyers except that their armaments were concealed. The Q boats lagged behind a convoy. Not wishing to waste a torpedo on a seemingly lame duck, a U-boat would rise to the surface to use its gun. A Q boat would beat it to this and four U-boats were sunk using this method. Unfortunately, the Q boats also sunk J2 when a piece of rag on the conning tower made the J look like a U. One problem that British submarines had was actually getting into their home port after a patrol before being attacked by over-zealous coastal protection vessels. To the defenders, a seen submarine had to be classed as an enemy submarine until it could prove itself otherwise.
Another way of disrupting the U-boats was to attack their bases. The largest U-boat base was at Zeebrugge in Belgium. This was attacked in March 1918 but to no avail.
Depth charges, mines, disguised boats etc were all valuable means of defeating U-boats. Some ideas were not so good. One idea that remained on paper only was to train seagulls to land on periscopes at sea thus giving away the position of a submarine! The Admiralty did, however, train sea lions to locate an underwater submarine and surface to give its position away. When the training was taken one step further - to lochs in Scotland - the sea lions escaped, presumably to the rich fishing grounds of the Atlantic. Such was the plight of Britain due to the success of U-boats, that the Admiralty even used a psychic lady to try to locate them in the North Sea. The plan failed.