Constructed using light

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

nets, indicator nets were often anchored at various depths to the sea bed around Allied

naval base A naval base, navy base, or military port is a military base, where warships and naval ships are docked when they have no mission at sea or need to restock. Ships may also undergo repairs. Some naval bases are temporary homes to aircraft that u ...

s during both world wars. They were intended to entangle

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role ...

traffic of the enemy, even though the

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s often managed to disentangle themselves and escape before being blown up by

depth charge A depth charge is an anti-submarine warfare (ASW) weapon. It is intended to destroy a submarine by being dropped into the water nearby and detonating, subjecting the target to a powerful and destructive Shock factor, hydraulic shock. Most depth ...

s. Predominantly deployed by the

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against F ...

as a means of discouraging enemy submarines from entering Allied waters, indicator nets were used extensively during

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

. Individual nets were sometimes as much as in length. Instead of being used as the sole anti-submarine measure, indicator nets were often mixed with extensive

minefields A land mine is an explosive device concealed under or on the ground and designed to destroy or disable enemy targets, ranging from combatants to vehicles and tanks, as they pass over or near it. Such a device is typically detonated automati ...

and patrolling

warship A warship or combatant ship is a naval ship that is built and primarily intended for naval warfare. Usually they belong to the armed forces of a state. As well as being armed, warships are designed to withstand damage and are usually faster a ...

s. Sometimes mines were attached directly to the nets, thus reducing submarine survival chances. After a submarine became entangled in the net, a marker

buoy A buoy () is a floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. Types Navigational buoys * Race course marker buoys are used for buoy racing, the most prevalent form of yac ...

attached to the net drifted along the water's surface indicating an enemy below. The first example of indicator nets causing the destruction of a U-boat occurred at

Dover Dover () is a town and major ferry port in Kent, South East England. It faces France across the Strait of Dover, the narrowest part of the English Channel at from Cap Gris Nez in France. It lies south-east of Canterbury and east of Maidstone ...

when the ''U-8'' became entangled on March 4, 1915. The British also used indicator nets attached to converted fishing boats. The boats were armed with guns and depth charges to attack any submarine that became entangled in their net. A number of these boats were deployed in the Mediterranean as part of the

Otranto Barrage The Otranto Barrage was an Allied naval blockade of the Otranto Straits between Brindisi in Italy and Corfu on the Greek side of the Adriatic Sea in the First World War. The blockade was intended to prevent the Austro-Hungarian Navy from escap ...

, to block the

Strait of Otranto The Strait of Otranto ( sq, Ngushtica e Otrantos; it, Canale d'Otranto; hr, Otrantska Vrata) connects the Adriatic Sea with the Ionian Sea and separates Italy from Albania. Its width at Punta Palascìa, east of Salento is less than . The st ...

. On 12 May 1917, the Austro-Hungarian submarine ''SM U-6'' was entangled in one of the nets. The captain surfaced in an attempt to cut the net free and the surfaced submarine was subjected to ineffectual gunfire from the fishing boats. The submarine was scuttled as its propellers had been fouled by the net and escape was impossible. All the crew were rescued. The British continued to develop indicator nets between World Wars I and II, though the Americans did not. In 1939, the British shared their expertise with the United States Navy. Before and during the

attack on Pearl Harbor The attack on Pearl HarborAlso known as the Battle of Pearl Harbor was a surprise military strike by the Imperial Japanese Navy Air Service upon the United States against the naval base at Pearl Harbor in Honolulu, Territory of Hawaii, j ...

, Japanese midget submarines were able to enter the inner harbor via an open gate and avoid the submarine nets. The standard submarine net at Pearl Harbor consisted of 300 foot panels of 1 inch mesh rope with 8 foot diagonal openings. The midget submarines were designed to penetrate this type of net with features including a small size, net cutters and a tail with no projections which would snag the net. After the attack, a light indicator net was deployed at Pearl Harbor. In the 21st century, the rising use of

unmanned surface vehicle Unmanned surface vehicles (USVs), also known as unmanned surface vessels, autonomous surface vehicles (ASVs) in some cases, uncrewed surface vessels (USVs), or colloquially drone ships, or robot boats, are boats or ships that operate on the ...

s and

unmanned underwater vehicle Unmanned underwater vehicles (UUV), sometimes known as underwater drones, are submersible vehicles that can operate underwater without a human occupant. These vehicles may be divided into two categories: remotely operated underwater vehicles (ROU ...

s has brought indicator nets back into consideration as a defensive strategy.Ukrainian ingenuity is ushering in a new form of warfare at sea
The Economist, 7 Dec 2022

References

{{DEFAULTSORT:Indicator Net World War II military equipment of the United Kingdom Anti-submarine warfare

See also

References