Naval trawlers are vessels built along the lines of a fishing trawler but fitted out for naval purposes; they were widely used during the First and Second World Wars. Some—known in the Royal Navy as "Admiralty trawlers"— were purpose-built to naval specifications, others adapted from civilian use. Fishing trawlers were particularly suited for many naval requirements because they were robust vessels designed to work heavy trawls in all types of weather, and had large clear working decks. A minesweeper could be created by replacing the trawl with a mine sweep. Adding depth charge racks on the deck, ASDIC sonar below, and a or gun in the bow equipped the trawler for anti-submarine duties.
History
Armed trawlers were also used to defend fishing groups from enemy aircraft or submarines. The smallest civilian trawlers were converted to danlayers.
Contemporary
Some nations still use armed trawlers for fisheries protection and patrol. The Indian Navy used naval trawlers for patrol duties during its involvement in the Sri Lankan civil war. North Korea has notoriously used armed trawlers as spy ships. In 2001 the Japanese sank a North Korean naval trawler after a six-hour battle known as the battle of Amami-Ōshima. Somali pirates have commandeered trawlers and armed them for attacking freighters off the Horn of Africa; the action of 18 March 2006 involved a naval trawler used by pirates.
Trawler classes
Portuguese class
Type 139
Around the world
Belgium
In the aftermath of the First World War, the Belgian Corps de Marine purchased several British war surplus naval trawlers. They were operational during the 1940 Battle of Belgium, and one of them, , evacuated a large quantity of the National Bank's gold reserves to Britain shortly before Belgium's surrender.
Brazil
As with Portugal, the British Royal Navy had a number of trawler-type warships on order from Brazilian shipyards. With the declaration of war by Brazil against Germany in 1942 these vessels were transferred to the Brazilian Navy for anti-submarine and escort duties.
China
People's Liberation Army Navy (PLAN) uses naval trawlers as spy ships, as well as fishing when PLAN was allowed to conduct business activities to supplement the huge military expenditure. In addition, prior to Chinese economic reform, it was relatively easy for PLAN to commandeer private vessels for military use in emergency situations when everything was government-owned in planned economy, but this has become increasingly difficult to do after the reform due to private ownership. However, PLAN has to keep a very large number of auxiliary minesweepers to prepare for war, and as a result, when naval trawlers had retired from their intelligence gathering role, they were converted to auxiliary minesweepers and placed in operational reserve of PLAN. In addition, environmental problems have caused constant geological/geographical/hydrographical changes in Chinese waters, hence creating huge survey requirement, thus a number of these naval trawlers retired from their spy ship roles have also been converted to survey vessels to meet the extremely heavy hydrographic survey requirement. Chinese naval trawlers include Type 113, Type 801, Type 8105, Type 8101, Type 8154 and Type 792 naval trawlers.
France
The French Navy used trawlers requisitioned from civilian use in wartime. In the Second World War 480 trawler-type vessels were in service as auxiliary mine-sweepers, and a further 60 as auxiliary patrol vessels.
Germany
During the Second World War the Kriegsmarine operated trawlers as Vorpostenboot (outpost boats) and as weather ships; the was an example. It also used a large number of Kriegsfischkutter, trawlers built after the 24m long model "G" of the scientifically developed fishing cutter models (seven "Reichsfischkutter"-models A to- G), redesigned for naval uses such as anti-submarine warfare, but intended for conversion to fishing vessel after the war.
The weather trawler programme was a major disaster for the German war effort; it has even been suggested that it was one of the major contributors to Germany's defeat. The British Royal Navy monitored and pursued them relentlessly, capturing or sinking many. The reason was not just the strategic importance of weather data, but that the trawlers were carrying Enigma encryption machines and information, which when captured helped the British to crack the Enigma code, enabling them to read Germany's secret communications; the Germans discontinued the use of weather trawlers as they were too vulnerable, though they had not understood how their weather missions compromised Enigma.