the last Tribal class destroyer is HMCS Athabaskan sh was moored at Lake Ontarion in 1968, I saw her then but was unable to go abaord. (If I'd known then etc) DF or Direction finding first started prior to the Great War, litrally as soon as W/T became viable people started Signals inteligence. Shore based Huff Duf stations, manned by WRENS were one of the most succesful though least known, of the anti U boat measures. A great deal of smokescreen disinformation and garbage was thrown about the time. The U Boats operated under control of doneitz and radio comms were esential if wolf packs etc were to be co-ordinated. Especially whilst the Air Gap existed in the mid Atlantic HF/DF was essential. even when such aircraft as the Liberator could operae all the way across, it helps if you know hwere to damm well go! It's a big ocean! The listening side was equally important, Enigma Cipher, important point Enigma was 'CIPHER' machine not a 'CODE' machine. Ciphers change all the time, codes dont. So if a mesage was sent to German Legation say in mexico, the message would be intercepted in cipher, then maybe Mexican authority would send part or all of the message in weak code or clear. Also the U Boats sent Meterolgical information, vital as most of Europes weather comes across the Atlantic, so if you want to plan a bombing raid on London, or launch D-Day you have to know what is going on in the Atlantic, no sattillites remember. As weather info has to be in a standard form, the U boats 'Weather code' transmissions could be compared to the info sent by Allied ships in the same area, providing HF/DF could triangulate the U Boat. You know what the mesage in code says. this information was then sent to the Luftwaffe who obligingly sent it out in thier Enigma variation, see the links? A lot of responsobility on those young women, any slip of the tounge would have led to the systems being changed. Not to mention Frequency analiys, and sig int, all of which those women helped pionner, and is still classified at the highest level today. You don't have to carry a gun to win a war.