[WalterBright]

Not long range methods other than breaking Enigma messages which said where/when U-boots were to go. This is what made breaking Enigma so important.

[Retric]

That's what we actually did, however RADAR was a new thing allowing a small number of British aircraft to regularly intercept Bombers. Without any evidence it must have seemed probable for something similar to be locating subs.

[lostlogin]

Regularly intercept in daylight - it performed poorly roughy that at night interception rates were abysmal.

[Retric]

Detection worked just fine at night. The problem was: "Although the RAF control stations were aware of the location of the bombers, there was little they could do about them unless fighter pilots made visual contact." https://en.wikipedia.org/wiki/Radar_in_World_War_II