very interesting T...
Airplanes have the advantage of height so bearing in mind you can see calais from dover i'd say visual spotting at 25 miles+ or so for sub hunters if surfaced in favourable conditions or somewhere in that range (afterall they had a crew with binoculars scanning the sea) conditions will of course play a part, Haze, low lvl cloud, waves, surfaced or at periscope depth.
that in turn would of course scare the pants off any sub crew that see a plane as it will already be on an attack run. lets just hope the watchcrew with their
or the radar man and his
picks em up in time hehe.
Joking aside single engine i'd say 120 sounds good if flying lvl (afterall the wings get in the way especially for Hurricanes) and fighter based aircraft were generally preocupied with other aircraft jumping em so i doubt they would be to interested with ground lvl antics.
Catalinas on the otherhand well near as damnit to 360 as you can get, the pilot / copilot wont miss whats in front and if both pods are man'd then you just about have all angles covered.