[Aktungbby]

Blackswan'40s extremely detailed site on what is the "wizard war" a radar/counterradar stuggle where German Uboats were preoccupied with their radar detection devices: metox etc. actually giving away the uboats' position; caused Uboats to remain submerged at night transiting Biscay and surface in daylight to charge batteries and chance being able to visually spot the inbound radar aircraft. The advent of the schnorkel, while not entirely proof against airborne radar with an extended schnorkel, was the best defense overall. Ultimately, by 1944 and during the invasion, only Uboats so equipped were sent out from the French ports.

Originally Posted by Bubblehead1980

Did they depart/arrive on surface at night only by this point? OR given radar, leigh light, etc were they safer on surface in day time or submerged in the bay?

The Germans take stock
After several months of failure by the Germans to find the cause of their misfortunes, it was apparent that the U-boat Command and its commanders had lost faith in their search receiver. In the Bay of Biscay U-boats began to spend the nights underwater. Preferring to take a chance on visual sighting of an enemy during daylight for the period of time necessary for ventilating and charging the batteries. In July 1943 the Director of the Naval Communications Division, German Naval War Staff, admitted in long communication concerning the search receiver service that the "invisibility" of the U-boat was gone, that the element of surprise in U-boat attacks had been eliminate by new Allied location methods which had so far proved inaccessible to interception. The Director outlined plans for overcoming the German deficiency, including increased use of radar itself by naval and coastal defense forces. Taking note of many serious mistakes made in the past, he announced extensive changes in organization and procedure aimed at strengthening radar and radar interception research and practices, and stated that the Commander in Chief of the navy had promised his special support toward attaining that end, as indeed Admiral Dönitz had promised U-boats in May..... During the invasion, Schnorchel was used to the limit of its capacity and allowed operation by U-boats against invasion traffic at a time when air coverage was as effective as to necessitate the recall to port of all U-boat's not equipped with Schnorchel. The device was considered of such importance that its installation was designated the chief task of U-boats in port in August 1944. Although Schnorchel was not immune to detection by radar, it was much less susceptible than a surfaced U-boat and permitted boats to operate close to coastal traffic concentrations with little fear of detection for weeks at a time. Unquestionably, Schnorchel was the most effective counter-device developed by the Germans in the Battle of the Atlantic.