Back to Bay of Biscay offensive, it looks to me like the preferred method was for U-Boats to travel at high speed day/night.
In OP ENCLOSE II (april 5-13, 1943) majority of U-Boats were detected at night:
Quote:
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5 - 13 April using 86 aircraft. 25 U-boats ran through the same patrol ribbon during the period, the RAF had estimated them to be 28. In 980 flying hours 11 U-boats were detected, for the first time the majority during the night, and 4 boats were attacked. U-376 was sunk and U-465 was damaged. Even with fewer aircraft and less hours the results were almost the same as in the previous operation.
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i.e. 25 crossed the area, 11 (44%) were detected, 4 (16%) were attacked and 2 (8%) sunk/damaged.
and even with massive Allied Air coverage, majority of U-Boats went undetected.
OP DERANGE
Quote:
Operation Derange was a larger-scale version of Enclose II with a larger patrol area between 8 ½ and 12 degrees west. It began on 13 April and was to continue until decided otherwise. Coastal Command used 131 aircraft, its entire available fleet in the operations, only some of which equipped with Leigh Light and 10cm radar.
To the end of April 81 U-boats crossed the Derange ribbon, either outbound or inbound. The RAF aircraft flew 2,593 day and night flying hours. 36 U-boats were located and 22 of them were attacked. One U-boat was sunk (U-332) and two outward bound boats (U-566 and U-437) so badly damaged that they had to abort their patrol.
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https://uboat.net/history/fight_back_order.htm
note the stats, 81 U-Boats crossed the area from 13 to 31 april 43, 36 were detected (44%) , 22 were attacked (27%) and 3 were heavily damaged/sunk (4%), so traveling at high speed on the surface and clearing the area as quickly as possible was still the safest course of action.