The U-boats had the "good time s"ca 1941-42 and then the bad times as the Allied sank the U-boats like little stones.
The first Happy Time was from July 1940 till October, 1940 or April, 1941(depending on source). The Second Happy Time was Operation Drumbeat, roughly January-August, 1942.
One of the good times was when Hitler declared war and the US Cities kept the lights on. Unfortunately "easy meat" for the U-boats. They were seen. By U-boats easily by silhouette.
OK. I'm well aware of OPERATION DRUMBEAT. CNO Adm. Ernie King didn't exactly burnish his reputation during all that. I was thinking of the first happy time.