‘Operation Drumbeat’ is a good account of the early days of German submarine warfare off the east coast. It only took a handful of German subs to wreak havoc. At one point all oil shipments were stopped due to u-boats sinking so many tankers.
Today, with 24 self-guided 24,000 yd torpedoes in a sub, one sub could sink 20 ships at 100,000 tons each ... in one mission.
And in WWII, only a few very, very good submarine skippers would sink 100,000 tons in the entire war.
The Germans found such easy pickings during the early part of the war off the eastern coast of the US they called it “The Happy Time’’.