Answer:
The US started sending out destroyer ships and aerial hunter-killer PBY's, Consolidated PBY Catalina. A type of amphibious aircraft. They were essentially flying boats, used as patrol aircrafts. The big advantage of these PBYs were that they could refuel on fueling ships out on the ocean-- they did not need to return to land to refuel, so they were essentially constant. The US had naval and aerial patrols that were 24/7 that could accompany liberty ship convoys. There were up to 10 liberty ships per convoy-- they brought all the needed supplies to the British Isles.
Sophisticated sonar equipment for first detecting, then classifying, locating, and tracking the target submarine is a key element of ASW (Anti Submarine Warfare). To destroy submarines, both torpedoes and naval mines are used, launched from air, surface, and underwater platforms. ASW also involves protecting friendly ships.
As soon the US destroyers detected submarines with their sonar, they would alert PBY's. Once alerted of the location, they would release depth-charges, a big 55-gallon drum, full of explosives, that has pressure triggers. Once the depth-charge reached the depth it was set at to explode, it would detonate and destroy the submarines. They would wait for the enemy U-Boat to surface, knowing they'd hit the target.
Since these patrols were constant, the German U-Boats never stood a chance.
