In WWII, the Allied forces were scared to death of Germany's submarine force. Before the U.S. even joined the war, over 500 British military and merchant ships were destroyed. Germany's plan was to destroy British merchant ships (including those going to and from the U.S.), to destroy their economy and get them to surrender. Our Navy spotted several German submarines off our east coast and ran them off. Germany hadn’t declared war on us and we hadn’t declared war on them at the time. So, what stopped their submarines from winning the war for Germany?
1) In the beginning, their submarines could only stay under water for a day before having to surface to run the diesel engines and recharge the batteries they used to propel the sub underwater. They couldn't run the diesel engines underwater because the carbon monoxide would kill the sailors. Because the submarines were spending so much time on the surface, they began being spotted more often and destroyed because of an increase in air patrols by Allied forces.
2) New radar was placed on allied aircraft that helped them spot the submarines when they were under water. The aircraft would then drop depth charges on the subs. These bombs were set to explode at a depth of water that was set by the aircraft’s crew. For example, if they set it to explode at 200 feet underwater and the submarine was 400 feet underwater, they wouldn’t destroy the sub. It was a tricky process.
3) Although the submarines were fast enough on the surface to engage ships in battle, when they were underwater, most track athletes could out run them. This made it easier for the subs to get chased down and depth charges were dropped on them.
Germany ended up designing a killer sub that addressed all the problems above. They designed a submarine that was faster than some ships even when they were underwater. The subs didn't have to surface for four days as they added around 400,000 pounds of batteries (a car battery weights around 20 pounds), which allowed them to run longer. They created an air cleaning system (the same type my submarine used thanks to German technology), which also allowed them to stay underwater longer. Their torpedoes were bigger, faster, and deadlier, where it would take only one torpedo to sink a ship plus, they could fire off 18 of them in just 18 minutes.
It was a killer machine and could've completely changed the war. The problem, they couldn't build them fast enough and they didn't have enough qualified sailors to operate them. But more importantly, the new submarine wasn't launched in time to do any damage because Germany surrendered. Not a single torpedo from these new killing machines was fired. One of the new subs was just getting ready to fire a torpedo at a British destroyer when the captain got a message that the war was over. He left the destroyer alone and made his way back to Germany.
When Germany surrendered, they immediately began destroying all paperwork associated with their submarine program and they were able to destroy all their newly designed submarines but one that was captured; it was the very one that my nuclear submarine was designed after, the USS Stonewall Jackson. Germany's submarine engineers were around 40 years ahead of their time.