This is what it took to be a U-boat ace in WWII

We've all heard of fighter aces. We're talking legends like Robin Olds, Duke Cunningham, Pappy Boyington, James Howard, Jimmy Thach, and Swede Vejtasa. Germany had their own U-boat aces, and while Erich Hartmann and Adolf Galland are just some who attained immortality with their feats in the skies, others, like Otto Kretschmer and Gunther Prien, were renowned for what they did under the sea.

Kretschmer and Prien were both considered "U-boat aces," and according to uboat.net, they were part of an elite group. Out of 498 men in World War I, and 1,401 in World War II who commanded U-boats, only a total of 71 men sank more than 100,000 tons of enemy shipping. The tonnage totals are eye-popping in comparison to American commanders, many of whom were rotated out of front-line duty to train new crewmen, similar to the policy used for ace fighter pilots like Thach.

America's top sub skippers, like Eli Reich or Joe Enright, earned their notoriety on single missions. Reich sank the only battleship to be sunk by American submarines during the war, avenging fallen shipmates, while Enright holds the distinction of sinking the Shinano, the largest vessel ever sink by a submarine.

Germany's U-boat aces pulled some incredible feats, themselves. Prien, for instance, earned his fame by sneaking into the British naval base of Scapa Flow and sinking the battleship HMS Royal Oak. 825 British sailors died when the Revenge-class battleship was hit by three torpedoes.

Kretschmer was the top-ranking U-boat ace of World War II, sinking 46 ships totaling over 274,000 tons of displacement. Compare that to the JANAC total credited to USS Tang (SS 306), Medal of Honor recipient Richard O'Kane's command, which sank 24 ships totaling 93,824 tons of displacement.

The German U-boat aces were also survivors. All five of their top aces lived through the war, but one was accidentally killed by a sentry five days after the war, and another died in 1950, and of their top ten skippers, only Gunther Prien was killed in action during the war.