The Type-XXI (Type Twenty-One) was a model of U-Boat built by the Kriegsmarine during World War II. It was a true revolution in submarines, for a number of reasons.

It was able to remain submerged nearly indefinitely, for despite being powered by conventional diesel-electric engines rather than the experimental hydrogen peroxide turbine, it had schnorkels for both crew air and for the Diesel engines. As a result, it did not need to spend the 4 or more hours per day (or night) on the surface that previous models of U-Boat did. This was to contribute greatly to its survivability, as Allied naval patrol aircraft were (by the end of the war) increasingly armed with radar. This meant that the formerly safe practice of surfacing at night to charge batteries, a necessary chore for the U-Boat's survival, instead began to expose it to surprise aerial attack.

In addition, the Type-XXI was the first production military submarine to be streamlined for submerged performance, and hence to be faster submerged than on the surface. Whereas all previous submarines used in the Second World War had a top speed submerged of perhaps five or six knots, the Type-XXI could make seventeen while snorkeling on diesel. This was due primarily to its exterior design; acknowledging its nature as a stealthed predator, the hull of this boat had no protrusions or uneven shapes to cause drag. Gone were the deck gun, railings for crew handholds, nonessential masts, aerials, deck planking and bitts.

With its capacious fuel bunkers, the Type-XXI could easily made the long dangerous journey from its bases in Europe past the English and Allied blockades into the mid-Atlantic. With its incredible speed and endurance, it would have been nearly impossible for convoy escorts and aircraft to stop with their current tactics.

Unfortunately for the U-Boat service but fortunately for the allies, the war ended after only two of the new Type-XXIs had managed to clear harbor for patrol. They were captured and confiscated in port by the Allied militaries. Their design, however, lent a great deal to the first generation of true submarines, the nuclear-powered ships sailed by the American and Soviet navies.

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