Type VIIB - More powerful engines & with an additional 33 tonnes of fuel. 24 Type VIIB u-boats were built (1936~1940). Type VIIBs included many of the most famous U-boats of World War II, including U-48 (the most successful), Günther Prien's U-47, Otto Kretschmer's U-99, and Joachim Schepke's U-100
Type VIIC - the workhorse of German U-boat force (568 commissioned from 1940 to 1945). Type VIIC differed from VIIB only in the addition of an active sonar and a few minor mechanical improvements, making it 2 feet longer and 8 tons heavier. Speed and range were essentially the same. Many of these boats were fitted with snorkels in 1944 and 1945. The most famous VIIC boat was U-96, featured in the movie Das Boot.
Type IXC - long range patrol with an additional 43 tonnes of fuel. 54 were built.