She was part of the last series (Group 4) of her class, having some improvements including more torpedo tubes.

L-21 was later raised, completed and commissioned, making both torpedo attacks and mine-laying.

Among her victims was the Hansa, a neutral Swedish passenger ship travering from Nynäshamn to Visby.

84 innocent people were killed in the attack, which is commemorated by a memorial plaque in Visby Cathedral on Gotland, where the ship's bell is also preserved.

[2] L-21 also managed to sink a number of German warships, including two torpedo boats and a submarine.