U-boat - Wikipedia

German submarine U-175 - Wikipedia

What was the fastest submarine in World Wars 1 and 2? - Quora

Type VII submarine - Wikipedia

German submarine U-234 - Wikipedia

How deep could a German U-boat go? - Quora

Campanha de U-boat – Wikipédia, a enciclopédia livre

Are military submarines still called U-boats? - Quora

What is the definition of a U-Boat badge? Did Admiral Dönitz ever

How many torpedoes did 'U boats carry at full complement in WW2