Patrick Chovanec on Twitter: "The u-boat normally ran on the surface using its diesel engines (which needed oxygen and an exhaust outlet), reaching a much faster top speed of nearly 18 knots.
U-Boat Engine Start in the Kiel-Wik Engine Museum - YouTube
German Industries and the U-boat offensive - Technical pages - German U- boats of WWII - Kriegsmarine - uboat.net
Engine Room, with twin diesel engines. - Picture of German Submarine U-995, Laboe - Tripadvisor