The submarine was powered by two MWM RS 127 S four-stroke, six-cylinder diesel engines of 700 metric horsepower (510 kW; 690 shp) for cruising, two Siemens-Schuckert PG VV 322/36 double-acting electric motors producing a total of 460 metric horsepower (340 kW; 450 shp) for use while submerged.

[2] U-20's first three patrols involved observation (in August 1939) and the laying of mines in the North Sea and off the British east coast.

She sank Magnus about 40 nmi (74 km; 46 mi) east northeast of Peterhead in Scotland.

U-20 sank a steady number of ships on her sixth and seventh patrols, (her eighth foray was relatively quiet), but a series of changes were on the way.

She was transferred to the U-Ausbildungsflottille as a school boat on 1 May 1940, then the Black Sea, avoiding the heavy British presence at Gibraltar and throughout the Mediterranean by being transported in sections along the Danube to the Romanian port of Galați.

She was then re-assembled by the Romanians at the Galați shipyard and sent to her new home in the Black Sea so she could serve with the 30th U-boat Flotilla.

[3] The boat's first patrol in the new environment, but her ninth overall, almost ended in disaster when she tried to torpedo a Soviet submarine chaser; the vessel responded by dropping eight depth charges.