The Type I U-boat was the first post–World War I attempt to produce an oceangoing submarine for Nazi Germany's Kriegsmarine. The type was based on the Spanish Type E-1, designed by Ingenieurskantoor voor Scheepsbouw. Only two Type IAs were built. They were not a successful design : due to its single rudder they had a large turning circle and were not very manoeuvrable. The gravity center of the U-boat was too forward so on surface the type I had its propellers exposed when she was pitching. Whilst submerged there were problems with depth keeping and stability as air bubbles in fuel tanks wobbled back and forth. Diving was very slow : at full speed and with six tons of negative buoyancy it took forty seconds to reach ten metres depth. As a result, the type was discontinued and a new type IX class oceangoing U-boats was designed.

The S-class or Srednyaya submarines were part of the Soviet Navy's underwater fleet during World War II. Unofficially nicknamed Stalinets, boats of this class were the most successful and achieved the most significant victories among all Soviet submarines. In all, they sank 82,770 gross register tons (GRT) of merchant shipping and seven warships, which accounts for about one-third of all tonnage sunk by Soviet submarines during the war.