The Type I U-boat was the first post–World War I attempt to produce an oceangoing submarine for Nazi Germany's Kriegsmarine.

Whilst submerged there were problems with depth keeping and stability as air bubbles in the 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.

[5] The two boats produced, U-25 and U-26, were primarily used as training vessels and for propaganda purposes to fly the Nazi flag.

The attack on this ship led to severe depth-charging by two British warships, including HMS Gladiolus.