"A merchant submarine is a type of submarine intended for trade, and being without armaments, it is not considered a warship like most other types of ...
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submarines. The intended use would be blockade running, or to dive under Arctic ice.
Strictly speaking, only two submarines have so far been purpose-built for non-military merchant shipping use, outside of criminal enterprises, though standard or partly converted military submarines have been used to transport smaller amounts of important cargo, especially during wartime, and large-scale proposals for modern merchant submarines have been produced by manufacturers. Criminal enterprises have also built transport submarines to avoid authorities, such as narcosubs." - (en.wikipedia.org 30.01.2021)
