When one thinks of a ship’s propulsion system, the conventional propeller and rudder setup is what comes to mind. While there are many other forms of powering a craft (such as waterjet propulsion, sail power etc.), a rather uncommon type of propulsion system is the Flettner rotors.
Rarely used on commercial vessels, it is an experimental concept in wind-based power generation that has been in development since the 1920s.
You can spot a Flettner rotor ship easily by the large stacks or rotors that...
https://www.marineinsight.com/naval-architecture/flettner-rotor-for-ships-uses-history-and-problems/