Figure 3. Flettner rotors are vertical, cylindrical sails spinning around their axis

From

Various Innovative Technologic Devices in Shipping Energy Saving and Diminish Fuel Consumption

Yaser Sharifi, Hassan Ghassemi, Hamid Zanganeh

International Journal of Physics. 2017, 5(1), 21-29 doi:10.12691/ijp-5-1-4