to kite sails, Flettner rotors often offer considerable efficiency gains when compared to the size of a sail or kite, versus the size of the rotor and prevailing wind conditions. Examples of 2018 Flettner rotor installations include : Cruise ferry Viking Grace became the first passenger vessel with a rotor. The liquid bulk tanker Maersk Pelican in was retrofitted with two rotors. The ultramax bulk carrier Afros received four rotors, which can be moved aside during port operations. Rising trends The efficiency gains of these three propulsion assistance mechanisms are typically around 15–20% depending on the size of the system. The main reason for the use of these mechanisms not being more widespread is mostly the hesitancy of shipping companies to install untested equipment. With government initiatives to encourage a decrease in carbon emissions, and with rising fuel costs, it appeared probable that these propulsion systems would see more widespread use in the coming years. Projects Smart Green Shipping Alliance Vindskip: A ship design using its hull as sail Wind Challenger Project Ventifoil Oceanbird Sailcargo Organizations International Windship Association See also Viking Grace - rotor assisted cruise ship Wind Surf wind assisted cruise ship Hydrogen-powered ship Nuclear marine propulsion Internal drive propulsion Integrated electric propulsion Combined nuclear and steam propulsion Astern propulsion Marine propulsion Air-independent propulsion References Marine propulsion Wind