The invention relates to vessel (1) comprising a hull (3) and a deck (2), a substantially cylindrical rotor (6) having a peripheral wall (8) rotatable with respect to the deck (2) around a longitudinal center line (20), the rotor (6) being mounted on the deck (2) in such a manner that in an operational state the rotor (6) is substantially vertically oriented, characterized in that a flap (18) is arranged near the rotor (6) extending substantially in a plane which is parallel to the rotational axis of the rotor (6), and the length of the flap chord (Rfc) is between 20%-90% of the rotor (6) diameter (Dr), wherein the position of the flap (18) can be adjusted with reference to the longitudinal center line (20).