A propulsion unit is disclosed with a hollow strut having an upper portion with an upper end portion passing through a passage (P1) formed between a first outer bottom and a second inner bottom in a vessel. The upper end portion is rotatably supported with a slewing bearing and sealed with a slewing seal towards the vessel. A cooling arrangement includes a cooling air duct system, at least one fan and at least one cooling unit. The slewing seal can include an upper slewing seal and a lower slewing seal at a vertical distance (H1) from each other with a space formed between the slewing seals. The upper end portion has openings (O1) into the space between the slewing seals. A first cooling air duct is directed to this space, whereby cooling air (L1) can be circulated through the first cooling air duct (400) into the space and through the openings (O1) in the upper end portion to the interior of the strut, or return air (L2) can be circulated from the strut in an opposite direction.