A vessel (1) and a stern extension (4) for being mounted on the watertight hull (2) of a vessel as an extension to the stern, the stern extension comprising: a stern unit (5) extending in a rearwards direction from the stern, the stern unit comprising side faces (6) adjoined along a vertical center plane thereof, the side faces (6) providing a barrier (8) partly defining a stern unit enclosure (9), wherein the side faces (6) constitute a continuous extension of the hull shape of the stern; and a mounting mechanism for securing the stern unit to the hull of the vessel, wherein the stern unit is externally mounted on the watertight hull, thereby providing an extension of the stern without prolonging the watertight hull (2).