An apparatus and a method for installing a propulsion unit to a hull of a ship. The propulsion unit comprises an upper part of the propulsion unit attachable to the hull of the ship and a lower part of the propulsion unit where to a propeller shaft is rotatably supported, and the upper part of the propulsion unit having flanges to attach the flanges tightly to the hull, and wherein there is an aperture in bottom of hull where to the propulsion unit is to be installed. The apparatus comprises a plurality of watertight covers (32,34) covering openings (24,28) in the bottom of hull and a plurality of watertight covers (50,52) covering openings (46,48) in the top of the upper part of the propulsion unit, wherein the apparatus comprises a removable hull-side cover (32) covering at least the opening (24) under it, and at least one removable propulsion unit side cover (50) covering the opening (46) under it and facing the opening (24), and the removable covers (32,34,50,52) are bolted from the hull-side.