An engine room arrangement (10) for a marine vessel (100) comprises a fin (12) formed at a rear portion of a hull (14) of the vessel (100). The fin (12) projects downward from the hull (14) and protects a propeller (24) of the vessel (100). A gear box (16) is arranged at least in part in the fin (12) and transmits a rotation of a propulsion shaft (18) coupled to a main engine (20) to a propeller shaft (22) drivably coupled to the propeller (24). The fin (12) provides extra buoyancy for the vessel (100). The propeller shaft (22) is formed as a relatively short shaft, while the propulsion shaft (18) is formed as a high speed/low torque shaft having a smaller diameter.