Why can't the single propeller of the twin-engine twin-propeller ship reach the rated power?

Power will be affected by ship's attitude, sea conditions, load and other factors. In practical work, the power output of a ship with two engines and two propellers is often affected by many factors, such as the ship's attitude, sea state and load, which will affect the ship's propulsion efficiency and power output, while the power output of a ship with one engine and one propeller may not reach the rated power under certain working conditions, and is also affected by the ship's attitude, sea state and load.