Because the forward power of the ship comes from the propeller, the forward rotation of the propeller has the characteristics of sucking forward and pushing backward to push the ship forward, and vice versa, similar to braking. The density of water is related to the forward power (hysteresis) generated by the compression of some ships after the stationary propeller rotates, especially for ships with large displacement. In addition, the generation of kinetic energy (the greater the tonnage, the greater the inertia) can not be overcome by reversing. As the 20,000-ton ship stops by express train, it can continue to glide for 2-3 nautical miles (1 nautical mile equals1.85km), so it is impossible for the ship to stop immediately. The sinking of the Titanic proved that it was too late for the captain to find that the iceberg had turned upside down, and the general large-tonnage ships basically did not move in the harbor. It's normal to have a tugboat to help you get out of the port. Tugboats in general ports are 6000 horsepower. It is normal for a 60,000-ton aircraft carrier to assist in leaving Hong Kong. There is no need to build our aircraft carrier. The same is true of American aircraft carriers. There is a tugboat at the port to help leave the dock. Don't let laymen talk about him, and don't let others laugh at their ignorance.