If the boat wants to exercise on the water, it needs to provide external force to push it, so that it can overcome the resistance during exercise. In the past, there were many ways to provide thrust, which can be roughly divided into four categories. One is manpower, such as paddles, fences, and slaps; the second is animal power, such as pulling fibers; the third is wind power, such as sails; and the fourth is mechanical power, such as paddle wheels, propellers, etc.
With the continuous development of human science and technology, propellers stand out among many propulsion devices. In 1752, physicists reported that the propeller was superior to various propellers that existed before it. Especially after the emergence of high-speed steam engines, propellers replaced many other propellers and gradually gained popularity.
The principle of the propeller is relatively simple. The paddle relies on the water movement to make the water flow produce a speed opposite to the thrust direction, and the water flow will give the propeller a reaction force, that is, thrust. The momentum theorem of fluid mechanics can also be used to explain that when the water flows through the propeller disk, the water flow will accelerate due to the action of the blades. In this way, the momentum of the water flow changes. Due to the momentum theorem, the momentum of the fluid changed within a certain period of time is the same as the force it receives, and the reaction force that changes the force received by the internal fluid is just the thrust of the propeller.