Propeller is the main propulsion tool of modern ships, and now most ships are propelled by propeller. There are many kinds of propellers. Depending on the number of blades, the propeller has 2, 3 or 4 blades, or even more. Generally, the more blades, the greater the absorbed power.

According to different structures, propellers can be divided into fixed-pitch propellers and variable-pitch propellers. The pitch of fixed-pitch propeller is fixed, which has the characteristics of simple structure and light weight, so it is widely used in ships. The pitch of the variable pitch propeller can be adjusted, and the pitch can be adjusted by hydraulic or electric drive through the propeller variable pitch mechanism.

At first, a double-pitch propeller was used. Use treble at high speed, bass at low speed, and then gradually increase the number of pitches.