Two-blade propellers are usually used in propeller aircraft, and three-blade propellers are usually used in ships.
From the perspective of the size of the propellers, we usually see large two-blade propellers and small three-blade propellers. They are mainly used for different media with different densities. The density of air is less than the density of water. In order to increase the reaction force flowing backward to the aircraft, the gap between the blades is large. If the clearance of the aircraft blades is small, the airflow will be small. There were also three-blade propellers used in airplanes in the early days, mainly because people didn’t know much about fluid mechanics and made progress in practice.
The density of water is 800 times that of air. Therefore, the reaction force of water on a ship with a three-blade propeller will be great, which is why a large propeller is not needed. If the propeller is too large, the power in the edge area of the blade will be ineffective, which is the same principle as the impeller steamer.
The number of blades is related to the pitch of the propeller. The number of blades is firstly related to power. Considering different powers, it can be three blades, four blades, and five blades. The number of blades is also related to the vibration of the ship, that is, the frequency or multiplier of the propeller blades and the natural frequency of the structure will resonate; the other is to consider air bubbles, which will cause noise and blade corrosion.