Marine propellers operate, essentially, the same way that aircraft propellers do: the propellers generate forward thrust by pushing the fluid back, and the resulting force moves the object forward. This is due to Newton’s third law of motion— for every action, there is an equal and opposite reaction. The propellers are connected to a rotary shaft that is powered by an engine. But marine propellers come in many different specs, all classified and sorted by the number of attached blades and the blade pitch.
An engine may have two to five propeller blades. They have a minimum of two blades for optimal efficiency, and more are added if they have to support heavier loads. A three blade propeller costs less to manufacture, has good high-speed performance, and solid acceleration characteristics. However, these propellers are not efficient for low-speed handling. A four blade propeller costs more to manufacture than the three blade propellers but have better strength and durability, give good low-speed handling and performance, have better holding power in rough seas, and has better fuel economy than other types. Five and six-blade propellers are also more expensive, but they have better holding power in rough seas and support heavier loads. They are often used for heavy container ships.
Propellers may be fixed pitch or controllable pitch. The blades on a fixed pitch propeller cannot be altered because they are permanently fixed to the hub. They do not have great maneuverability compared to a controllable pitch propeller, but they are less complicated and cost less to manufacture. Controllable pitch propellers, also known as variable pitch propellers, can be altered during operation to accommodate various operating requirements. Therefore, they have increased maneuverability and improve engine efficiency. However, due to the mechanical and hydraulic arrangements that allow them to move, they are more expensive to manufacture.