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Propellers of a ship

Ships use propellers to generate pressure difference and provide the motion in water. Nickel-aluminum bronze  alloy is used to make them corrosion resistance.  (80 Cu; 10 Al; 5 Ni; 5 Fe) is appropriate alloy combination when the propeller used in sea water.  Its coefficient of thermal expansion is 9.4 millionth per Fahrenheit.    
Bolt fitted propeller
Three blade propeller
Number of blades depend on the capacity and the ships mass so that enough acceleration can be produced from the propulsion system. For heavy load and high speeds the blades are bolted rather than direct welding to the shaft hub.
Three blade propeller ready after polishing
A four blade propeller model shown here is categorized by direct welding the blades to the shaft hub rather than bolting. Such model are preferred only for low speed and medium capacity ships. The word propeller is used for ship driving system because they make the fluid flow in axial direction. The shape of the blades is one of the important identity of propeller which looks entirely different from turbines and anchors. The angle of the blades are always less than 90 degrees.  
Model of four blade propeller used for ships
Four blade propeller