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.     Three blade propeller Number of blades depend on the