The engine of a ship develops 600h.p. and transmits the power by a horizontal propeller shaft which runs at 120 rpm. It is proposed to design a hollow propeller shaft with inner diameter 60% of the outer diameter. Considering torsion alone, calculate the diameter of the propeller shaft if the stress in the material is not to exceed 630kg/cm2 and also the angular twist over a length 250 cm is not to be more than one degree. The modulus of rigidity of the shaft material is G = 0.8 ×105 kg/cm2.