10.Longitudinal vibration of propulsion shafting systems is often excited by the variable thrustdeveloped by the propeller due to the non-uniform wake pattern in which it operates. Thepredominant periodic component of the thrust developed by a propeller occurs at blade-ratefrequency, i.e., the number of propeller blades times the rotational frequency of the shaft. For a four-blade propeller running at 60 rev/min, what is the predominant periodic frequency that excitesthe longitudinal vibration of the propulsion shafting systems?
(A)1 Hz
(B)2 Hz
(C)3 Hz
(D)4 Hz