21.推力軸承中為了防止推力塊隨著推力環轉動，在正倒俥推力塊的上方均設有什麼裝置？ (A)振盪器 (B)止動器 (C)滑動器 (D)刮油器

10.Longitudinal vibration of propulsion shafting systems is often excited by the variable thrust developed by the propeller due to the non-uniform wake pattern in which it operates. The predominant periodic component of the thrust developed by a propeller occurs at blade-rate frequency, 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 excites the longitudinal vibration of the propulsion shafting systems? (A)1 Hz (B)2 Hz (C)3 Hz (D)4 Hz

21.推力軸承中為了防止推力塊隨著推力環轉動，在正倒俥推力塊的上方均設有什麼裝置？
(A)振盪器
(B)止動器
(C)滑動器
(D)刮油器

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