Class 4 (4th Engineer) SC&S 📅 Sep 2022

Exam Question

(a) Explain what is meant by synchronous or resonant vibration (6)

(b) A ship's speed is increased by 20% above normal for 8 hours, reduced by 10% below normal for 10 hours and for the remaining 6 hours of the day the speed is normal. Calculate the percentage variation in fuel consumption in that day from normal. (10)

Reference Answer

### Part (a): Synchronous or Resonant Vibration (6 Marks)
**1. Definition of Resonance**
Synchronous or resonant vibration is a critical dynamic phenomenon that occurs when a periodic exciting force acts on a system at a frequency that is equal or very close to one of the system's natural frequencies. When this condition, known as resonance, is met, the energy imparted by the exciting force is efficiently absorbed by the system. This results in a rapid and significant amplification of the vibration amplitude, which is limited only by the inherent damping within the system. In a marine context, this can lead to excessive stresses, noise, and potential catastrophic failure of ship structures and machinery.
**2. Sources of Excitation (Forcing Frequencies) on a Ship**

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