118.Resonance & Critical speed.

Critical Speed Range (or Barred range)

-It is the range of speed at which resonance may occur.

-Resonance is a phenomenon experienced when the engine’s operating frequency       coincides with the natural frequency of the hull which results in vibrations of higher   amplitudes than normal which is very dangerous.

-The critical speed range should be passed as soon as possible.

-In order to avoid operation of the main engine in critical (or barred) speed range, a quick-   pass function is provided.

 If the bridge command is within these areas, the lamps for critical rpm limit on the bridge   and in the engine control room will illuminate.

-During acceleration, keep the engine running below the lower critical speed,

-During deceleration it will keep the engine running above the upper limit.

Indications of Critical speed range :

- More vibrations can be experienced.

-The lamps for critical rpm limit on the bridge and in the ECR will illuminate.

 *The speed range is marked RED in all Manoeuvring Panels. (Bridge, ECR, Local)

 *The critical speed range is also labeled adjacent to the Manoeuvring Panels or near the   telegraphs.

Means to reduce the amplitude of vibrations:

-Sectionalizing of the shafting and

-Interposing special couplings between sections.

-Using vibration absorbers fitted to the crankshaft.

Means to avoid resonance is to adjust:

-Speed of the engine

-Mass of the flywheel or

-Engine’s firing order.