Vibration and Forces - Important element in ship vibration

Page 9
Index

Important element in ship vibration

The four elements of importance in ship vibration are:
Excitation,
Stiffness,
Frequency Ratio, and
Damping


Any of the following contribute to vibration reduction:

1. Reduce exciting force amplitude (F).
In propeller-induced ship vibration, the excitation may be reduced by changing the propeller unsteady hydrodynamics. This may involve lines or clearance changes to reduce the non-uniformity of the wake inflow or may involve geometric changes to the propeller itself.

2. Increase stiffness, (K). 
Stiffness is defined as spring force per unit deflection. 
In general, stiffness is to be increased rather than decreased when variations in natural frequency are to be accomplished by variations in stiffness.

3. Avoid values of frequency ratio(W/Wn) near unity, i.e the resonant condition. In resonance, the excitation is opposed only by damping. 
The frequency ratio (W/Wn) can be varied by varying either excitation frequency W or natural frequency Wn. 
The spectrum of W can be changed by changing the RPM of a relevant rotating machinery source, or, in the case of propeller-induced vibration, by changing the propeller RPM or its number of blades. Or is changed by changes in system mass and/or stiffness; increasing stiffness is the usually preferred.

4. Increase damping. 
Damping of structural systems in general, and of ships in particular, is small; (<<1). 
Therefore, except very near resonance, the vibratory amplitude is approximately damping independent. 
Furthermore, damping is difficult to increase significantly in systems such as ships; damping is, in general, the least effective of the four parameters available to the designer for implementing changes in ship vibratory characteristics.

Experience has shown that attention to vibration in concept design of large ships can usually be paid to the following items:
1. Hull girder vertical vibration excited by the main engine.
2. Main machinery/shafting system longitudinal vibration excited by the propeller
3. Superstructure fore-and-aft vibration excited by hull girder vertical vibration and/or main propulsion machinery/shafting system longitudinal vibration.



Q. Axial vibration damping
Q. Torsional vibration dampening
Q. Reasons of vibration in newly overhauled pump
Q. List of vibrations due to propeller shafting
Q. Vibration in engine during astern moment

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