Question

With respect to Induced Vibrations in a ships hull:

(a) State FOUR sources of excitation that may induce vibration into the main hull girder.

(b) Suggest methods for reducing the vibration levels induced by EACH of the exciting forces in (a).

Answer 1

Part (a)

Propeller excitation: the propeller produces fluctuating forces (blade frequency and multiples) due to the varying pressure field and the wake, transmitted through the shaft and the water to the hull, causing vibration of the stern and the whole hull girder.
Main engine excitation: the reciprocating and rotating parts of the engine produce unbalanced forces and moments (primary and secondary, and their harmonics) at the engine's firing frequency and multiples, transmitted through the engine seating to the hull.
Wave excitation: the periodic wave forces on the hull (at the encounter frequency) excite the hull girder, particularly in heavy seas, causing springing and whipping.
Auxiliary machinery and other rotating equipment: pumps, generators, compressors and other machinery produce unbalanced forces that are transmitted to the hull through their mountings.

Part (b)

Propeller: increase the clearance between the propeller and the hull (aperture), use a larger blade area and a suitable blade number to avoid resonance, fit a skew or a different blade design, and use a propeller boss cap fin; also avoid operating at resonant speeds.
Main engine: balance the engine (fit balance weights, use a suitable firing order), fit a flexible coupling and a resilient engine mounting, tune the engine speed to avoid the hull's natural frequencies, and use a tuned vibration damper on the crankshaft.
Wave excitation: reduce speed and change course in heavy seas to avoid the resonant encounter frequency, and design the hull to have adequate stiffness and damping; use a hull stress/vibration monitoring system.
Auxiliary machinery: fit resilient (anti-vibration) mountings, balance the rotating parts, isolate the machinery from the hull, and avoid operating at resonant speeds.

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