Hull Vibration

The natural frequencies of ship hulls are relatively difficult to predict accurately and are also influenced by the loading condition. MAN B&W Diesel and Sulzer two-stroke engines do not generate any free forces; and free moments only are generated for certain numbers of cylinders.

Hull vibration can only be excited by a Sulzer RTA low speed engine if it is located on or near a node of critical hull vibration, and if the frequency of this vibration mode coincides with the first or second harmonic of the engine excitation. Furthermore, the magnitude of the troublesome free moment has to exceed the stabilising influence of the natural damping in the hull structure.

If resonance is foreseen, Sulzer notes the following solutions (Figures 1.19 and 1.20):

• Lanchester balancers, either on the engine or electrically driven units usually located on the steering gear flat, compensate for ship vibration caused by the 2nd order vertical moment.

• Counterweights on the crankshaft often represent a simple and effective solution for primary unbalance, which is only relevant to four-cylinder engines.

Second-order

Second-order

Figure 1.19 Provisions for vibration control and balancing on Sulzer RTA low speed engine
Figure 1.20 Typical attachment points for transverse stays of Sulzer RTA low speed engine

• Combined primary/secondary balancers are available for RTA engines to counteract entirely both primary and secondary unbalance.

• Side stays can be fitted between the engine top and hull structure to reduce vibration caused by lateral moments, a situation mainly arising for four-, eight- and twelve-cylinder engines. Hydraulic-type side stays are preferable.

Acknowledgements are made to MAN B&WDiesel and Wartsila Corporation (Sulzer) in the preparation of this chapter. Both designers have produced valuable papers detailing the theory of vibration in shipboard machinery installations, analysis techniques and practical countermeasures.

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