Self-Excited Vibration of Propeller Nozzles Experienced on Ocean-Going Tugboats : 1st Report

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Despite every means available at the time of construction applied to prevent vibration, violent vibrations were reported on two ocean-going tugboats constructed by IHI, of size that places them among the world's largest. Measurements made of the vibration of aft part including nozzles and analysis of measured data revealed that the vibrations of nozzles :- were constant in frequency, irrespective of propeller or engine revolution, and attained values of acceleration reaching 8,000 Gal; -were generated only under certain conditions of operation, and diminished to very low levels under all other conditions ; - corresponded to what is known as flow-induced vibration, and were closely associated with the water flow around the nozzles. Two possibly ascribable mechanisms were considered as cause of the self-excited vibration. (1) Locked-in vibration induced by Karman's vortex street ; (2) Occurrence of negative damping (known as "galloping"). Neither of these two mechanisms have so far proved conclusively assignable as cause : (1) Karman's vortex streets on account of uncertainty in estimating the exciting force under lock-in condition, and (2) galloping for lack of definite tendency of the negative slope of nozzle lift vs. attack angle curve. The vibrations observed being of such amplitude as to permit no delay in remedial action, measures were taken to counteract both the mechanisms considered above: (1) Karman's vortex streets by attaching fins to the nozzles to decrease their trailing edge thickness, and hence to modify the shedding frequency of the vortex streets, and avoid resonance; (2) galloping by strutting the two nozzles together so as to constrain them to in-phase vibration mode, which was considered from observation of the damping behavior to be conductive to marked modification of the structural and mass damping performance, and consequently convert the damping to positive value. Vibration measurements reperformed after applying the above measures proved the violent vibrations to have been completely eliminated, evidencing ample effectiveness of the measures adopted. Further token of the effectiveness of the measures is provided by the complete absence of reports from either tugboat on violent vibrations, throughout the two years that have since elapsed, during which the tugboats have each marked over 4,000 hours of active service.

Self-Excited Vibration of Propeller Nozzles Experienced on Ocean-Going Tugboats : 1st Report

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