PROPELLER-EXCITED VIBRATION: A CAVITATION CRITERION FOR THE ASSESSMENT OF SCALED MODEL WAKES

For some years, B.S.R.A. has been systematically investigating the subject of propeller-excited hull vibration. The author (of B.S.R.A.) gives some general information on this work and examines some aspects relating to propeller cavitation wake non-uniformity, and the production of high surface-forces on the hull plating. Cavitation and cavity pulsations are discussed as a primary source of vibration excitation and a criterion, in the form of a diagram in which cavitation number is plotted against a wake non-uniformity parameter, is presented as a simple means of predicting the possibility of the occurrence of large excitation forces. The diagram is based on full-scale and model experimental data, and it is mentioned that it should be up-dated as further data becomes available; its principal application is in the assessment of a model wake distribution when propeller diameter and rpm have already been determined. For preliminary design, a table is given which indicates the type of afterbody shape necessary to produce a value for the wake non-uniformity parameter compatible with pre-determined propeller diameter and rpm. The method of use of the diagram and table, and the effects of a change in propeller diameter and rpm (e.g., with a change of engine-type during the building of a standard series of ships), are explained with the aid of a numerical example. If high excitation forces are predicted in the diagram, they may be reduced by decreasing propeller rpm (so increasing the cavitation number), or by improving the wake distribution by modifying the stern form or fitting flow-improvement appendages.