Contribution to Improved Accuracy in the Calculation and Measurement of Torsional Vibration Stresses in Marine Propeller Shafting

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Abstract
Methods of calculating torsional vibration stresses in marine propeller shafting have differed mainly in the apportionment of damping energy between engine and propeller, and for this purpose a propeller damping constant of average value has commonly been assumed. The author has long considered this simplifying approximation unjustified, since no account is taken of variations in geometrical design factors as affecting propeller damping properties. As the propeller is usually by far the major damping influence in the single-node mode, the accuracy of stress prediction must often suffer correspondingly. The paper details a convenient method of deriving damping coefficients for modern three- and four-bladed bronze propellers. Curves are plotted, for a series of pitch ratios, against a non-dimensional power coefficient involving only quantities usually known in the design stage, namely, horse-power, propeller revolutions and diameter. Allowance for disk area ratio is by interpolation between limiting values. Attention is also drawn to possible serious errors in interpreting torsiograph records taken from the crankshaft end of marine heavy-oil engine systems, arising from the phase difference between engine and propeller vibration at resonance—a consequence of heavy propeller damping. Here again improved knowledge of propeller damping characteristics is desirable if maximum accuracy is to be achieved. The paper concludes with a worked example from practice, indicating the order of magnitude of errors involved, with details of a simple vector method of allowing for the phase effect in any given instance.

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