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SUPPRESSION OF SHIP VIBRATION BY FLOW CONTROL L.A. Baier, Professor and Chairman Dept, of Naval Architecture and Marine Engineering University of Michigan. Jesse Ormondroyd, Professor of Engineering Mechanics University of Michigan. SUMMARY A survey of the probable causes of a peculiar type of vibration frequently occurring at the stern of ships, together with a review of hull vibration, is presented in a non-mathematical manner. A study of flow conditions in the vicinity of the propeller was undertaken by the use of self-propelled models in the circulating water tank at the David Taylor Model Basin, Carderock, Maryland. A simple non-mechanical flow control device was designed by the authors and to date has been installed on some 22 vessels with uniform success in the practical elimination of stern vibration. SHIP VIBRATION In the design of a ship the naval architect is concerned with a large mass which is self supporting in a fluid subject to dynamic forces and propelled through this medium by the conversion of torque into thrust. The movement of this mass and the creation of thrust likewise introduce hydrodynamic and physical influences which frequently develop serious problems. One consequence of particular complexity is the phenomenon of hull vibration. Such a condition is often of considerable discomfort to the operating personnel and induces structural failures through material fatigue. The more common sources of vibration are unbalanced rotating parts of the propulsion machinery, shafting or propellers; torsional crit-icals; wave impacts; hull and propeller cavitation. The resulting hull vibrations may be typed as torsional, horizontal linear or vertical linear, in the proppller-shaft-machinery system vibrations may be identified as linear in the longitudinal direction of the shaft; and torsional. These types of ship vibration have been the subject of numerous theoretical and experimental investigations. A rather complete bibliography for this field of study is given of page l1* of Reference (5). It may be of interest to outline the general problem of ship vibration with some further detail. Two fundamentally different kinds of vibration can arise from the rotation of the propulsion machinery in a ship. The first of these comes from mechanical unbalance in the rotating units. If the propeller shaft is bent or the propeller blades damaged, or statically or dynamically unbalanced, a disturbance with a frequency of once per propeller revolution acts on the ship. This can manifest itself in vertical or lateral hull vibration. In multi-pro-pellered ships, where all the wheels are off the fore and aft center line, this unbalance disturbance can give rise to torsional vibration of the hull. Unbalance also can excite bending shirling of the propeller shafting. Hull and shaft vibrations caused by unbalance remain relatively unimportant unless the rotational frequency coincides with a natural frequency of the hull (vertical, lateral or torsional), or a natural whirling torsional or fore and aft frequency of the propeller shafting. When the coincidence of rotational frequency and natural frequency occurs (resonance) large vibration am- 3