Hull vibratory forces transmitted via the fluid and the shaft from a submarine propeller

Abstract

Disturbing forces due to a rotating propeller are transmitted to a ship or submarine hull via both the propeller shaft and the pressure field that is created in the surrounding water. This pressure field has hydrodynamic and acoustic components, because water is a compressible medium with a finite speed of sound. The work described in this article is a pilot study to investigate the combination of forces generated by the propeller motion that are transmitted to a submerged body. Fluctuating propeller forces are transmitted along the shafting system to result in structural excitation of the submarine hull. These forces are also transmitted to the surrounding fluid and result in an external hull pressure field, because they correspond to acoustic dipoles. Both types of hull excitation contribute to hull vibration and underwater radiated noise. To demonstrate the fundamental nature of the problem, the case of a propeller located on the principal axis of a finite cylinder is considered, which represents a typical submarine arrangement. The analysis in this work has been simplified by considering only stationary fluctuating forces at the propeller hub and by regarding the propeller, hull and shaft as rigid structures. The relative magnitudes and phases of forces transmitted by the fluid and propeller shaft change with frequency in the range containing both low multiples of propeller blade passing frequency and broadband random components owing to turbulent flow in the submarine wake. The effects of a finite speed of sound are found to be significant at low frequencies, causing changes in the hull forces that would be computed by assuming that water is incompressible. Results are presented describing the properties of acoustic dipole sources, which have both hydrodynamic and acoustic pressure fields that change in relative importance with distance from the source.