An investigation into the effects of submergence depth, speed and hull length-to-diameter ratio on the near surface operation of conventional submarines

The strategic requirement for naval submarines to operate in near surface and littoral environments continues to increase as they are more frequently employed to support joint force missions. The operational workload for helmsmen and the risk to mission safety and success are significantly increased due to the interaction between the submarine and the free-surface. This thesis reports on an experimental investigation into the effects of submergence depth, speed of advance and length-to-diameter ratio on the interaction between a streamlined axisymmetric body and the free-surface when travelling in a near-surface condition. A broad review of the existing literature indicates that little work has been published to date on the effects of length-to-diameter on the near-surface performance of streamlined axisymmetric bodies. Furthermore, there is little or no experimental data available describing the vertical force and moment that act on a shallowly submerged body moving beneath the free-surface. Nonetheless, there is evidence to confirm that the requirement for submarine near-surface operation is significant and that a wellfounded understanding of submarine near-surface performance is needed. A model scale experimental program was conducted to measure and observe the resistance, lift force, trimming moment and wave field generated by a series of submerged bodies of revolution moving at constant forward speed. The Joubert conventional submarine geometry was tested in its bare hull configuration in three length-to-diameter ratio formats: 7.30, 8.50 and 9.50. The three geometries possess the same maximum diameter. The models were tested at velocities that correspond to a Froude number range of 0.10 to 0.50 inclusive and at centreline submergence depths of between 1 and 3.5 hull diameters. The results of the experiment indicate that the wave resistance, lift force and trimming moment all vary periodically with speed and are directly influenced by the wavelength of the free-surface wave field generated by the submerged body. The steady-state wave field itself is a direct function of the submerged body’s length-to-diameter ratio and speed of advance (Froude number). The magnitude of the forces and trimming moment were found to diminish exponentially with an increase in submergence depth. Considering the submergence depths and speeds experienced by naval submarines when conducting near-surface operations, it is concluded that the effect of wave resistance is a secondary issue and that the vertical lift force is of the greatest operational significance. Based on the test results of the three Joubert hull geometries, it was observed that for near-surface operation a larger length-to-diameter ratio is preferable for achieving minimum resistance and vertical plane motion (lower relative lift force and trimming moment). An evaluation of the SHIPFLOW 4.7 potential flow software program was completed using the experimentally measured data in conjunction with additional published experimental and numerical wave resistance data. Good correlation was observed for the predictions of lift force and trimming moment. Mixed results were achieved when comparing the numerically predicted and experimentally measured wave resistance. Nonetheless, it is concluded that the potential flow method offers an inexpensive and suitable approach to evaluating the near-surface performance of submarine type geometries.