The Hydrodynamic Development of a Low-Drag Autonomous Submersible.

Three generic hull forms were compared on the basis of the energy required and the energy available to perform a trans-oceanic water sampling and data collection mission. It was concluded that the most feasible solution for this type of mission is a submersible with a laminar flow hull shape. Having defined the speed, volume and range of the vehicle a computational optimisation of the hull, fins and propeller was undertaken. A previously unpublished aftend shape was joined to the Unmanned Free Swimming Submersible (UFSS) (Johnson (1980)) fore/mid body shape and the combined hull optimised for minimum drag. The stick free and stick fixed static stability of the fins and hull were investigated and the optimum solution chosen. A drag-matched, single stage propeller was designed to produce the required thrust to propel the vehicle taking into account the hull boundary layer in which it will be turning. The computations were verified by a series of wind tunnel experiments. The wind tunnel tests on the hull showed that the body exhibited 70% laminar flow and had a drag factor 1/4 that of a torpedo shape. Initial test results exhibited significant flow interference between the aftend of the model and the mounting strut. A second series of tests, employing a revised mounting system, resulted in a full set of static hydrodynamic characteristics for the hull and fins. The propeller was tested both on and off the hull model but no usable results were recorded due to mechanical failure and some inconsistencies in the computational process. The errors in the propeller characteristic calculations are discussed in full. Tests were also conducted in a second wind tunnel with a significantly higher background turbulence. These tests showed some degradation in the laminar boundary layer, moving transition forward to the 45% position. The experimental results show that the laminar flow hull can achieve the very low drag predicted. Even in a more turbulent flow the low drag form still gives 1/2 the drag of a more conventional torpedo shape