An extensive study of crashback was performed in the U.S. Navy's William B. Morgan Large Cavitation Channel (LCC), located in Memphis, TN. Propeller 4381 was attached to a standard axisymmetric submarine hull model (DTMB Model 5495-3) which was then suspended in the LCC. Forces and moments were measured on both the body and the propeller for steady and unsteady crashback conditions. The unsteady conditions were simulated by allowing the propeller to 'windmill' in the forward direction and then engaging the propeller in the reverse direction, with the tunnel velocity held constant. Forces and moments were measured during the change in propeller speed. A further simulation was undertaken whereby the tunnel motor was shut off once the windmilling propeller engaged in reverse rotation and the tunnel speed was allowed to coast down to zero. Forces and moments were measured during this simulation also. Laser Doppler velocimeter (LDV) measurements of the flow field around the propeller during steady crashback were obtained in addition to extensive flow visualization measurements. The dimensionless force and moment data were found to collapse when plotted against the ratio of the actual propeller speed to the propeller speed required for self propulsion in forward motion. The force and moment coefficients were found to have a local maximum when this ratio was around -0.8. Unsteady crashback maneuvers also were investigated with two different types of simulations in which the propeller and tunnel speeds were allowed to vary. In some cases the peak off-axis force and moment coefficient magnitudes exceeded those observed during the steady crashback measurements. The LDV studies showed that the ring vortex structure appeared to migrate upstream and outward as the reverse propeller speed increased in magnitude. (145 figures, 17 refs.).


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