Wake-adapted ducted propeller for full-scale generic underwater vehicle: parametric study on blade skew for unsteady propeller thrust

A parametric study of blade skew for unsteady propeller thrust in the wake of an underwater vehicle (BB2) is presented. Unsteady thrust forces transmit via the propeller shaft and produce undesirable noise and vibration, particularly at frequencies corresponding to multiples of the blade number. The effectiveness of blade skew in reducing noise and vibration has been reported in previous studies, though not for underwater vehicles. The amplitudes of the dominant harmonics of the unsteady propeller thrust are determined using a Fourier series. The balanced skew reduces the dominant fifth harmonic more significantly than the biased skew at identical total-skew angles. The total-skew angle strongly influences the unsteady propeller thrust magnitude for balanced skew designs, whereas the effect of the maximum forward-skew angle is negligible. Significant fluctuations in one-blade loads are observed when the blade passes through the downstream wake of the vehicle's sail and casing. Increasing the blade total-skew reduces the blade load fluctuation when the blade approaches the top dead centre position, but has limited influence on the fluctuations when the blade moves away from this position. The nominal wake distribution at the propeller plane is analysed, and the fifth harmonic is identified as having the largest amplitude. This suggests that an increase in the number of blades would reduce the unsteady propeller thrust of the full-scale BB2 underwater vehicle.