Open Water Tow Tank Testing of a Surface Piercing Propeller

Open water tests of a surface piercing propeller with a pitch to diameter ratio of 1.9 have been performed in a non pressurized towing tank. Flow visualizations of the ventilated cavities formed behind the propeller and Shaft line force/torque measurements were taken at tip immersion ratios of 0.50 and 0.33, yaw angles from 0° to 30° and inclination angles from 0° to 15° for advance ratios ranging from 0.8 to 1.9. Three wake parameters, the wake pitch, wake pitch angle and wake diameter, were identified and semi-automatically measured using composite images of the wake of the surface piercing propeller. The symmetry of the wake at non zero yaw angles, as well as the time evolution of the width of successive ventilated cavities in the wake were explored. It was found that the wake pitch angle changes proportionally to the advance ratio and does not change significantly as yaw angle changes; the ratio of wake diameter to SPP diameter increases as the advance ratio decreases; and the wake was most symmetric at a depth of immersion of 33%, a yaw angle of 15 degrees, zero shaft inclination angle, and an advance ratio of 1.2. The SPP was found to operate in a fully ventilated condition for scaled advanced ratios of 0.905 – 0.721. Asix-componentt load cell was integrated with the rotating propeller shaft line to provide measurements of propeller thrust, torque, side forces and moments. An examination of the efficiencies determined from these measurements and subsequent analyses reveal that, while the thrust measurements are near values expected from empirical relations, there appears to be an error in the torque measurements, likely due to an instrumentation problem. Nevertheless, the time histories of the load cell data are instructive in qualitatively illustrating the relation between carriage speed, propeller speed and the transient forces/moments acting on the propeller.