Ship propulsion

Numerical simulations of waterjet inlets have been conducted in order to understand inlet performance during ship turning maneuvers. During turning maneuvers waterjet systems may experience low efficiency, cavitation, vibration, and noise. This study found that during turns less energy arrived at the waterjet pump relative to operating straight ahead, and that the flow field at the entrance of the waterjet pump exhibited a region of both low pressure and low axial velocity. The primary reason for the change in pump inflow uniformity is due to a streamwise vortex. In oblique inflow the hull boundary layer separates when entering the inlet and wraps up forming the streamwise vortex. These changes in pump inflow during turning maneuvers will result in increased unsteady loading of the pump rotor and early onset of pump rotor cavitation.

The relationship between cross-flow at a waterjet inlet and delivered thrust is not
fully understood. A direct thrust measurement system was designed for a waterjet
propelled, free running USV. To induce sway velocity at the waterjet inlet, which was
considered equivalent to the cross flow, circles of varying radii were performed at
Reynolds Numbers between 3.48 x 106 and 8.7 x 106 and radii from 2.7 to 6.3 boat
lengths. Sway velocities were less than twenty percent of mean forward speed with slip
angles that were less than 20°. Thrust Loading Coefficients were compared to sway as
a percent of forward speed. In small radius turns, no relationship was seen, while in
larger radius turns, peaks of sway velocity corresponded with drops in thrust, but this
was determined to be caused by reduced vehicle yaw in these intervals . Decoupling of
thrust and yaw rate is recommended for future research.

The use of surface piercing propellers (SPPs) shows promise for high speed operation by virtually eliminating appendage drag, which can be as much as 30 percent of the total drag on a vehicle at high speeds. The scarcity of available systematic test data has made reliable performance prediction difficult. The primary objective of this research is to obtain experimental performance prediction data that can be used in SPP design. In a series of open water tests in a non-pressurized towing tank facility, force transducer measurements were taken at tip immersion ratios from 0.5 to .33, yaw angles from 0° to 30° and inclination angles from 0° to 15° over a range of advance ratios from 0.8 to 1.8. Force transducer measurements were taken for thrust, torque, side forces and moments. These results will help develop a baseline for the verification of SPP performance prediction.

In the present study, 3 wake parameters are semi-automatically measured in 63 composite-labeled images of a surface piercing propeller tested at yaw angles 0-30 degrees, pitch angles 0-15 degrees, propeller immersion ratios of 0.33 and 0.50 and scaled advance ratios 0.656-1.927. A fourth wake parameter is measured in four composite labeled images of yaw angles 0-30 degrees, pitch angle 0 degrees, immersion ratios of 0.33 and 0.50 and scaled advance ratios 1.363-1.927. Measurements are plotted against propeller's angular position. Major findings include the behavior of wake parameters as the values of scaled advance ratio, yaw angle, pitch angle, and immersion ratio vary.