A direct thrust measurement system for a waterjet propelled, free running USV

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.