An Equilibrium-Based Design Framework for Pitch Stabilization of a Forward-Towed Towfish

Forward-towed towfish systems offer practical advantages for uncrewed surface vehicle (USV)-based operations because they are inherently compatible with simplified launch-and-recovery procedures and reduced deck intervention. However, locating the towing point near the forebody introduces a persistent stern-sagging trim bias under steady towing, which can degrade sensor alignment and hydrodynamic performance. This study presents a quasi-static equilibrium-based framework for passive steady-state pitch-trim design in forward-towed towfish systems. The trim problem is formulated as a pitching-moment balance about the towing point together with a vertical-force design constraint imposed on a fore–aft wing subsystem, yielding closed-form expressions for the required wing installation angles as explicit functions of towing speed. The resulting solution reveals an inverse-square dependence of the required angle magnitude on towing speed and clarifies the geometric role of wing placement in restoring-moment generation. A full-scale towfish prototype equipped with a 1°-resolution passive wing-angle adjustment mechanism was developed and tested in a towing tank at three representative towing speeds. Pitch behavior was evaluated using mean pitch, standard deviation, and RMS metrics. The experimentally identified minimum-response settings shifted toward smaller wing-angle magnitude as towing speed increased, consistent with the equilibrium-based trend. Pair-space comparison further showed that the analytical fore–aft angle pairs lie close to the experimentally preferred equal-magnitude settings over the tested towing-speed range. These results indicate that the proposed framework captures the dominant restoring-moment requirement governing steady-state trim and provides a practical first-order methodology for passive trim design of forward-towed towfish systems without resorting to high-fidelity multiphysics simulation.