Abstract: A typical inventive device is self-contained and is designed for adjunctive placement at the unconnected end of a towed body in order to test dynamic motions of the towed body. The inventive device includes an IMU, a computer, wired/wireless communication electronics, a fluid-dynamically shaped member, and a cylindrical housing (which encloses the IMU, the computer, and the communication electronics). The housing is connected to the towed body aft of the towed body, and the shaped member is connected to the housing aft of the housing. Data is acquired that is indicative of the inertial dynamics of the towed body while in motion through a fluidic medium. Differently shaped members can be independently connected to the housing at different times, each influencing in its own way the inertial dynamics of the towed body and yielding its own set of data. Comparisons can thus be drawn between/among differently shaped members.