Effectiveness of a two-part tow for decoupling ship motions

Two-part tow configurations are becoming more widely used for stabilization of towed bodies, but there have been few experiments to quantify performance. Two-part tows consist of a depressor at the bottom of the main tow cable, followed by a connecting cable and a towfish which are both close to neutral buoyancy. Because the angle between the two cables at the depressor is large, ideally a right angle, heave of the main tow cable caused by ship motions is markedly attenuated before affecting the towfish. This paper describes a full scale, detailed study of motions of the connecting cable and towfish with vertical excitation of their tow point. For oscillatory excitation, the amplitude ratio of the towfish heave with respect to the tow point heave was found to decrease with increasing frequency or cable length and with decreasing tow velocity. The trial results are compared to simulation with DYNTOCABS, a fully dynamic three-dimensional cable simulation program