Motion characteristics and T-foil based optimization of marine towed-cage in swell

Abstract

Unmanned surface vehicles (USVs) are widely used in variety of ocean operations and are typically launched and recovered from a mother ship. However, their recovery can be challenging, especially when they encounter ocean swells. In this paper, the main research objective is to analyze the performance of a cage-type davit launch and recovery system, with a specific focus on the marine towed-cage used in this process. The study utilizes Computational Fluid Dynamics (CFD) to conduct a detailed analysis of the longitudinal motion characteristics of the marine towed-cage during towing conditions. It has been observed that while encountering ocean swell, the marine towed-cage, as a hollow-out surface towed vehicle, faces difficulties such as stern trimming, oscillations, and an unstable attitude. This instability is mainly attributed to an insufficient pitch restoring moment. In order to address these issues and to optimize the longitudinal motion of the marine towed-cage, the study proposes two aspects of optimization: geometric optimization and cable length selection. This involves designing a modified marine towed-cage equipped with a T-foil and using a constant and sufficiently long cable. Subsequent to implementation of the proposed optimization scheme, the modified marine towed-cage exhibits significant improvements such that high-frequency oscillations are reduced, large-amplitude trim by the bow is minimized, especially at high towing speeds, and the system becomes more adaptable to changes in towing speed. The research presented in this paper provides a better understanding of the seakeeping performance of the marine towed-cage under towing condition and contribute to the safer and more efficient recovery of USVs.