Towing operations are widely applied in various fields such as maritime accident rescue, assisting large vessels entering and exiting ports, and transporting large ocean platforms. Tugboats and the towed objects form a complex multi-body system connected by flexible cables, and during operations, they are subjected to the effects of complex marine environmental loads. Current research focuses on using numerical simulations and model tests in water tanks to study the motion response of towed objects and cables under the action of environmental loads. There is a lack of research that combines the mechanical response and structural strength with the load conditions of towing operations. Taking cables as an example, most studies focus on the mechanical properties of cables without considering the impact of towing conditions. After reviewing the literature, this paper summarizes the shortcomings of the existing research and points out several potential research directions in the field of towing: the mechanical response of cables during the initial stage of towing, experiments on towing by multiple tugboats, research on composite fiber cables using experimental and finite element simulation methods, and structural optimization of components related to towing operations.

To research the stern flap (SF) and waterjet–hull interaction, unsteady Reynolds-averaged Navier–Stokes (URANS) simulations for a waterjet-propelled trimaran considering sinkage and trim are performed. Uncertainty analysis of the numerical results for the bare hull (BH) model is presented. At the design speed Froude number (Fr) of 0.6 and under displacement state, the model-scaled trimaran, installed with stern flaps of varied angle and length, tests the BH and self-propulsion (SP) performance based on URANS simulations. For the resistance, the global effects due to motions and the local effects of SF, waterjets (WJ), and the coupled term between SF and WJ on the hull are separately analyzed. Taking the waterjet propulsion system into account, an SP model with reasonable stern flap effectively reduces the trim, the resistance acting on the hull and the waterjet thrust deduction which contributes to energy-saving and high-efficiency propulsion. The mechanism of the improved performance of the waterjet-propelled trimaran with stern flaps is discussed. For the resistance increment, the global effects, the local effects of SF and WJ are the major reason for resistance increase, and the nonlinear coupled term of local effects contributes to the resistance reduction most. In addition, the different resistance components of frictional, hydrostatic, and hydrodynamic are separately researched, which shows that the pressure resistance components plays a leading role in the total resistance reduction in the SP model with the suitable SF.

Bionic flapping hydrofoil motion is increasingly used in bionic underwater robots. However, many scholars have focused their attention on the navigation problem in deep-water environments, thus ignoring changes in the hydrodynamics of hydrofoils under the action of the near-free-surface effect. This paper studies and explores the hydrofoil motion in near-free surfaces through the RANS viscous flow numerical simulation method, combined with overset mesh and adaptive mesh technology. Three different forms of motion are studied respectively, including a stationary fixed, a single-degree-of-freedom pitch and a two-degree-of-freedom heaving-pitching coupled hydrofoil. The effects of varying the immersion depth d on the lift and thrust generated were analyzed. Results indicate noticeable differences in the free surface action among different motion forms. When the water depth is less than one chord length C, the lift and thrust of the three motion forms decreased rapidly decrease. When d/C=1~1.5, the static fixed hydrofoil lift and thrust gradually approach the deep-water state. When d/C>2, the pitching motion of a single degree of freedom also tends to be stable. The two-degree-of-freedom motion is d/C>3. This finding shows that the effect of the near-free surface is closely related to the vertical motion. The greater the vertical motion is, the more severe the effect.