Experimental Study on the Drag Resistance of Tunnel Towing Navigation Facilitating Upstream and Downstream Connectivity in Mountainous River Bends

Navigable tunnels serve as an effective method to connect upstream and downstream navigation structures in mountainous regions with sharp bends. The towing resistance of ships in navigable tunnels, a key technical indicator for towing equipment development, demands focused research. Utilizing the innovative top towing method for tunnels, this study develops a physical model for towed navigable tunnels, conducts ship model tests, and measures and calculates the total resistance of ships towing through navigation under various conditions. Through resistance test results, it analyzes factors influencing the total resistance of ship navigation. The findings reveal: (1) regarding towing speed, at speeds exceeding 1.5 m/s, resistance spikes by 100 kN to 560 kN; (2) concerning water depth, at depths lower than 5.5 m, the impact on a ship navigation’s total resistance is pronounced, reaching 5 to 13 times that of calm water; (3) in terms of flow velocity, at velocities over 2 m/s, the impact on a ship navigation’s total resistance is substantial, amounting to 1.5 to 2 times the resistance at a flow velocity of 1.5 m/s; (4) in comparative analyses, the total resistance of ships towing through navigation in narrow tunnels is significantly higher than calculations based on existing formulas, increasing by 7 to 138 times.