Free-running tests for DTC self-propulsion – An investigation of lateral forces due to the rudder and the propeller

Course-keeping ability of a ship maintains its stability during surge. Although a ship might have a symmetric hull form, the propeller rotation creates asymmetry in the flow leading to path deviations in straight-ahead condition. When there are additional asymmetries involved in other appendages (such as the rudder), this deviation might be greater. In this study, we investigate such a case: the Duisburg Test Case (DTC) has a twisted rudder which significantly decreases its course-keeping ability, as previous studies have shown. In this context, the propulsion parameters of DTC Container Ship at 1/80 model scale were investigated in this study with free-running tests conducted experimentally using a remotely-controlled model in towing tank and numerically via CFD. Experiments and numerical simulations were both conducted for four-degree-of-freedom ship motions excluding sway and yaw for straight-ahead condition. An extensive set of experimental test matrix was created including resistance tests, open-water propeller tests, and free-running tests. Resistance and open-water propeller tests were used as inputs to the self-propulsion estimation (SPE) method to generate estimates for propulsion parameters and verify numerical and experimental free-running test results. All results were found to be in good accordance. After validation of the numerical approach, propeller side force and twisted rudder (at zero rudder angle) normal force were computed at different ship velocities using CFD to computationally highlight the asymmetries due to these two appendages. The DTC propeller generated a side force of 3-4% of thrust at each case. Additionally, it was found that the propeller side force was dominant over the twisted rudder normal force at zero rudder angle.