Experimental and numerical study of ship-to-ship interactions in overtaking manoeuvres

The study on close-quarter manoeuvring of vessels is of great importance for the safety and efficiency of maritime operations. In this paper, the hydrodynamic interactions between two vessels in moderate-speed overtaking manoeuvres are studied. Computational investigation by free-surface panel method is performed, and the results are assessed against experimental measurements from towing-tank model tests. The influences of overtaking speed and the speed difference between vessels on the hydrodynamic loads are studied. It is found that the free-surface deformation, on account of the blockage effects of the bodies, wave-making properties of the vessels, and the interference of unsteady wave patterns between the vessels, considerably affects the hydrodynamic interactions. In addition, it is also discovered that the influence from the unsteady heave and pitch motions of the hulls on the hydrodynamic loads can be non-negligible. Furthermore, it is found that the slower vessel to be overtaken generally experiences larger loads with more variation than the overtaking vessel. The loads on both vessels become more similar to those of a steady-state di-hull system when the speed difference between vessels is small.

Method for the Calculation of the Underwater Effective Wake Field for Propeller Optimization

A quasi-steady prediction model of propeller hydrodynamic performance was established here using the surface panel method to determine the effective wake field of a propeller. The apparent wake field was accurately determined in advance by CFD (Computational Fluid Dynamics). The average of the induced velocity near the front of the propeller was determined by coupling the steady calculation and the unsteady forecast to render the induced velocity field more consistent with the actual situation when the propeller works in a non-uniform flow field. By superimposing the induced velocity near the front of the propeller with the apparent wake field, the effective wake field was able to be determined. Then the induced velocity field was calculated again to determine the new effective wake. An iterative calculation method was used until the hydrodynamic performance converged. The case described here shows that the effective wake obtained by this method can better predict the hydrodynamic performance of the propeller, and it can provide a basis for the design and optimization of the propeller. It was found that the results of the prediction were consistent with the experimental values.

Design and Performance Investigation of the Energy Recovering Rudder Bulb-Turbine Device

This study presents the performance investigation of a rudder bulb-turbine device (RBTD), which is designed to recover the rotational energy into torque to drive a generator inside the rudder. The blade of turbine is designed first by using lifting line theory, which is modified by lifting surface theory. The induced velocities between the forward propeller and turbine are obtained through the surface panel method (SPM). Hydrodynamic performance for the system is predicted by using SPM. An iterative calculation method is used until the hydrodynamic performance of the system converges. Through the design of turbine behind propeller of a large container ship, the influence of rotational speed of turbine on hydrodynamic performance of propeller-turbine system is observed. Then the size of rudder bulb is determined by calculating the influence of rudder bulb geometry on the hydrodynamic performance of the propeller-rudder system. Based on CFD technology, the hydrodynamic performance of the propeller-rudder system without/with RBTD is also simulated, and velocity distribution is observed. The results show that RBTD is successfully designed and as a ship energy saving device is feasible.

Experimental Study and Numerical Simulation on Hydrodynamic Performance of a Twisted Rudder

To analyze the energy-saving effect of a twisted rudder, this work presents the simulated and experimental results of propeller-rudder systems. In this article, a surface panel method (SPM) and computational fluid dynamics (CFD) are introduced to simulate the hydrodynamic performance of propeller-rudder systems. The thrust coefficient Kt, torque coefficient Kq, open-water efficiency η of the propeller, and thrust coefficient Kr of the rudder as a function of the advance coefficient J are obtained and plotted. The energy-saving effect of the twisted rudder is analyzed by comparing the results of numerical simulation and a cavitation tunnel experiment. The experimental energy-saving effect is 2.23% at the design advance coefficient J = 0.8. The pressure distributions of the propeller blade and rudder are plotted by two methods, and the difference of the force on an ordinary rudder and a twisted rudder is discussed. This study improved the experimental twisted rudder model. The change makes the rudder take advantage of propeller wake and improves the energy-saving effect of a twisted rudder. After improvement, the energy-saving effects obtained by the two methods are 0.448% and 0.441%. To analyze the energy-saving mechanism, this study compares the pressure distributions and efficiencies of different systems.