Numerical study on the thrust vector of a propeller-rudder unit in the bollard pull condition

In this paper, in order to enhance the DP capability of a four-screw shallow-drafted ship with large moulded breadth and block coefficient, the thrust vector for the inner-sided propeller-rudder unit in the bollard pull condition is studied by RANS-based CFD method. To account for the blockage effect of the hull on the propellerrudder unit, numerical simulations of hull-propeller-rudder interaction are carried out. To capture accurately the propeller-induced violent free-surface flow in the stern area, an adaptive refined mesh method is adopted. Validation study shows good agreement between the CFD and EFD results in terms of the open-water propeller characteristics, the thrust vector angle, and the free-surface flow in the stern area. Based on the numerical study, a reliable relation between the thrust vector angle and the rudder angle is established. Detailed investigations into the thrust vector angle, the hull-propeller-rudder interaction, the propeller and rudder hydrodynamic forces, and the propeller-induced free-surface flow show that as long as the requirement of the theoretical critical Reynolds number is met, the thrust vector characteristics are almost unaffected by the propeller shaft speed. Thus, the thrust vector angle obtained by the CFD method at model scale can be used for full-scale ship without further corrections.