Analysis of the asymmetric behavior of propeller rudder system of twin screw ships by CFD

The interference between the hull, propeller and rudder remarkably affects the control and maneuvering capabilities of marine vehicles. In case of twin screw/twin rudder ships, the asymmetric evolution of the wake past the hull causes the asymmetric functioning of the propeller rudder system. Systematic investigations on this aspect for twin screw ships are limited. Available experimental data carried out on simplified hull propeller rudder system and captive model tests do not allow to completely understand the fluid mechanism at the basis of the hydrodynamic interaction that should be taken into account in simplified maneuvering mathematical models for preliminary predictions. In this paper the hull-propeller-rudder interactions phenomena for a twin screw/twin rudder model are investigated by URANS simulations, with a particular focus on the asymmetry of the propeller rudder system. To this aim, captive model tests consisting of pure rudder and coupled drift-yaw motions corresponding to the steady phases of turning circle maneuvers at different rudder angles (delta = 15 degrees divided by 35 degrees) are performed at the speed correspondent to Fr=0.265. Moreover, a free running maneuvering simulation is also performed to gain more insight on the transient phase of the maneuver. An identity-rudder lift methodology is applied to synthesize the hull-propeller-rudder interactions by means of a flow straightening coefficient; the analysis highlights that these effects are weak and invariant with respect to the rudder angle on the windward shaft, whereas on the leeward side these effects are extremely sensitive to the evolution of the hull and propeller wake.