The rudder normally operates in the wake of the propeller in the stern of the ship. This is needed in order to create enough lift for its main role, maneuvering, with the high speed flow leaving the propeller. When the ship is in free sailing condition, the rudder is only responsible for doing minor course corrections, normally carried out by the autopilot. Due to the pressure distribution over the rudder, different forces appear, mainly a longitudinal force that could be drag or push force and a transverse force affecting the ship’s course.
The wake adapted rudder is normally designed so as to improve the propulsive and maneuvering performance of the propeller – rudder unit. The amount of energy which is recovered will depend on the form and thickness of the profile of the propeller, the aspect ratio, Rn, the spatial distribution of the velocity upstream and the turbulence of the flow. It is expected that a wake adapted rudder has less transverse force than a conventional one at 0º angle when a vessel sails in free run operation, therefore reducing the need for autopilot corrections.
In single screw vessels there are also a force unbalance due to the asymmetry of the propulsion system which modifies the flow differently on port and starboard side depending on how the propeller rotation is.
The flow is not stationary, mainly due to the rotation energy supplied by the propeller; also the presence of the rudder modifies the performance of the propeller compared to the open water case, partially blocking the water flow downstream the propeller. Analyzing this system by CFD means that the behaviour of the flow would imply the solution of an evolutionary problem on a moving mesh. These problems have high computational costs and are not suitable to be solved on a routine basis. A lot of articles consulted in the literature demonstrate that they can be dealt with in a quite precise way by means of Reynolds Averaged Numerical Stationary simulations so as to obtain integral values on the different regions of interest, among other can be cited Caldas et al.
Numerical simulations of a conventional and a twisted rudder with the same area are performed behind the running propeller of a vessel in order to determine how large the transverse forces at 0º angle are. A parametric study was done analyzing small variations of angle in order to search the optimum angle that minimizes net transverse. The simulations carried out included an analysis of the forces from each part of the ship i.e. rudder, propeller and hull. All the calculations are carried out using commercial CFD code STAR-CCM+.