Investigation of the manoeuvrability characteristics of a Gate Rudder system using numerical, experimental, and full-scale techniques

The Gate Rudder is a recently introduced twin rudder system whose major advantages stem from its energy-saving properties. The two foil-shaped blades of the rudder, placed aside of the propeller, act as a partial duct in the wake of the hull producing additional thrust. However, since the rudder is a primary safety device on any vessel, investigating the manoeuvrability performance of the Gate Rudder is a critical aspect that needs careful and detailed consideration. Owing to its peculiar working principle and location, the standard manoeuvrability prediction methods cannot produce reliable results since they are applied to rudders acting behind the propellers. This paper presents the first comprehensive investigation into the manoeuvrability performance of a Gate Rudder system, which includes the development of a modified MMG model, towing tank experiments and full-scale measurements. The modified MMG model was conceived to predict the manoeuvring motions of a ship with the Gate Rudder system. A generalised prediction method is defined based on this modified MMG model and detailed CFD analysis of the flow pattern around the Gate Rudder for two commercial hull-forms. The simulation model of the Gate Rudder is validated by means of towing tank tests and full-scale manoeuvring trials. The sea trials were conducted onboard two sister container vessels, the first fitted with the first-ever Gate Rudder system and the second with a high-performance flap rudder. This also allowed to compare the two different steering systems.