# The Prediction of the Performance of a Twisted Rudder
###### Abstract
[PERSON] [28] carried out an experimental investigation of the effect of rudder location on the propulsion efficiency of a single-screw, single-rudder container ship. [PERSON] et al. [29] experimentally analyzed the hydrodynamic characteristics of the propeller-rudder system at low-speed operation. The problem of rudder cavitation has been an issue due to the type of rudder that has been adopted to most large ships [30]. Thus, full-spade rudders have been used for high-speed vessels and researched to avoid cavitation problems. [PERSON] [31] provided an empirical formula of inflow angle for twisted rudder generation and discussed the effectiveness of reaction rudder on rudder cavitation. [PERSON] et al. [32] proposed a rudder design method to improve rudder cavitation performance and tested a twisted rudder in a large cavitation channel concerning rudder surface cavitation and cavitation inception speed. [PERSON] et al. [33] and [PERSON] et al. [34] developed twisted full-spade rudders based on incoming flow angles for a large container ship to recover the swirl energy for the propeller slipstream. [PERSON] et al. [35] designed three twisted rudders for a large container carrier and verified the speed performances of the ship improved by those twisted rudders through model tests in a towing. Furthermore, they analyzed the change in self-propulsion factors by the twisted rudders. [PERSON] et al. [36] developed a twisted rudder to overcome the cavitation problems of large container carriers in the way of the leading edge also considering manufacturing productivity. They called this rudder the X-Twisted rudder and verified its hydrodynamic performances through various model tests such as resistance, self-propulsion, cavitation, and maneuvering tests. [PERSON] et al. [37] performed numerical simulations and cavitation tunnel experiments to predict the hydrodynamic performance of twisted rudders and verify the energy-saving effect. [PERSON] et al. [38] designed a twisted rudder by using the genetic algorithm and investigated cavitation behaviors on the rudder surface. A wavy twisted rudder superior to a conventional full-spade twisted rudder with regard to lift-drag ratio and stall delay was proposed and verified by [PERSON] et al. [39]. In general, most research on twisted rudders has been carried out for single-screwships. However, this study focused on a twin twisted rudder of a surface combatant and investigated its effects on hydrodynamic performances. In the current work, a novel design of a twisted rudder has been proposed to improve the rudder performance of an existing surface combatant. Our approach involved prediction and correction of twist angle distribution which compensate for the effect of inflow angles induced by the propeller. While developing the design of the twisted rudder, our main focus was to improve the rudder cavitation performance without any loss of lift performance. The primary design criteria are a decrease in rudder moment and a decrease in the drag force of the twisted rudder up to a certain level which is not harmful to the ship's self-propulsion performance. Three twisted rudders with three different twist angle distributions were designed using the results of the CFD simulations and tested both numerically and experimentally to estimate their hydrodynamic performances at a relatively high ship speed. Using the same test setup and procedures, a reference flat rudder was also tested to validate the effect of twist angles on the rudder performance. The numerical and experimental evaluations mainly focused on the rudder surface cavitation, rudder forces, and moment. The initial twist angle distribution was predicted from numerical simulation and its accuracy was estimated. A method of correction of the predicted initial inflow angles induced by the propeller was proposed and applied to obtain the final corrected distribution of twist angles.