# The Prediction of the Performance of a Twisted Rudder
###### Abstract  
A new design approach using the concept of a twisted rudder to improve rudder performances has been proposed in the current paper. A correction step was introduced to obtain the accurate inflow angles induced by the propeller. Three twisted rudders were designed with different twist angle distributions and were tested both numerically and experimentally to estimate their hydrodynamic characteristics at a relatively high ship speed. The improvement in the twisted rudders compared to a reference flat rudder was assessed in terms of total cavitation amount, drag and lift forces, and moment for each twin rudder. The total amount of surface cavitation on the final optimized twin twisted rudder at a reference design rudder angle decreased by 43% and 34.4% in the experiment and numerical prediction, respectively. The total drag force slightly increased at zero rudder angle than that for the twin flat rudder but decreased at rudder angles higher than 4\({}^{\circ}\) and 6\({}^{\circ}\) in the experiment and numerical simulation, respectively. In the experimental measurements, the final designed twin twisted rudder gained a 5.5% increase in the total lift force and a 37% decrease in the maximum rudder moment. Regarding these two performances, the numerical results corresponded to an increase of 3% and a decrease of 66.5%, respectively. In final, the present numerical and experimental results of the estimation of the twisted rudder performances showed a good agreement with each other. twisted rudder; rudder performance; cavitation performance; CFD; large cavitation tunnel 40.  
fluid dynamics (CFD) simulations and/or model tests performed in a towing tank and a cavitation tunnel. It is reported that discrepancies have been found between CFD and ITTC 1978 prediction concerning the scale effects on ESDs' performances estimated at model scale [16], and the need for CFD simulations at full-scale conditions and sea trials to assess these scale effects has been discussed [17]. There is limited sea trial data in the public domain, but it is clear that several ESDs have been fitted to operational ships and attempted in full-scale trials [18]. [PERSON] et al. [18] investigated the scale effects on ships with ESDs using CFD simulations and reported the reliability of CFD data at full scale compared to model scale. [PERSON] et al. [19], [PERSON] et al. [20], and [PERSON] et al. [21] studied a rudder bulb, or an additional device in a fin on the rudder bulb to retrieve the energy loss behind the propeller. These studies showed that rudder bulbs devised increase propulsion efficiency by reducing hub drag, which is caused by decreased separation and pressure pulse. [PERSON] et al. [22] showed the energy-saving effect of the combination of rudder bulb and rudder thrust fin is better than that of rudder bulb at full-scale trials. [PERSON] et al. [23] proposed two kinds of newly developed rudders that obtain the low drag effectively comparing with the conventional rudder. [PERSON] et al. [24] employed a parametric geometry design using the non-uniform rational B-splines (NURBS) technique to obtain the best rudder geometry for improving propulsive efficiency, where rudder optimization was performed based on CFD. [PERSON] et al. [25] developed rudder gap flow-blocking devices to suppress rudder gap cavitation and carried out cavitation observation and pressure measurement in a cavitation tunnel for the estimation of their cavitation performances. [PERSON] et al. [26] investigated the unsteady cavity patterns around the gap of the conventional and newly developed semi-spade rudders for marine ships using a high-speed CCD camera, time-resolved particle image velocimetry (PIV) analysis, and pressure measurements. The relationship between the cavitation phenomenon on the rudder surface and the rudder angles was analyzed by [PERSON] et al. [27], where PIV technology was adopted to observe the flow field between propeller and rudder.