# The Prediction of the Performance of a Twisted Rudder
## 5 Conclusions  
The present paper reported a design approach to develop a twisted rudder to improve the rudder performance of an existing surface ship. An accurate twist angle distribution was determined by introducing prediction and correction steps for the inflow angles induced by the propeller. The performances of three twisted rudders with different twist angle distributions were evaluated through both experimental tests in a large cavitation tunnel and CFD simulations. The hydrodynamic characteristics of the twisted rudders were discussed in terms of the total cavitation amount, drag and lift forces, and moment for each twin twisted rudder by comparing with those of a reference twin flat rudder. Due to the symmetry of the twin rudder, the compensation of the induced inflow angles may be unfavorable in the starboard rudder or port rudder for a given rudder angle; such a situation was considered in the current rudder design for a twin twisted rudder. The total amount of surface cavitation on the final optimized twisted rudders on the starboard and port side decreased largely at the reference design rudder angle of \(\delta_{\text{R}}=-15^{\circ}\) by about 43% and 34.4% in the experiment and numerical prediction, respectively. This shows the importance of compensation of the effect of twist angles and the validity of the present correction method for the inflow angle distribution. The total lift forces for each twin twisted rudder at the reference design rudder angle were slightly larger than that of the twin flat rudder by about 6-11% found through experimental measurements and by about 3-11% obtained via numerical simulations. Finally, in the final designed twin twisted rudder, there were about 5.5% and 3% increase in the total lift force in the experiment and numerical simulation, respectively. This indicates that the use of twisted rudders is more effective without any loss of the ship's maneuvering performance. The maximum moment coefficients of all the twisted rudders were smaller than that of the flat rudder. The decrease in the maximum rudder moment for the final designed twisted rudder was 37% and 66.5% as obtained from the experiment and numerical simulation, respectively. The total drag force for each twin twisted rudders at a rudder angle, \(\delta_{\text{R}}=0^{\circ}\), increased compared with that of the twin flat rudder. However, the increase in the final designed twin twisted rudder was the least. In addition, the total drag force at rudder angles higher than \(4^{\circ}\) and \(6^{\circ}\) in the experiment and numerical simulation, respectively, was smaller than that of the flat rudder, but those of the other twisted rudders were not. This shows that loss in the ship's self-propulsion performance can be minimized by using an optimized twisted rudder. However, the self-propulsion performance should be tested in a towing tank. By using a twisted rudder, rotational energy can be recovered, which in turn, should lead to improvement in the propulsion efficiency. Conceptualization, I.P. and B.P. ; methodology, I.P., J.A., and J.K.; software, J.K.; validation, J.A. and I.P. ; investigation, I.P. and J.A. ; writing--original draft preparation, I.P. ; writing--review and editing, I.P. and J.K.; supervision, B.P. ; project administration, B.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the Ministry of Trade, Industry, and Energy and Defense Acquisition Program Administration and the Agency for Defense Development. This study was supported by grants from the dual-use technology project, \"Three-dimensionally curved twisted-rudder technology\" of KRISO (PNS3450). **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.