Research On Speed Loss During Ships' Turning

With the increase of large ships' quantity, speed loss during ships' turning has been paid more and more attention. Generally speaking,speed loss should be evaluated by the nonlinear mathematical model of ship motion. Both Abkowitz nonlinear model and the MMG model are complex and of much calculation,although they can be applied to predict the ships' turning parameters including speed loss. So approximative method is usually applied in practice. Several charts about speed loss and evaluation formulae have been deduced by Davidson , Schoenherr and other foreign scholars. Nevertheless, these charts cannot explain the difference among ship's different loading conditions. Furthermore,the charts and the formulae can only be used to forecast the ratio of velocity V during stationary circle phase and initial velocity V₀ before turning and cannot predict the velocity of transition phase which is often involved when ships turn around to avoid collision. Consequently the charts and formulae have some limitation.In another way, two relatively accurate formulae of K and T are acquired through multinomial regression of 47 ships' K,T statistical data. After analysis of some speed loss data of ships' turning experiments,the speed loss coefficient is expressed by ship's turning index K', rudder angleδand course changingΔψ. The limitation of the formers' research can be overcome to an extent by this method. Prediction of speed loss during ships' whole turning process is realized.In this paper a VLCC named OPALIA is taken as an example to predict the speed loss during her turning. Compared with the formulae and charts above, the prediction result is satisfactory. The maximum error is 14.6% and the minimum error is 0.3%. It can be concluded that the multiple nonlinear regression analysis is of feasibility to predict speed loss during ships' whole turning process.