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In the first report, the unusual phenomena appeared on the two full-bodied ship models are investigated. Although both models are ore-carriers with similar ship form, one of them shows the unusual phenomenon more typically and radically. To investigate the unusual phenomenon more deeply, two ways of approach were done using this model. First, as described in the second report, the full-scale trial was performed and it is found that the unusual phenomenon also appears on the actual ship as well as on its model ship. In this paper as the second approach, the capative model tests were again carried out aiming at the investigation of the stern flow field and the effect of propeller or rudder on the phenomenon. In the result, following conclusions are derived: 1) The unusual phenomenon is caused by the unusual (additional) hydrodynamic force acting on the stern. 2) This force is induced by the large flow separation which occurs either starboard or port side of the stern (see Figs. 1, 2 and 3). 3) This separation is produced only when the propeller is rotating, and its rotating direction is secondary importance for the separation. 4) The basical cause concerning to this one-side separation is the unstable flow field around the stern, where flow from the bottom or the side is not apt to reach smoothly to the upper part of the stern. This circumstances becomes more or less severe according to the fullness of the upper part of the stern and the region of the propeller suction. 5) When the clearance above the propeller or the rudder is large and if the stern is flat, this unstable flow field developes into the one-side flow separation. 6) To avoid the unusual phenomenon, a center fin shutting the clearance is effective from the practical point of view. 7) The mathematical model of ship motion accompanied with unusual phenomenon, proposed in this paper, can represent the motion fairly well.