It has been passed more than ten years since so-called unusual phenomena in manoeuvrability were first reported by Nomoto. At the early stage of research, the phenomena were regarded as the unusual scale effect, because it appeared only in model ships. According to the tendency of growing the fullness of ships, however, these phenomena slided even into real ships. Besides, quite many types or varieties were appeared from the results of free-running tests. For these several years authors have treated many cases appeared on either model ships or real ships, carrying out free-running tests, captive model tests or full-scale trials. Some of these results were published. Through the detailed examination of these experiences, the comprehensive explanation of the phenomena is made as the main purpose in this paper. It is known from the captive model tests that in model ships with the phenomena the measured hydrodynamic forces contain two different characteristic forces : one is the ordinary force proportional to the motion of ships and the other is the abnormal force which appears only in ships with the phenomena. This abnormal force is found to originate from the flow separation at either side of the stern by the flow observation and divided into two main types ; binary type and breakline type (see Fig. 5). Although this abnormal force is the direct cause of the phenomena, such many types of the phenomena don't seem to be derived from only the two types of the abnormal force. Another factor closely relative to the phenomena is directional stability of ships. Almost all the unusual phenomena ever found are shown to be well explained by the combinations of directional stability with two types of abnormal force.