For these ten years, the author has investigated about unstable phenomena for high-speed planning craft experimentally and theoretically. In this paper, some of the unstable phenomena are introduced. The measured results of the motion of dynamic instabilities are shown and the characteristics of them are explained. The estimation method developed by the author to assess the possibility of their occurrence and some results are shown. And the mechanism of the instabilities is also explained in detail from the point of view based on motion equations.


Planning craft are sometimes suffered from various unstable motions that are completely different from those for conventional displacement type vessels. Some of them are known as non-zero heel, bow steering, bow diving, chine walking, corkscrew, porpoising and so on. Occurrence of the phenomena has close relation with high forward speed. Therefore, it can be safely said that the phenomena occur only when hydrodynamic forces are dominant comparing with hydrostatic forces like buoyant forces. Then, the phenomena are generally referred to as dynamic instabilities (Blount & Codega, 1992). Although the phenomena are dynamic, the occurrences of them are fundamentally governed by stability characteristics created by hydrodynamic forces at high forward speed. The author has investigated the phenomena experimentally, revealed the mechanisms of the occurrences of some of them, and proposed several methods to assess the occurrences of them using measured hydrodynamic forces acting on a model towed steadily at high forward speed. In this paper, especially, some of the dynamic unstable phenomena for planing craft, that measured by Katayama et al. (1996, 1997a, 1997b & 2001) and Suzuki et al. (2004), are introduced. The measured dynamic instabilities are shown. The unstable phenomena are called porpoising and transverse porpoising, and the characteristics of them are explained.

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