An Experimental Study on Instability of a Super High-speed Planing Craft with Outboard Engine at Straight Running

Recently, there are super high-speed planing craft, which can run at more than 100 km/h, owing to engine improvements and reductions in hull weight. At high-speed range, where most of hull is above the water surface, even if it runs in calm water, there are some kinds of peculiar unstable phenomena. For example, porpoising, corkscrew, transverse pure stability loss, directional instability, dutchroll, and so on. In this study, the stability of a craft with lower-hull of an outboard engine, whose real craft causes unstable phenomena at super-high forward speed regions, is investigated experimentally. Especially, the effects of lower-hull on the stability are focused. First, in order to investigate the roll stability, the measurement of roll restoring moment is carried out. The results show that there is rarely the possibility of occurrence of the instability except for the special running attitude caused by special loading condition, etc. Second, in order to investigate maneuverability, the measurements of maneuvering hydrodynamic forces are carried out and the indexes of maneuverability are obtained. The results show that straight line stability increases but yaw checking ability decreases by attaching lower-hull of outboard engine and there is the possibility of occurrence of directional instability. Finally, in order to investigate stability of straight running based on sway-roll-yaw coupling motion, the hydrodynamic forces of the motion equation are measured. As the results of investigation by Routh–Hurwitz stability criterion, for a hull with lower-hull, it is indicated that there is the possibility of roll-yaw coupling self-excitation motion if roll and yaw damping moments are small enough.