Inhibition on porpoising instability of high-speed planing vessel by ventilated cavity

Abstract

Porpoising instability is a tough nut bewildering ocean academia for years. This study provides the first experiment found that the cavity formed by artificial ventilation can significantly inhibit the inherent porpoising instability of planing vessel under high/ultra-high forward speeds. Restricted by the occurrence of porpoising instability, a Froude number (volume of displacement based) upper limit of 5.91 is reached at the current conventional ship model calm water towing experiment. However, this instability phenomenon disappears when a cavity is formed at the bottom of ventilation ship model within all experimentally Froude number range (5.91−6.82). The inhibition effect can be tracked from the timing sequences value of trim. The decrease in trim value, to the degree below the threshold of porpoising instability, is responsible for the disappearance of this longitudinal instability. The enhancement in longitudinal stability is believed to be directly related to the increase in volume of displacement led by the cavity, which greatly reinforces the impact of hydrostatic lift in the vertical force balance. The strengthening of hydrostatic lift attenuates the influence of dynamic instability caused by fluid dynamic pressure. Therefore, if an external disturbance is given to the vessel, the disturbing moment around the center of rotation is weakened, which prevents the strong periodic alternation between inertial and restorative forces.