Measurement of Hydrodynamic Sway Force Acting on Two-Dimensional Models of Small Ship

Abstract

In the previous papers authors studied capsizing phenomena of a small pleasure boat in beam seas. Small ships, which usually have large breadth/draft ratio, are apt to sway with a large amplitude in waves and to drift at a high velocity in wind. It was clarified that the large lateral motion can lead to a large drag force and moment because their complicated under-water hull forms can easily make flow separation. In a heeled condition the rolling moment by the separated flow works asymmetrically to the direction of sway motion, being larger when the ship moves to the direction of heeling than to the other direction. This asymmetries helps to enlarge the heeling angle and finally to capsize.In this paper a large amplitude forced sway motion test was conducted to investigate the effect of hull forms to the non-linear sway force and moment. Three two-dimensional round-chine models with dead-rise angle of 10, 20, 30 degrees were used. Totally the test was carried out on nine hull forms, using the three round-chine models with/without hard-chine pieces and skegs. It is clarified that a conventional linear integral equation method can almost evaluate the acceleration component of sway force and moment, but cannot evaluate the damping component, because drag component occupies the main or non-negligible part of sway damping. It is also clarified that the rolling moment by the sway motion gets very asymmetric when flow separation occurs locally at the vicinity of the bottom center or of the chine. The asymmetry becomes small when the separated flow covers the whole bottom, which happens on the models with skeg or with large dead-rise angle.This paper also deals with the force and moment of drift motion at a constant speed. The characteristics of the heeling moment is similar to that of sway motion. The moment works to increase the heel angle when the ships moves to the direction of heeling. But, it becomes small if the model makes a large separation region.