This paper reports on an experimental study of applying pendulum theory to determine whether a rectangular hull model would overturn. A pontoon used as a non-self-navigating work vessel was considered in this study. Its model begins to list when an unbalanced load is applied. It can then either stabilize in an inclined state or continue overturning, throwing the unbalanced load midway, and finally stabilizing in the inverted position. In a mid-air experiment, this correlation was made between two motion scenarios, pendular or rotational motion. The two motions were reproduced by changing the distance between the hull center and the rotary axis and the mass of the weight. The experimental results closely matched those of an analysis performed based on the Runge-Kutta method, which confirms the validity of the proposed analysis method.