A series of experiments were carried out to investigate the occurrence of the Mathieu-type instability. The main objective of this study is utilization of an auto-parametrically excited oscillation for wave energy converters. In this paper, the subject is the auto-parametrically excited oscillation of a spar-buoy type point absorber with two degrees of freedom. A small spar buoy model with a ballast controlling system was made and the model experiments were conducted to realize the large oscillating motion based on the Mathieu-type instability. The ballast controlling system is installed in the buoy model and the vertical movement of the ballast produces a certain change of the pitching natural period. Using the controlling system, the pitching motion in regular waves under the heave resonant period was measured. In some experiments, it was observed that the large pitching motion occurred suddenly, and the time histories showed different excitation pattern from the theoretical Mathieu-type instability. Based on the model experiments and considerations of the theory of Mathieu-type instability, the occurrence of the large pitching motion is discussed.

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