The heeling moment against a small ship, caused by a billow breaking at the crest, is not yet sufficiently investigated. To investigate the characteristics of the billow, and then the dynamical mechanism of the heeling moment, the movements of water particles in a breaking billow are observed in the water tank. From the kinematical analyses of the movements of particles, the experiments on an abrupt water-jet, which is a model of wave breaking, are carried out. These experiments prove the following results, (1) In the breaking region of a billow, the general concept of the effective surface can not be applicable to the heeling moment of a ship. (2) The heeling moment M_q formed by the breaking is impulsive such that is caused by the abrupt water-jet. (3) Strength of M_q depends on not only strength of water-jet but also <KG>^^^-, form and size of a ship.
