The forced oscillation method is a very suitable method to find out the characteristics of oscillations, and several types of forced oscillation generater have been developed. However, most of them are designed for the model towed by or running under the tank carriage, which can be used as a platform to endue the force. For the investigation of the rolling characteristics, it is desirable to use a completely free running model, for example in a square shaped basin, because the experiments by the model can be carried out without any wall effects and any complicated techniques. For the completely free running model, very few oscillation generater can be used except the weight shifting type. At first, we also used this type, however, the oscillater produces the movement of the centre of gravity together with the inclining moment, that makes the scope of analysis of the oscillation characteristics limited.
Recently, we have completed a new type forced oscillation generater to use on the free running model test. As is shown in Fig. 2, the new oscillater is composed of two gyroscopes, which have their axis vertical and are turning in opposite direction. The angular velocity is given to the gyroscope around the y axis oscillatory, then the forced oscillatory moment is produced around the x axis as the precession moment. As the direction of rotation and the given angular velocity of each gyroscope is reverse each other, only the forced moment on the same direction around the lateral axis x is produced, while the reaction moments are cancelled each other. Moreover, even if the model perform the pitching motion, the rolling moment is not produced. The superior character of this method to Gerritsma's and Golovato, s methods is that the oscillation can produce the pure sinusoidal forced moment by giving the sinusoidal angular velocity to the gyroscopes. The characteristics of oscillation can be obtained by measuring and analysing the motion of the model.
Here, as the first attempt, we have carried out the forced rolling experiments by the free running model to obtain the rolling characteristics, especially when she is advancing with forward speed. The results are summarized as follows ;
1. The rolling characteristics of ship while she is advancing are essentially the same with that while she is drifting. Even in rolling, it is sufficient to consider the effect of the nonlinearity of damping term in the equation of motion, and the non-linearity of restoring term can be neglected, so far as the amplitude is modelate.
2. Comparing the forced rolling tests in still water with the rolling tests in regular beam waves, it is concluded that the amount of forced moment of the wave affecting on the ship with advance speed is almost the same as that affecting on the ship in drift condition.
3. The effects of forward advance speed on the roll damping of the ship were made clear quantatively as well as qualitatively. The damping of roll increases with the increase of the advanced speed of ship, and around the service speed of the ordinary ships, say at Froude number of 0.2 to 0.3 it becomes about 1.5-1. 8 times the damping in drift condition. It is very interesting to find that the damping of ship has usually a hump and a hollow at the same range of the Froude number.