On the swaying,yawing and rolling motions of ships in oblique waves

In this paper an approximate method is introduced to calculate the swaying force, yawing and rolling moments acting upon a ship navigating in oblique waves. It next treats of linear coupled equations of swaying, yawing and rolling, where the ship has a finite velocity, in order to derive an approximate method for solving the problems of forced oscillation.

As the first step for studing such coupled motions in three degrees of freedom in lateral plane, the author investigated mutual coupling actions among swaying, yawing and rolling when the ship has zero velocity.

That is to say, the author worked out a numerical calculation for three kinds of hull forms by means of the linear coupled equations of swaying, yawing and rolling which he derived in 1965 for the case of zero ship velocity.

The results can be epitomized as follows:-

In case of a hull form with cut up stern, there is a remarkable coupling action between yawing and rolling.

Also in this case, if yawing is restrained, rolling will find its maximum value in quartering seas.

Depending on hull shapes, it may be possible that we may find rolling at its maximum in quartering seas because of the coupling actions of swaying and yawing.

When a ship has a cross section of large beam/draft ratio, the coefficient of effective wave slope γ sometimes becomes much larger than that of Dr. Watanabe’s theoretical calculation.