² three rotations about these axes:
– roll about the x-axis, positive right turning,
– pitch about the y-axis, positive right turning, and – yaw about the z-axis, positive right turning.
These de…nitions have been visualized in …gure 3.1.
In many cases these motion components will have small amplitudes. Any ship motion is build up from these basic motions. For instance, the vertical motion of a bridge wing is mainly build up by heave, pitch and roll motions.
Another important motion is the vertical relative motion, de…ned as the vertical wave elevation minus the local vertical motion of the ship. Thus, this is the motion that one observes when looking over the rail downwards to the waves.
3.1.1 Axis Conventions
Three right-handed orthogonal coordinate systems are used to de…ne the ship motions:
² An earth-bound coordinate system S(x0;y0;z0).
The (x0;y0)-plane lies in the still water surface, the positive x0-axis is in the direction of the wave propagation; it can be rotated at a horizontal angle ¹ relative to the translating axis system O(x;y;z) as shown in …gure 3.2. The positive z0-axis is directed upwards.
² A body–bound coordinate system G(xb;yb;zb).
This system is connected to the ship with its origin at the ship’s center of gravity, G. The directions of the positive axes are: xb in the longitudinal forward direction, yb in the lateral port side direction and zb upwards. If the ship is ‡oating upright in still water, the (xb;yb)-plane is parallel to the still water surface.
² A steadily translating coordinate system O(x;y;z).
This system is moving forward with a constant ship speed V . If the ship is stationary, the directions of the O(x;y;z) axes are the same as those of the G(xb;yb;zb) axes. The (x;y)-plane lies in the still water surface with the origin O at, above or under the time-averaged position of the center of gravity G. The ship is supposed to carry out oscillations around this steadily translating O(x;y;z) coordinate system.