Aqwa also provides a facility for hull drag force and moment on a diffracting structure to be calculated in a similar way to that for Morison element, i.e. the structure motion in six degrees of freedom is taken into account in the drag force calculation, such as

(7–17)

where is a 6×1 matrix consisting of three Morison hull drag force components and three Morison hull drag moment components, is a 6×6 Morison drag coefficient matrix, and (_j = 1, 6) is the relative translational or rotational velocity component in the structure local axis frame.

By default, the translational relative velocity (_j = 1,3) in Equation 7–17 is the difference between the steady fluid velocity (current speed only without fluid particle velocity due to waves) and the structural motion velocity,

(7–18)

where and are the current velocity and structure motion velocity along the j-th direction of structure local axis frame.

This default definition of translational relative velocity can be optionally changed by only taking structure motion velocity components:

(7–19)

However, the rotational relative velocity components in Equation 7–17 are always in the form of

(7–20)