Hydrodynamic interaction concerns the influence of one body's flow field on another's. The importance of interaction will depend on both the body separation distances and the relative sizes of the bodies. The hydrodynamic interaction includes not only the radiation coupling but also the shielding effects as well.

Two points regarding hydrodynamic interaction should be emphasized:

First, the response amplitude operators (RAOs) for each of the hydrodynamic interacting structures will be different from those that would have resulted if each of these structures were on its own. The RAOs are not a physical property of a structure but, as can be seen from the equations of motion, depend on the radiation and diffraction forces. The radiation as well as the diffraction forces change in the case of hydrodynamic interaction and therefore the RAOs of the structures in the equation of motion will also change.

Second, when hydrodynamic interaction is employed, special attention is needed when you move the structures relative to each other. In Aqwa, the RAOs of hydrodynamic interaction structures are always evaluated relative to the fixed reference axes (FRA). Therefore, if different positions of one or more hydrodynamic interacting structures are defined in two consecutive Aqwa radiation and diffraction analyses, the results between these two runs will not be comparable.

In the multiple structure hydrodynamic interaction case, the total degrees of rigid body motions are 6×M where M is the number of structures; the total unsteady potential is usually expressed as a superposition,

(4–32)

where is the isolated space dependent incident, is the diffraction wave potential, and is the amplitude of motion of the j-th degree of freedom of the m-th structure. is the radiation potential due to the unit j-th motion of the m-th structure while other structures remain stationary, mathematically it is defined by the boundary condition on the wetted hull surface:

(4–33)

where, similar to the definitions in Equation 4–5,

(4–34)

in which is the mean wetted hull surface of the m-th structure with its center of gravity at in the FRA.

By employing Equation 4–32, Equation 4–33, and Equation 4–34, the source distribution approach discussed in Radiation and Diffraction Wave Forces can be used directly. Once the unsteady potential is calculated, the wave exciting forces and radiation force related added mass and damping coefficients are expressed as

(4–35)

where the subscripts m, n correspond to the m-th and n-th structures, and the subscripts j, k refer to the motion modes.