Eq.(29) is motion equations for 2 ships moving in close proximity. Added mass and moment of inertia of 2 ships are explicitly represented in the equation and mutual hydrodynamic interactions are considered through changes of the added mass and moment of inertia.

 For reference, here, we consider a case of no existence of Ship 2 in eq(29). Then, all of the terms of time derivative of added mass and FDk vanish. Also taking the flow symmetry around the ship into account, the following equations are obtained as:

　

These equations are coincided with the motion equations derived by Lamb [7].