Multi-body dynamics is important in many fields of engineering. For the at-sea transfer of cargo between ships multi-body dynamics is particularly important. There are several methods of transferring solid cargo between vessels and these include by crane or by ramp. Each method is extremely sensitive to the relative motions between the various vessels. An accurate modeling of the vessels' motions is critical in determining limiting sea state conditions and in suggesting how to improve the given system. There are various levels of approximation which are commonly employed to model vessel hydrodynamics and we hope to eventually determine what level of approximation is appropriate for a given situation. In this paper, we will compare the effects of considering as well as ignoring the multi-body hydrodynamic interactions using a constant coefficients approximation to the time domain radiated wave force. Copyright © 2009 by TEES.