In this work, a set of methods, namely, a time domain transient Green function solver, alternatively a time domain hybrid solver for zero speed problems and the adopted hybrid solver for nonzero forward speed problem, are developed to study the seakeeping behavior of ships and floating structures in waves. It is proved that by properly selecting a solver the developed package is capable of simulating large amplitude motions of ships or floating structures in waves thus it can serve as an advanced tool for the assessment... Obtained results include radiation/diffraction forces, wave making resistance, motion simulation and added resistance (drift force). The reliable prediction of the seakeeping behavior of ships in real seas is a demanding task for naval architects and of great interest to ship owners/operators, as it affects both the design and operation of ships. Validation cases cover both simple mathematical geometries and practical ship hulls, either without or with forward speed.