From the view point of marine safety, it is of importance to evaluate the ship manoeuvring motions, such as the berthing in a harbour, the drifting behaviour of a disabled ship. For the prediction of ship manoeuvrability with high accuracy by numerical simulation, it is required to estimate the accurate hydrodynamic forces acting on ship not only in relatively high speed motions with small drift angle but also in low speed motions with large drift angle. At larger drift angles the flow around the ship's hull will be complicated due to vortex shedding and separation, but it is considered that the manoeuvring characteristics of a ship will be significantly influenced by hydrodynamic forces, initiated by the cross flow underneath a ship from the inflow to the leeside, and cross flow effects become dominant as the ship's drift velocity becomes relatively larger compared to the ahead speed. In this paper, special attention is given to the cross flow around each cross section of a ship during oblique motions with large drift angle, from lateral shifting motion to ahead motion. A distribution of bound vortices and free vortices is used to represent 2-dimensional separated flow around ship's section. The separation points are defined from velocity distributions for non-separated flow. The representation of the latest free vortex shed from a separation point at any incidence is discussed. Numerical results for a container ship hull in drifting motions are presented. By comparing with the measured results of oblique towing model tests at increasing forward speed, the calculated lateral forces and yaw moments acting on a ship agree well with experimental ones. Therefore, this approach will be useful for more accurate prediction of ship's manoeuvrability at low speed.