In the previous report, the longitudinal forces acting on a ship under accelerating or decelerating motion were treated and an estimating method of the transient motion was proposed from a practical point of view. In this report, the lateral forces acting on a ship in the transient motion, and accordingly manoeuvrability are investigated. The model ship tested is a Series 60 parent type ( L / B =7.0, C _B =0.7) and experiments were carried out in a towing tank using a PMM. As a propeller working effects on a hull and on a rudder are different from each other, it is reasonable to divide the lateral force of a ship into two parts, the rudder force including rudder induced force on a hull and the remainder. Propeller working effects are unexpectedly strong on a hull and can be represented by a simple function of the apparent advance ratio J _s , as seen in Fig. 6. The damping force acting on a rudder, in its dimensionless form, is proportional to the parameter k =√1+ k 8/π· K _T / J ², while the turning force is proportional to k ². As a consequence, both longitudinal and lateral hydrodynamic forces on a ship are possible to be represented using J _s . Hence, course stability of a ship and rudder effectiveness depend strongly on J _s in the transient motion. The results are also confirmed by the free running tests of the model