This paper discusses a conventional standard of tugboat assistance in a harbour using a mathematical model. Diagrams show required tug-power for each element of the harbour manoeuvres and for each ship type. The turning and drifting motion in a harbour is the most typical combined ship handling manoeuvre and the final position of this motion is important for this ship handling plan. As a result of this study, formula are presented to estimate the moving distance from start to final position during turning and drifting motion and it is confirmed that these formula accurately agreed with the simulation results.