The objective of this paper is to afford a more rational method of calculation for hydrodynamic forces on a ship in manoeuvering motion. Applying the slender body theory and extending the Barche's method to a ship form associated with free vortex layers which represent 3-dimensional separation vortices, the problem can be reduced to some 2-dimensional problems. More precise hull form or ship condition can then be taken into consideration for hydrodynamic force calculation. The author calculates by this method normal force coefficient C_N and moment coefficient C_m _??_ for various ship models, and compares the calculated results with tank test results. These comparisons affirm the validity of this method. He also measures the pressure distribution on a ship hull in steady oblique towing, the results of which show a good agreement with those of potential flow calculation except in small region located at lee side and aft part of the ship.