This paper presents results of free-running model tests concerning ship maneuvering motions due to assist tugboats and simulation calculations based on the mathematical model proposed in this paper. The free-running model tests were carried out by the use of wind fans on the model deck which simulate the tug forces. The mathematical model for such maneuvering motions was developed on the basis of captive model tests which consist of the static drift test with whole range of drift angle and the yaw rotating test with several combinations of towing speed and yaw rotating rate. The simulation calculations were compared with the free-running test results for typical maneuvers with tugboats. Through the present study, the following conclusions are obtained.
(1) The free-running model tests clarify the basic characteristics of ship maneuvering motions due to tug forces and indicate that such maneuvering motions largely depend on ship forward speed.
(2) The simulation calculations show satisfactory agreement with the free-running test results. This indicates that the mathematical model proposed in the present paper is available for describing a so-called low speed maneuvering motion dealt in this study.