Abstract: Free-running model test using a VLCC model ship were carried out in regular waves. Ship speed, oblique angle, counter rudder angle together with periodic pitch and roll motions are analyzed for straight running test in waves. Drifting speed, drifting direction, average yaw rate, oblique angle and ship speed during turning motion are obtained for 35deg turning test in waves. Overshoot angles in zigzag test in waves are compared with those in calm water. Stopping distance and final state heading angle are also shown as functions of wavelength and initial encounter angle to waves. Effect of wavelength and encounter angle to waves together with effect of loading condition upon manoeuvring motion is discussed based on these experimental data. An example of theoretical calculation for manoeuvring motion of the model ship in wave is provided for further discussion.

　

 Manoeuvring standard adopted in the International Maritime Organization requires sea trial to be carried out in calm sea condition. However, in general, external forces due to wind and waves are inevitable at actual seas. Researches on wave effect on manoeuvring motion are not as numerous as those on wind effect on manoeuvring motion. In order to estimate effect of external forces on manoeuvring motion correctly, researches for studying wave effect on manoeuvring motion of ships must be promoted.

　

 Experimental study for investigating wave effect on manoeuvring motion is reported in this paper. In order to clarify wave effect on manoeuvring motion of ships, free-running model test using a VLCC model ship was carried out in regular waves. Two kinds of loading condition,full load and ballast conditions were applied for the experiment.

　

 Ship speed, oblique angle, counter rudder angle together with periodic pitch and roll motions are analyzed for straight running test in waves. Drifting speed, drifting direction, average yaw rate, oblique angle and ship speed during turning motion are obtained for 35deg turning test in waves. Overshoot angles in zigzag test in waves are compared with those in calm water. Stopping distance and final state heading angle are also shown as functions of wavelength and initial encounter angle to waves. Effect of wavelength and encounter angle to waves together with effect of loading condition upon manoeuvring motion is discussed based on these experimental data. An example of theoretical calculation for manoeuvring motion of the model ship in wave is provided for further discussion.

　

2.1 Model Ship Dimensions and Steady Turning Characteristics in Calm Water

　

 Coordinate system of a ship manoeuvring in waves is shown in Fig, 1. Ship motion is described using ship speed U, oblique angle β and yaw rate r. Rudder angle is represented by δ. Wave encounter angle is written by x.

　

 Principal dimensions of a VLCC model ship for free-running model test are listed in Table 1 for both full load and ballast conditions. Displacement in ballast condition is 55 percent of that in full load condition.

　

　

　

　

　

 Steady turning characteristics in calm water are shown in Fig. 2 and Fig. 3 for full load condition and ballast condition respectively. White circle and white triangle represent non-dimensional yaw rate r(L/U) where L represents ship length. Black circle and black triangle represent non-dimensional ship speed ratio U/Uo where Uo represents approach ship speed. Triangle marks stand for data measured in reversed spiral test. Fig. 2 and Fig. 3 tell that full load condition is directionally unstable while ballast condition is directionally stable. Other characteristics in calm water condition such as overshoot angles in zigzag manoeuvre are shown later together with those in waves.

　

2.2 Test Condition

　

 Free-running model test were carried out at a square tank of which size is 80m by 80m with 4.5m depth in National Maritime Research Institute, Japan. This tank is an out-of-door facility. Therefore, experiment was carried out in calm wind condition. Ship trajectory, ship speed, oblique angle, yaw rate and heading angle were measured using RTK-OTF (Real Time Kinematics on the Fly) positioning system, a kind of GPS [1]. Pitch and roll motions were measured using a fiber optical gyro.

　

 Experimental conditions in waves are shown in Table 2. Wavelength ratio λ/L was set to be 0.4, 0.6 and 1.0 in full load condition and 0.6 in ballast condition. Wave amplitude was set to be 0.02m. Propeller revolution was 19.11rps for full load condition and l7.26rps for ballast condition. In zigzag test and stopping test, initial wave encounter angle χi is one of test parameters. In head wave condition, wave encounter angle χ is equal to 180 deg. In a condition in which χ is equal to 90deg the model ship faces waves on starboard side. Rate of change of propeller revolution in stopping test is elaborated later.